The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: olev on 22.11. 2008 05:49

Title: crankcase pressure etc
Post by: olev on 22.11. 2008 05:49
Ok, here is a bit of light reading for the technically minded.
We have a quality classic motorcycle magazine in Australia called OBA (Old Bike Australasia).
OBA no.8 had an article about crankcase breathing written by Rex Bunn. Rex is an author and inventor who is a member of the BSA owners club in New South Wales and in New Zealand. (must be ok) Rex also sells a breather kit for classic singles and twins. I have permission from OBA and Rex to post the article here. I have no barrow to push with Rex, an email is the only contact I have ever had with the man. I personally would be interested to read any thoughts the forum members have on this lot.
Rex also has a blog site that deserves a read.   Item 43 shows a Norton twin and what can occur on particular makes and models.

http://www.a7a10.net/oba.pdf
http://bunnbreather.bigblog.com.au/index.do

Erling, thanks for posting the pdf on the server.
I also suspect that oil leaks from A10 rocker boxes is caused by over pressurisation of the crankcase.

cheers
Title: Re: crankcase pressure etc
Post by: tombeau on 22.11. 2008 12:43
I think I might have to look into this.
My A10 has always leaked from the rockerbox and flung a fair bit out the exhaust. a complete top end rebuild that necessitated giving up high compression pistons years ago, did nothing for this and left me skint and sickened.So I've ignored it since.
I'd be interested to hear how people get on with this and fitting it to an A10.
Cheers,
Tombeau
Title: Re: crankcase pressure etc
Post by: scotty on 09.12. 2008 17:50
I read the links with interest Olev...thanks for posting them.

I'm in the process of re building my 55 A10GF engine and seriously thinking of installing one of the bunbreathers ....however I just read the following in BSA Twins & Triples by Roy Bacon. Page 158

" The breather valve of the A7 & A10 was improved near the end of its life. The newer item, part number 12967-0987 could be fitted to all the earlier engines but again required that the sealing cork washer fitted between it and the pinion face be under compression"

Anybody out there have experience with the "improved"  breather valve as fitted by BSA ?

Cheers

Scotty
Title: Re: crankcase pressure etc
Post by: NIGES on 17.12. 2008 23:46
Hi Scotty
I have the same problem with a 53 A10 engine I have just fully rebuilt and find I have loads of crankcase pressure so much so the breather pipe emits smoke continously I feel I have a big problem but I do not know the cause, is it pistons I wonder, only it has been rebord to +60 rings gaped correctly etc so it cannot be that, the oil is returning to the tank really well but I to get leaking from the rocker boxes I assume this is to do with the pressure and oil in the exhausts.
Anybody out there know the answer any advise gratfully received 
Title: Re: crankcase pressure etc
Post by: groily on 18.12. 2008 07:35
Oil in the exhausts? It can only get there by one of 2 routes - via sloppy valve guides on the way down from over-filled or over-oiled rocker box, or on the way up from the cylinders and out of the exhaust valves.  There's been some discussion here about the correct size of the rocker feed pipe unions at the head, although this sounds extreme for that.
And the breather is sending out continuous smoke? Presumably the oil consumption is pretty high as well. For smoke to be coming out of the crankcase, you're talking about burnt oil. If the oil level in there were simply too high and under pressure, you'd get an oil slick rather than smoke I would have thought.
Don't know, but if it was mine I'd have the head and barrel off again, and the breather bits out too, and look at the lot. My number one thought would be pistons and rings not bedded in, or rings of incorrect size or fit or oval bores.
A problem here could account for all of the things you note -
Excessive crankcase pressure possibly preventing the top end oil from draining effectively
Smoke from the breather which would be products of combustion getting down past the rings as the piston goes up on the exhaust stroke, thereby pressurising the cases and then venting to atmosphere, and
Oil in the exhaust from the small lake that will form on top of each piston as it descends and oil is forced past them from below, which will in turn take the direct route out when the exhaust valve opens and the piston ascends.

Don't take this as an answer NIGES, there'll be others with maybe better thoughts . . . but that's my thinking from what you say.
Title: Re: crankcase pressure etc
Post by: NIGES on 18.12. 2008 14:25
Thanks for the reply very interesting thoughts, I did have the barrels as I say rebored to + 60 And I used a used set of pistons with new rings fitted, but as you say the oil pool on top of the piston would burn and go out through the exhaust, you can see what you call oil pools on the piston crown, do you think the rings would bed in given a bit of time or not.
Thanks again
Title: Re: crankcase pressure etc
Post by: RichardL on 18.12. 2008 14:28
Niges,
Groily's points well taken, though, to me, guides in conjunction with excess oil feed and high pressure seem more likely. As you've said, the pistons, rings and bores were all redone but you did not mention the guides. Another thing that occurs to me is that it may be possible to use too thick a cork gasket at the rotating breather, leading to no turning and no breath. For the pistons, a compression test. For the rotating breather, outer timing cover off.

All, said, there is more sage advice here than mine.

Typos courtesy of Blackberry.

Richard L.

 
Title: Re: crankcase pressure etc
Post by: NIGES on 18.12. 2008 15:20
Hi Richard
I did wonder about the cork washer being a bit thick but dismissed this as the breather is turning, if it were not there would be score marks where the small spigot engages with the breather, a compresion test is a good thought I have a tester and will give it a go any idea what the readings should be?
Thanks for your thoughts

Regards Nigel
Title: Re: crankcase pressure etc
Post by: RichardL on 18.12. 2008 19:41
Niges,

By "spigot," I assume you mean the pin in the timing idler gear (also assuming a '53 A7 is like a '55 A10). I don't really understand what you mean by score marks on the breather from too thick a cork. I would think the cork might hold off the breather completely, though there might not be enough space for that. Nevertheless, if you're confident that it's turning, then, next steps.

As for the compression readings there is a long discussion of this on the forum at http://www.a7a10.net/forum/index.php?topic=381.0

If this leaves you crosseyed, someone here will surely be able to provide a PSI value for your engine. What compression pistons do you have (or did you already tell us?).

Richard L.
 
Title: Re: crankcase pressure etc
Post by: bsa-bill on 18.12. 2008 19:45
I have read somewhere ( a memorey would be useul ) that there are several versions of the breather, the difference being where the hole is relation to the pip.
Perhaps this is where the improved version differs from the early ones altering the timing of the breather.
Those more knowledgable than me might like to comment - enquiring minds need to know

All the best - Bill ( northeast England but not quite in Scotland)
Title: Re: crankcase pressure etc
Post by: Richard on 18.12. 2008 20:26
Should this be under chat?
Anyway
for my two penny worth there are loads of different mods for classic engines that have been so say proven and have all the theory guff to go with it but back in the late sixty's we had the same problems with the rocker box gasket leaking and we put a push bike inner tube valve stem in the rear rocker cover minus the valve core put a plastic pipe on it and took it to the back mudguard problem solved no more rocker box gaskets blown out due to excessive crankcase pressure building up in the top end.
The pressure is still being produced but relieved in the top end by the breather tube.
This is what I have done on my Super Rocket and it seems to have cured the rocker box gasket from leaking and has not cost me anything to do
Sometimes I think we are induced to spend our hard earned on unnecessary mods because we are told that they are the bee's knee's when in fact the bikes have an easier live now than when they were our main means of transport and we were younger then and thrashed them a hell of a lit more than now.
When BSA made the original bike they ran ok and any known problems were sorted out or we always knew what to do to sort them so why spend hundreds on oil feed mods and breather mods which are unnecessary
(More like 5 bob's worth but had to say it)
Richard
Title: Re: crankcase pressure etc
Post by: bsa-bill on 18.12. 2008 21:44
Should this be under chat?

Hold on there Richard, seems to me all replies on this thread relate to the original post.

Tend to agree with you about the mods though, as I recall we did'nt have the money to throw around in those days.
I never had a problem with rocker box leaks but there again mine was a 61 Flash so probably it had the later improved breathing.
My mate had a 59 A7 SS and it had serious rocker box leaks, but he admits himself to having filed the mating faces to improve the leak and in fact it got worse - don't think we had heard of the sheet of glass thing then though.

All the best - Bill
Title: Re: crankcase pressure etc
Post by: NIGES on 19.12. 2008 07:01
Hi Richard

What I mean by score marks on the breather is if the pin on the camshaft gear that drives the breather had a to thicker cork and was not engaging the breather fully the breather would maybe tend to skip over the pin and this would cause scoring to the rear face of the breather.

Nigel
Title: Re: crankcase pressure etc
Post by: terryk on 31.12. 2008 09:24
If you have engine pressure problems just drill a hole in the inlet side rocker cap they are easy to replace at swaps etc and tap a tread in it and run a pipe from it to the back of bike or wherever. Make sure that nothing can come up the pipe like water or dirt. Modern bikes have one way valve plastic flaps closed at end let pressure out but nothing in.
Title: Re: crankcase pressure etc
Post by: trevinoz on 31.12. 2008 20:56
Terry, put a PCV valve in the hole in the cover. A mate used one off an old car on his AJS and fixed his problem of oil being forced into the generator.
 Trev.
Title: Re: crankcase pressure etc
Post by: KeithJ on 07.10. 2016 11:12
Having some pressure problems after 200 miles with my A10 S/A with a completely rebuilt engine, I found the posts on the Bunn breather.  What I don't understand, among many other things is, if it requires to have a valve to allow air in and one to allow air out, why not just have a tube with no valves on it?  Is it because at some time there is a "dwell" when both valves are shut?  I thought there was a period where the engine required a specific time to be in either positive or negative pressure, can't recall which.

Appreciate some pearls of wisdom.

Keith
Title: Re: crankcase pressure etc
Post by: rocker21 on 07.10. 2016 13:24
many years ago Eddie Dow used to sell a pair of rockerbox  covers that were finned and the rear one had a breather in it and you fitted a pipe to it and took it out to the rear of the bike, problem solved
Title: Re: crankcase pressure etc
Post by: KeithJ on 07.10. 2016 13:47
Had "vent tubes" on my engine for years before I rebuilt and it did leak some oil form the engine breather but I think that was due to worn bores.  Will probably go back to some breather on the rocker box and see how it works.

ATB

Keith
Title: Re: crankcase pressure etc
Post by: Tomcat on 07.10. 2016 22:42


Ive read about the Bunn breather too, and came to the same conclusion.  Triumph have a huge breather pipe into the primary case which I assume is to neutralise the pressure and vacuum.
What about racing engines? Heavy breathing must be a serious problem when you thrash the f*** out of a vetical twin? Rocket Racer and Muskrat???
Title: Re: crankcase pressure etc
Post by: KiwiGF on 08.10. 2016 02:25
I'm not sure where the allow air in bit comes in, but the basic principle I think of the breathers on our type of engines (e.g. with pressure fluctuations in the cases) is to allow air out when the pistons have compressed any gases at around bdc, and to prevent air entering the cases when the pistons reach around tdc, and could potentially suck air in through the breather outlet.

The standard timed breather theoretically does the above and hence a vacuum develops in the cases after a few revs after start up, once the vacuum has developed the breather will just let any gases from piston blow by out at bdc, these gases should be minimal....all this can be seen by putting a tube in the breather outlet, with one end in a jar of water and starting the engine.

Creating a breather in the rocker box (which should share the same vacuum as the cases) would seem to negate the design principal behind the timed breather, but it seems to work for some.

From what I've heard it's very difficult to make a non return valve (which is what the bunn breather essentially is) that can operate reliably at high speed (which I guess is why BSA designed the timed breather) the bunn breather claims to operate at the required speed, but there are differing opinions on whether it works well in practice (and can replace or supplement the std breather) or not.....or is it solving a non existent problem  *dunno* or making things worse *dunno*

I've no experience of a bunn breather myself but a friend had one fitted but he eventually reverted back to the standard breather after a major investigation into his continuing his "blow by" and crank case pressure problems, this meant addressing blow by from all possible sources (Pistons, rings, bores, valve guides etc).
Title: Re: crankcase pressure etc
Post by: BSA_54A10 on 08.10. 2016 10:33
back in the days when engines had no oil seals and peak revs of 4000 rpm engine designers used the pumping pressure under the engine to create a partial vacuum in the crankcase thus at all of the places with oil slingers and all of the shafts with scrolls sucked air in thus reducing the amount of oil leaks.
No one bothered to challenge this idea for decades so it was thought you need a low pressure zone in the cases,
BSA originally used an unsprung flapper valve and these work fine on low reving long stroke engines with modest compression ratios and slow acceleration rates
By the time BSA started making Gold Stars, it became apparent the simple flapper could not cope, in fact flappers end up working backwards at high speeds and get totally out of phase with the engine, they also spew out oil vapours.

Thus BSA changed the position of the flapper on the singles and went to a timmed breather on the twins and latter unit singles.

However air has mass and there fore inertia, it will also compress before it starts to move so again the port ends up being out of time with the flow of gas.
BSA did a fairly reasonable job considering the amount of money available for this type of research remembering it was slide rules in those days.
No thermionic cameras and no computers and no one doubted you need to have a low pressure in the crank case.

However it was found that a flow through systems actually works better and remains in time with the engine pressure pulses.
Thus the Bunn Breather, a system that had been used in cars for decades but no one thought it would be beneficial on motorcycles.

The next problem was finding a valve that did not resonate anywhere within the operational range of the engine and that was hard.
On an A 10 you close off the timed breather in favour of two non return valves, one lets air in once a specific low pressure signal has been achieved while the other lets air out.

On some engines the valves can be the the same but on a lot of engines the valves need to be different and on others still no valve is needed on the inlet, it is all engine specific.
Then we come to the outlet.
Outlet tubes need to be tuned, same as an exhaust pipe so the diameter, length & stiffness of the outlet tube all need to work together to get the best results.
He did get up to 2 Hp increase out of some engines.
Royal Enfield & Harley both endorsed the system and originally  offerred them as a factory approved dealer fitted accessory.
Enfield then modified the breathing on subsequent new models based on the work Rex did for them.

If you normally ride sedately you may or may not derrive a benefit from fitting one.
BSA's own breathing seems to work reasonably well at speeds between 30mph & 60 mph on most models with standard gearing.
However I can testify that a Bunn will work better and a lot of previous leaky bikes have become oil tight through fitting one.
With A 7 & 10's the big advantage is the end of rocker box weeping.
Some will find a pair of over the counter check valves work just as well as the Bunn, it is factor of the actual bike and the way the owner rides it.
However the Bunn will work with everything because the resonance of his valve is outside the operating speeds of your bike.

I is exactly the same principle as resonance in reed valves ( which are a one way check valve ) fitted to 2 stroke inductions.
What works on one bike and makes a huge difference can actually make a different bike run worse. 
Title: Re: crankcase pressure etc
Post by: nimrod650 on 08.10. 2016 16:26
rocker box breather in past years i drilled a hole in rear cover to take a brake or clutch cable adjuster two fibre washers and length of petrol pipe job done
Title: Re: crankcase pressure etc
Post by: muskrat on 08.10. 2016 20:50
+1 for the Bunn breather. Unfortunately no longer available.
Cheers
Title: Re: crankcase pressure etc
Post by: mugwump on 09.10. 2016 16:17
Early on in this post NIGES stated he used USED pistons and new rings. How USED were the pistons I wonder. If the ring grooves or lands were worn this could cause the rings to 'pump' oil into the comb. chamber.
Title: Re: crankcase pressure etc
Post by: Klaus on 10.10. 2016 18:45
Find some time to whrite some lines about cankcase pressure.
All my engines had the genuie timed valve with no extra holes and other breathers.
At any time I had no problem with oilleaks or heavy oilconsumption, doesnt matter if i ride road or racetracks.
There is only a little more than a pint air in the crankcase that will pressed and expand by the up and down from the pistons.
This air will find his ballance in the first seconds after fired up the engine.
There is only one small hole in the timingvalve giving the full port open some hundreds of a second.
Imagine you driving with a 4000 revs it is fantastic that a pint of air will find his way out.

Oilleaks causing in bad gaskets, loosen bolts or nuts. Oil out of the breatherhole is a result from worn out corkwashers fixing the breathervalve, and blow by from the pistons, that give a heavy preasure to the crankcase from the combustion.

Check the pistonring gap, and be sure that the rings with the right side up, the are signed.
I trust no dealer with aftermarked spares I check it all. If parts are scrabby I send it without exception back to demand a good quality or money back.

I have done with a mate a two and a half week trip to Ireland and back to Germany with a full milage of a little more than 4000km.
Both bikes (A10 engines) had an oilconsumption from only 1 litre oil, all the way.
Ok after this trip the rockerbox gaskets was worn out, but this was the only one was leaky.

cheers Klaus
Title: Re: crankcase pressure etc
Post by: KeithJ on 11.10. 2016 09:01
As far as I know, I have assembled the engine correctly.  I did check the points you mention and I should be OK.  As I have not run in the bike, should I be hopeful, it will get better?
ATB
K
Title: Re: crankcase pressure etc
Post by: Topdad on 11.10. 2016 13:30
Keith,I vented the rocker box and it stopped the rockers weeping, really easy bit's to use are free standing gas fire pipe connectors approx. diameter qtr inch hole drilled washers each side and tighten cost less than a £5from local p/merchants done in an hour,.
Title: Re: crankcase pressure etc
Post by: t20racerman on 20.08. 2017 11:36
Hi all

A very interesting thread. My A10 has long had oil leaks from the top end, but after a rebuild, copper rocker gaskets, surface preparation etc, its fairly oil tight... UNTIL you hold high speeds for long periods of time. If you hold 80mph (about 4000rpm on mine) for along distances it starts to leak from the top end - as well as out of the tacho cable connection, the inner timing case tacho cable fitting boss and even the oil tank filler cap!. Clearly seems to be a high revs only crankcase breathing problem, as described by many. I have a tube on the engine breather leading to the chain, and very little (if any?) oil comes out there since I sorted out the pistons, bore and valve guides (to which I added seals on the inlets).

As this is a higher revs problem, I was going to try the rocker cover mod, but am unclear from this thread if that badly affects the main timed breather, or just helps sort the problem out. I wondered:

1) Does anyone use the 'road draft tube' idea? This would be a pipe running from the valve cover down to an open end facing down and located in the vehicle's slipstream so that (ref Wikipedia):
 "When the vehicle is moving, airflow across the open end of the tube creates a draft that pulls gases out of the crankcase. The high location of the engine end of the pipe minimises liquid oil loss. An air inlet path to the crankcase, called the breather and often incorporated into the oil filler cap, meant that when a draft was generated at the tube, fresh air swept through the crankcase to clear out the blow-by gases."
As a physicist, (but not an A10 breathing expert!) this makes a lot of sense

2) If not the above, do you need a collector, or just vent to the air/chain etc via a tube

3) A PCV valve was suggested - is this necessary, and has anyone tried this? I don't have an airbox though, so it seems this would not work correctly.

I'm happy to modify and experiment, but am only looking for rocker box mod advice as everything else is spot on.

Just for background info, my A10 is heavily modified, runs 9:1 pistons, has gas flowed head, spitfire cam etc.

Thanks
Adrian

Title: Re: crankcase pressure etc
Post by: edboy on 20.08. 2017 21:55
there are lots of different problems mentioned and probable causes of oil leaking. my rockerbox doesnt leak oil as i prepare joining faces with a file and wellseal on all the joints. no air is drawn in and no oil is blown out. simple.
before engine strips i would clean and reset the PRV so oil can go where it was designed to go. bsa recomended every 1000 miles i believe. however i also have a nice weepy oil surface on the crankcases which deters corrosion.
Title: Re: crankcase pressure etc
Post by: Topdad on 21.08. 2017 11:39
I used a second hand gas fire fitting ,fitted to the rear tappet cover and joined to rubber tubing running down towards the rear mudguard didn't cost a penny and works.
Title: Re: crankcase pressure etc
Post by: bikerboy on 25.09. 2017 02:02
I feel qualified to comment on this post for sure because I had exactly the same problem after my rebuild you people might remember.

I had the bores resleeved, used old heppolite pistons and new rings. The work was done by an old engineer who really knows his stuff and he passed everything as suitably sized no problem. A plce called teh rebore shop very highly respected in the south of england so I have been told.

3 times I had that engine back apart due to oil and smoke out of the breather. Burning oil. Smoking quite badly out of the exhausts at times. I replaced the guides even tho they did not seem that bad, as I was doing guides I fitted new valves as well. I done rocker box breathers etc etc. There was a long running post on this over the course of months as we all tried to fathom the problem.

In the end in desperation I bought new pistons and took them back to the rebore shop as they were metric just to make sure.

On fitting the new pistons everything was cured instantly.

I came to the conclusion that the new rings nowadays just dont suit old pistons. I can think of no other explanation.

Out of curiosity what colour are your spark plugs after running a while? Mine were always black from burning oil. 30 miles after the new pistons went in I checked a plug and much to my joy I saw this biscuit brown colour.

3 strip downs and about 8 months it took me to cure the problem

https://www.a7a10.net/forum/index.php?topic=9198.0

 
Title: Re: crankcase pressure etc
Post by: Scott and Jay on 25.09. 2017 22:32
I can second this conclusion, but it took us 4 years to put the same solution in place. I had a thread called "Timed breather, what is the actual timing". Our reconditioners had also advised that the old pistons were ok, with only a hone to the bores. To compensate I went the Total Seal Gapless rings way, but I think that just exacerbated the crankcase pressure. So, we were distracted from the underlying excessive-blowby situation. I lengthened the breather valve holes, and narrowed the valve clearances by inserting K-liners. These weren't bad mods. The last time I had oil over the back wheel it turned out pressure hadn't pushed out the drive side oil seal (like it had in the past). But there was too much oil coming out of (where it should, in such a case) - the breather exit pipe, and high consumption. My oil was always black, though - indicating blowby contamination.
My engineer had detected an imperfect part of the bore (caused by rust from 30 years being left), but he was more insistent on new "decent" pistons. I believe the only ones up to the quality he was after are IMD. Adrian Wright, the proprietor, dealt with me personally, with all the specs. The rings that go with them are a big part of the advantage. They are deeper and narrower, and come with their own 3-piece oil ring. He said "you don't gap them" and I didn't - because I couldn't get them out. My compression isn't as low as the pistons are rated because there was barrel and head skimming involved (some of it unavoidable), in my rebuild. It's probably about 8.1:1, lower than the 8.5:1 my old domed pistons were rated at. It kicks over easier. I did the "stationery" running in procedure that Klaus described. From my first ride the bike was much smoother-running and more powerful than before. The plug colour is light brown - from idle through to full throttle.
Title: Re: crankcase pressure etc
Post by: t20racerman on 02.10. 2017 18:12
Thanks for the continued comments - appreciated.

Interesting tale about the pistons - I guess old designs aren't as good as new ones! My oiling problem is nothing like bikerboy describes though - its just a bit leaky at prolonged 4000rpm+ . Its fine at 3700rpm though, so I guess I just need better breathing. My plugs are lovely sandy brown by the way - with no evidence of excessive oil burning. I had inlet valve guide seals fitted when I had the head gas flowed and new 'unleaded friendly' valve seats fitted, and that definitely reduces oil burning.

Update to my earlier post: I fitted a breather to the rear rocker cover, with a tube out to the mudguard stay to catch the air flow. This seemed to make little difference, but my tube was too small a diameter I thought. I put a bigger tube on (about 8/9mm diameter) and that definitely helped to make it less oily - even when thrashed. However, the breather spigot I made only had a 6mm bore, which in hindsight is way too small. Topdad - what internal diameter is the breather you made?
I'll make a bigger one when I'm next working on the bike. I'll report back as and when

Re the tacho cable drive leak - this is STILL chucking out oil at 4000rpm+, despite me taking off the cable mount bracket (inner timing case kind) fitting a new gasket, and resealing it. The cable outer has a split in it at the bottom though, and I'm now sure this is where the oil is coming from - despite me putting silicon goo on it. I've just ordered a new tacho cable, and will ensure it is all sealing nicely.

I'll let you know how it goes.
Title: Re: crankcase pressure etc
Post by: JulianS on 02.10. 2017 19:02
The oil leak in tach drive will be coming from between the drive spindle and cable fixing bracket. If not sorted it will get into the tacho head which will do it no good.

If your setup is the original one there will be a tiny and flimsy o ring and later a ground spiral on the spindle which hoped to keep the oil in. It was no too successful because there are a 3 factory service bulletins on the leak.

The current pattern ones take a more substantial o ring on the inside of cable bracket, like the A65 setup, which is more effective.

Photo is an original drive from a 1962 Rocket engine.
Title: Re: crankcase pressure etc
Post by: t20racerman on 02.10. 2017 19:15
Thanks for that JulianS - very interesting, and undoubtedly correct. Mine is all pattern stuff - I got the tacho drive shaft from SRM about 30 years ago I think. My bracket (also a pattern) was bought at the same time - not sure where from. It doesn't have an O ring of any kind..... I've always had a well oiled tacho cable though!  ;)

As the shaft spins so quick, I didn't think an O ring would be a good idea, but would be very interested in a diagram or picture of how it should be - or how people have modified theirs. I was looking at this at the weekend and wondering if there was an oil seal small enough to fit in the bracket.

Is there anywhere nowadays selling modified brackets with an O ring or oil seal?
Title: Re: crankcase pressure etc
Post by: JulianS on 02.10. 2017 19:47
First photo shows an A65 cable bracket left and pattern item right.

Second photo same with dimension of diameter of recess in inches into which the o ring fits..

Third is the spindle fitted to my A10 together with the A65 bracket and O ring above.

Keeps the oil in.
Title: Re: crankcase pressure etc
Post by: t20racerman on 02.10. 2017 20:08
Those pics are really useful - thank you so much.

Mine is the same, but has never had an O ring. What size is your O ring - do you know?
Title: Re: crankcase pressure etc
Post by: JulianS on 02.10. 2017 20:43
Think it is 2 mm cross section x 6 mm inside diameter x 10 mm outside diameter.

It fitted better in the genuine item than it did in the pattern one.
Title: Re: crankcase pressure etc
Post by: chaterlea25 on 02.10. 2017 21:01
Hi All,
To sort the tacho drive oil leak I eventually made a complete new housing
I shrank a sleeve over the drive spindle and machined the outside to be a tight push fit into
a pair of sealed miniature ball races that are fitted in the new housing
The new housing needs longer screws to hold the lot in place  *wink2*

John
Title: Re: crankcase pressure etc
Post by: JulianS on 03.10. 2017 09:18
Rocker box breathers assisting the timed breather are limited by the 4 drillings , 2 by cam followers and 2 from exhaust valve compartment which allow the oil to drain to the crankcase.

Any breather in the rocker will suck air in as well as blow it out.

Seal a plastic freezer bag or similar to the rocker breather tube, run engine and see what happens to the bag - it does not inflate as a thought it would.
Title: Re: crankcase pressure etc
Post by: t20racerman on 03.10. 2017 17:24
Charterlea - that is definitely the best long term solution, and something that I will look into. In the meantime I'll try fitting an O ring.

JulianS - I know it moves the air both ways, but removing the peak pressure each cycle is the idea behind my breather. I'm looking into a one way pressure release valve to keep the peak negative pressure (sucks oil in!) and let the peak positive pressure out each cycle. That would blow your bag up - albeit slowly as it's pressure peaks I'm trying to reduce, not huge amounts of air flow.
Title: Re: crankcase pressure etc
Post by: Scott and Jay on 03.10. 2017 20:59
I'm glad Julian pointed out what I believe too. I also had a vent in my rocker cover at one stage. I know it's common and have met other people who did it. It certainly didn't seem to alleviate crankcase pressure for me. With the narrow crank cavity on the A10 (not open to the primary like A65s) - it seems to need the vacuum created on the piston upstroke. That means it's not having to push much air out of the breather exit, when the (standard) breather valve opens, on the downstroke. You won't get such a vacuum if it sucks air in through the rocker vent and there will be more pressure to be relieved..
Title: Re: crankcase pressure etc
Post by: t20racerman on 03.10. 2017 22:31
I'm glad Julian pointed out what I believe too. I also had a vent in my rocker cover at one stage. I know it's common and have met other people who did it. It certainly didn't seem to alleviate crankcase pressure for me. With the narrow crank cavity on the A10 (not open to the primary like A65s) - it seems to need the vacuum created on the piston upstroke. That means it's not having to push much air out of the breather exit, when the (standard) breather valve opens, on the downstroke. You won't get such a vacuum if it sucks air in through the rocker vent and there will be more pressure to be relieved..

That's why I'm trying to get one of these:

http://www.ebay.com/itm/Stoney-Racing-Oil-Catch-Can-One-Way-Non-Return-Check-Valve-PCV-Breather-Hose-/182462713511 (http://www.ebay.com/itm/Stoney-Racing-Oil-Catch-Can-One-Way-Non-Return-Check-Valve-PCV-Breather-Hose-/182462713511)

Unfortunately out of stock. ☹️
Title: Re: crankcase pressure etc
Post by: orabanda on 04.10. 2017 03:40
"That's why I'm trying to get one of these:

http://www.ebay.com/itm/Stoney-Racing-Oil-Catch-Can-One-Way-Non-Return-Check-Valve-PCV-Breather-Hose-/182462713511

Unfortunately out of stock. ☹️"


Here is another option:

https://www.mikesxs.net/catalogsearch/result/?q=breather+REED+VALVE

Title: Re: crankcase pressure etc
Post by: duTch on 04.10. 2017 03:49
 
I'd be more confident if it were called a 'PCV', instead of a 'PVC' valve
Title: Re: crankcase pressure etc
Post by: Rocket Racer on 04.10. 2017 04:04
I run bunn breathers on my road rocket, I am forced to run catch bottles on both the inlet (the timed breather) as well as the exhaust on the rocker gear. In my case I'm also trying to expel methanol fumes rather than have them gather in the crank where I'll end up with water if left unchecked.
I still have trouble fully sealing the top end rocker box 3 gaskets but partially put this down to the fretting rocker box suffers from due to vibration through the head steady at the rev's I'm running which are typically in the 6-7000 range but have been seen to hit 7500.

On my road going 60 SR I had to replace the tacho cable to remove leaks, but my race RR runs the taccho driven cable which doesnt suffer those problems

Yes many oil rings are rubbish.
Title: Re: crankcase pressure etc
Post by: t20racerman on 04.10. 2017 06:31
"That's why I'm trying to get one of these:

http://www.ebay.com/itm/Stoney-Racing-Oil-Catch-Can-One-Way-Non-Return-Check-Valve-PCV-Breather-Hose-/182462713511

Unfortunately out of stock. ☹️"


Here is another option:

https://www.mikesxs.net/catalogsearch/result/?q=breather+REED+VALVE

Looks ideal - but they want to charge me $117 for postage to the UK! 🤤
Title: Re: crankcase pressure etc
Post by: duTch on 04.10. 2017 07:57
Quote
......On my road going 60 SR I had to replace the tacho cable to remove leaks, but my race RR runs the taccho driven cable which doesnt suffer those problems. .........

 Is there something wrong with that statement ? doesn't quite make sense to me...but many things don't
Title: Re: crankcase pressure etc
Post by: coater87 on 04.10. 2017 15:23
"That's why I'm trying to get one of these:

http://www.ebay.com/itm/Stoney-Racing-Oil-Catch-Can-One-Way-Non-Return-Check-Valve-PCV-Breather-Hose-/182462713511

Unfortunately out of stock. ☹️"


Here is another option:

https://www.mikesxs.net/catalogsearch/result/?q=breather+REED+VALVE

Looks ideal - but they want to charge me $117 for postage to the UK! 🤤

 high postage cost? Welcome to the world of owners living in the U.S.

 Its ridiculous, and I am going to say 18-24% of the cost of my rebuild is due to postage. *sad2*

 Lee
Title: Re: crankcase pressure etc
Post by: muskrat on 04.10. 2017 20:32
Either way it would be better with two. One in and one out like the Bunn breather set up I have on my Cafe. The timed breather is disabled. https://www.a7a10.net/forum/index.php?topic=8292.msg58577#msg58577
 Cheers
Title: Re: crankcase pressure etc
Post by: Sluggo on 04.10. 2017 21:30
This all has been a very interesting topic and read it front to back.  It seems topical and resurges in interest often with different makes and models.  Some years back I worked a spell on a bike backlog for a local shop and they carried a number of bikes modern and vintage.  The owner was very interested in the BUNN Breather issue for some of the modern Enfields as we built hot rodded versions for some customers.  People tend to get tunnel vision and not look beyond their specific make and model.  At that time I amassed a huge amount of discussions and tech materials from a wide range of sources and shared it with the brain trust at that shop.
We concluded that the BUNN breather had merits but anyone could make their own versions and I suspect thats what did in the supplier.

This topic is hotly debated on Norton forums and seems to be several different guys supplying their perfect mousetrap for Nortons.  Some of them are quite clever and well engineered albeit expensive however some just modify and adapt a japanese bike metal PCV flapper valve and that seems to work for a fraction of cost.

I used to build some V8 performance engines and we installed a few with a vacuum pump setup that worked really well,  Eliminated leaks, increased ring sealing and slight boost in power. You can make your own but Moroso sells kits.  Not totally feasible on a A10 but the concepts are worthy of study for a better understanding.

I have some HD bikes and multiple Buells, As universally known HDs are known themselves for leaking oil.  Buells design for their breather system is I believe the final word on solving these issues in reasonable way.  The sportster engine design ran a number of case breather setups as well as eventually having breathers thru the cylinder heads with "Breather Bolts" that vented INTO the air cleaner which created a slight vacuum as well as met EPA emissions requirements.
But Buell changed that system into a baffled breather system out the TOP of the cylinder heads with PCV valves and hoses. (Again culminating into the intake)  Many people in the Buell world have setup alternatives where they go to a check valved catch can,  or vent openly but most agree the top of the head PCV valve setup works really well.  Many other V Twin models now copy this design as well or adapt this system.

I have greatly angered some BSA owners in the past arguing the timed disc setups in BSA Singles and twins is inadequate (FACTORY STOCK IS BEST you heretic!)  But simple testing shows thats not true.  I do think on a BSA Preunit twin that the venting from the case UP thru the head is problematic  but could be addressed easily enough but likely would be visible externally.    It seems logically that 2 large fittings (one at the head and one off the case) into a good PCV-check valve set up would be the way to go.
I am not going to tell you guys what you have to do, but instead encourage you all to look at what other makes and models do like I did and adapt what works best for your situation,.
Title: Re: crankcase pressure etc
Post by: t20racerman on 06.10. 2017 16:17
Either way it would be better with two. One in and one out like the Bunn breather set up I have on my Cafe. The timed breather is disabled. https://www.a7a10.net/forum/index.php?topic=8292.msg58577#msg58577
 Cheers

That is an excellent looking system muskrat - thanks. I'll definitely look into doing that :-)  A couple of things though:

 - Taking the air in from the top sounds good with respect to cooling (and air flow through the engine) but is the original venting between the crankcases and the rocker box up to the job for moving air quickly in and out? I wondered if it was a bit restricted? Did you mod this in your engine?

- Here's the best Bunn breather I can find to buy anywhere in the UK:
http://triumphbonneville.com/store/engi.html  (http://triumphbonneville.com/store/engi.html) (scroll down on the page a bit...)
This was recommended on the thread you pointed me to. Have you or anyone experience of this particular system? Looks good to me, and happy to buy it, but just wondered if it was what I need.

Thanks, as always to all, for the amazing help and technical info on this forum. :-)

Adrian

Title: Re: crankcase pressure etc
Post by: t20racerman on 06.10. 2017 19:04
Sluggo - a very interesting post. Combined with muskrat's observations, it seems clear that this is the way to go. :-)

I chatted with a Commando guy at work today and he was of the same opinion, but also pointed out that this was a VERY contentious subject for some of the 'you can't beat the std set-up' guys who inhabit a lot of Norton forums. Twas ever thus...  ;)
Title: Re: crankcase pressure etc
Post by: muskrat on 06.10. 2017 20:23
G'day t20.
Yes I have the Bunn breathers on my Cafe. It works great, the motor accelerates a lot quicker and no oil leaks. That kit from Shropshire is identical to mine. I'm surprised to see one for sale as the Bunn has been out of production for years.
Cheers
Title: Re: crankcase pressure etc
Post by: t20racerman on 06.10. 2017 21:28
G'day t20.
Yes I have the Bunn breathers on my Cafe. It works great, the motor accelerates a lot quicker and no oil leaks. That kit from Shropshire is identical to mine. I'm surprised to see one for sale as the Bunn has been out of production for years.
Cheers

Thanks so much for the reply muskrat - really appreciated. Have just ordered the kit.... :-)

Oh, one other thing muskrat - have you improved the internal breathing between the rocker cover and the crankcases - or is yours just as BSA made it?
Title: Re: crankcase pressure etc
Post by: muskrat on 07.10. 2017 10:42
G'day t20.
No alteration to the oil drain holes. All breathing is from top to bottom (in via ex cover out via std breather hole) so it helps rid the rockerbox of spent oil.
At idle you can hear the valves working.
Cheers
Title: Re: crankcase pressure etc
Post by: Topdad on 09.10. 2017 15:43
Hi T20 any chance of some pictures showing how you fitted it ? cheers bob
Title: Re: crankcase pressure etc
Post by: t20racerman on 10.10. 2017 20:07
Will do. Haven't got the bits yet though. I'm off work in two weeks, so should be sorting it then  - and hopefully fitting a 5 speed gearbox at the same time :-)
Title: Re: crankcase pressure etc
Post by: Cleve on 11.04. 2019 17:57
Hi

I'd be really interested to read the pdf but the link doesn't seem to work?
Title: Re: crankcase pressure etc
Post by: muskrat on 11.04. 2019 22:36
G'day Cleve.
I see the original didn't work but a link in another topic did (https://www.a7a10.net/forum/index.php?topic=3634.msg24978#msg24978). Try this http://www.a7a10.net/BSA/oba.pdf
Cheers
Title: Re: crankcase pressure etc
Post by: BSA_54A10 on 14.04. 2019 13:17
Quote
This all has been a very interesting topic and read it front to back.  It seems topical and resurges in interest often with different makes and models.  Some years back I worked a spell on a bike backlog for a local shop and they carried a number of bikes modern and vintage.  The owner was very interested in the BUNN Breather issue for some of the modern Enfields as we built hot rodded versions for some customers.  People tend to get tunnel vision and not look beyond their specific make and model.  At that time I amassed a huge amount of discussions and tech materials from a wide range of sources and shared it with the brain trust at that shop.
We concluded that the BUNN breather had merits but anyone could make their own versions and I suspect thats what did in the supplier.

The plumbing has been known for decades.
The trick was to find the right material for the valve that would not resonate anywhere in the frequency of the engine pulses.
Rex was a chemical engineer and eventually found a material that worked.
A sheet of it was a hock your house type price so he then hunted out users of the material , bought their scrap & hand cut all the valves from that in order to keep the price down.
Rex has a shortish attention span so after a while he was looking for a buyer but there simply was not the interest so he stopped making them and went into movie making producing 3 doccos and writing another book which includes everything he found out with the exception of the valve material.
Yes there are thousands of check valves out there.Rex and eventually the University of Auckland tested each & every valve available at the time but none worked over the entire rev range 1000 rpm to 10,000 rpm, temperature range & volume range
Some worked at low speeds & some worked at high speeds but none worked at all speeds except his.
What Rex failed to take into account was that while he wanted & was willing to pay for perfection the average British bike rider is way too cheap to pay $ 40 for his valve when they could go the el-cheapo car parts & buy a PCV valve for $ 3.95 plus 4 foot of PVC tube and that is exactly what happened .

Everything in the kit was unique to the kit
The valve , the thickness, density & size of the tubing were all specially made so they worked together.
All the current knowledge about engine exhaust went into the kits which is why they were supplied with very specific instructions about tube length for different bikes.
The kit of the B31/33 does not work on an A10 right out of the box and visa - versa.

Even the book he wrote with the results of 15 years of research never covered publishing costs.
Title: Re: crankcase pressure etc
Post by: berger on 14.04. 2019 17:06
does anyone know if a reed valve would work? *dunno2*
Title: Re: crankcase pressure etc
Post by: chaterlea25 on 14.04. 2019 21:31
Hi Berger,
Quote
does anyone know if a reed valve would work

They are fitted on XS 650 Yam's and I have read of people using them on their Brit twins

John
Title: Re: crankcase pressure etc
Post by: muskrat on 14.04. 2019 21:39
https://www.mikesxs.net/yamaha-xs650-reed-type-engine-breather-valve-pcv.html
Title: Re: crankcase pressure etc
Post by: berger on 14.04. 2019 22:24
 I have not been to the PUB! o yeah musky its all coming back to me now, brain wouldn't go ---cheers for the link.  [again I think *eek* *conf2*] musky and john to the rescue *beer*
Title: Re: crankcase pressure etc
Post by: BSA_54A10 on 15.04. 2019 00:08
Same problem.
Valves resonate so they get set to where they are most needed for that application.
Also the size of the holes are important
A small hole will have high velocity gas coming out which is good for opening a sprung valve
A big hole has low speed gas which is good for a flapper valve.
At last 50 people can tell you that the M20's breather works perfectly at any speed up to about 1/3rd throttle.
Engine is nice & oil tight (ish )
But WFO for any more than 3 or 4 minutes and all behind me are waterproofed.
The std A 10 timed breather is good to around 3000 rpm then it gets progressively more out of sync with the gas flows till it ends up backwards at 7000 rpm
Between 3 k & 7 k is generally when the rocker box leak becomes chronic .
This is because the crankcase fume has weight so the movement of the gas under the piston lags piston movement but the breather remains in sync with the movement of the pistons so it is out of time with the movement of the gas.
Then there is the resonance of the exhaust tube, just the same as the engines exhaust pipe it can end at a pulse node which will prevent the movement of oil .
Title: Re: crankcase pressure etc
Post by: muskrat on 15.04. 2019 00:17
 *pull hair out*
Title: Re: crankcase pressure etc
Post by: t20racerman on 15.04. 2019 09:13
The std A 10 timed breather is good to around 3000 rpm then it gets progressively more out of sync with the gas flows till it ends up backwards at 7000 rpm
Between 3 k & 7 k is generally when the rocker box leak becomes chronic .

This is exactly what I found, and why I fitted the Bunn valves to my A10. I could ride round all day up to about 3400rpm without much more than a gentle weep at worst out of the rocker cover - hold it at about 3800-4000rpm though on a dual carriageway or fast road, and it turned into an oil pump! If I slowed back down, the leak stopped. SO frustrating
I can now run it at 3800-4000rpm for long distances (75-80mph) without a problem thanks to the Bunn valves. its such a shame they aren't being made any more. :-(
Title: Re: crankcase pressure etc
Post by: berger on 15.04. 2019 11:20
ok chaps so put it to me in simple terms, is the yam 650 breather any good for  my 500 that will be revving [ hopefully] between 1000 and 6500 max, [6500 on a mental day] or do I need this non obtainium chocolate BUN *dunno* *help*
Title: Re: crankcase pressure etc
Post by: muskrat on 15.04. 2019 20:46
G'day berger.
Yes, one in one out.
Cheers
Title: Re: crankcase pressure etc
Post by: berger on 16.04. 2019 12:38
I went to the pub *countdown* musky you are a star, you told me that ages ago *whistle*--- I remember now you've said it again. brain needs a talking to *bash*