crankcase pressure etc

[bikerboy]

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

 

[Scott and Jay]

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.

[t20racerman]

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.

[JulianS]

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.

[t20racerman]

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?

[JulianS]

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.

[t20racerman]

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?

[JulianS]

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.

[chaterlea25]

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 

John

[JulianS]

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.

[t20racerman]

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.

[Scott and Jay]

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..

[t20racerman]

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

Unfortunately out of stock. ☹️

[orabanda]

"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

[duTch]

 
I'd be more confident if it were called a 'PCV', instead of a 'PVC' valve