Author Topic: crankcase pressure etc  (Read 6629 times)

Offline olev

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crankcase pressure etc
« 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

Offline tombeau

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Re: crankcase pressure etc
« Reply #1 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

Offline scotty

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Re: crankcase pressure etc
« Reply #2 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

Scotty
'56 A10 Red Flash
'54 B33
'98 HD FLHTPI
‘74 BMW R75-6

NIGES

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Re: crankcase pressure etc
« Reply #3 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 

Online groily

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Re: crankcase pressure etc
« Reply #4 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.
Bill

NIGES

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Re: crankcase pressure etc
« Reply #5 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

Online RichardL

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Re: crankcase pressure etc
« Reply #6 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.

 
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NIGES

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Re: crankcase pressure etc
« Reply #7 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

Online RichardL

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Re: crankcase pressure etc
« Reply #8 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.
 
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Online bsa-bill

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Re: crankcase pressure etc
« Reply #9 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)
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Richard

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Re: crankcase pressure etc
« Reply #10 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

Online bsa-bill

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Re: crankcase pressure etc
« Reply #11 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
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

NIGES

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Re: crankcase pressure etc
« Reply #12 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

Offline terryk

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Re: crankcase pressure etc
« Reply #13 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.
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Online trevinoz

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Re: crankcase pressure etc
« Reply #14 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.