The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: ellis on 02.11. 2016 19:42
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Could anyone tell me what the gap should be on the rings. I have a standard 70mm bore with AE hepolite 9 to 1 pistons in my A10 and the ring gaps are between 20 to 25 thou. Is this correct as I think they seem a little on the large side. *help*
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hi ellis,
yes 20 thou are to much, 0.9 to 13thou is recommented but not more.
cheers Klaus
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20 thou ring gaps aren't too big and won't do any harm.
Although I should ask whether they are old rings which have worn so the gaps have increased from <13 thou to 20 thou. In that case I would then ask how the engine has been running.
I sort of assumed that you are talking about new rings and pistons.
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The bike runs pretty good, it has had a new liners not too long ago as I can see faint signs of the honing marks. But it does smoke a little from cold but not too excessive. After both your feedbacks I will get a light hone to the bores and put a new set of rings in while it is stripped down and hopefully this may sort my problem. Thanks again lads any information is always useful. *beer*
Cheers ELLIS
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I have seen rings with 40 thou gap and still held reasonable compression. BUT, if you've got it down throw a new set in after a light hone. 10 on the comp rings 13 on the oil. Check piston clearance as well. 5 is OK, 6 is getting worn, 8 is time for a rebore.
Cheers
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Just out of curiosity can piston rings these days be bought and just put straight with the gaps more or less correct or do you still need to measure and file them to correct size like the old days ?
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G'day Dave. Not very often. But in saying that I just opened a box of Hastings rings for a +40 Trihard 650 (same size as +80 A10) and the gaps were 1 to 2 thou big.
Gapping rings isn't hard if your careful.
Cheers
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Just out of curiosity can piston rings these days be bought and just put straight with the gaps more or less correct or do you still need to measure and file them to correct size like the old days ?
I say no. Check each piston ring to correckt gap I have see pucke horses *eek*
Differend gaps in one piston ring set, it was all scrap and send it back.
Most time the aftermarked spare parts are not plug and play
and in poor quality. If I had a chance I will refit genuie parts.
There is much blow by if the gap is to much. This will cause any oil leaks by pushing gaskets off and havy oil consumption.
By doing a low milage a year, to hell with that, I do more than 10 thousend miles a year and I'll be sure to have realy good parts with correckt sizes in my engines. Three seasons hard racing with my A10 and the engine was not opened and totaly dry ...knocking on whood.... hoping next year is also out of failures.
Cheers Klaus
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There is much blow by if the gap is to much.
Cheers Klaus
You won't find a modern, knowledgeable reference to agree with that.
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There is much blow by if the gap is to much.
Cheers Klaus
You won't find a modern, knowledgeable reference to agree with that.
But the use of pistonrings is recommented ;)
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Whenever faced with this question I go for comfort reading to 'Torrens' of The Motor Cycle', whose long-ago book "The Motor Cyclist's Workshop" has a great chapter entitled "Is It Worn Out".
He says in the Seventh Edition, published in 1961, that:
"The term "worn out" is a poor one. The gap between the ends of a ring, contrary to widely-held belief, is not the deciding factor as to whether a ring should be scrapped or not. Much more gas leaks round the back of a piston ring than ever passes through the gap.
Small gaps are a fetish with some people . . . the only time they really matter is when they are too small and then there is the devil to pay. . . . err on the side of large gaps . . .whether to scrap depends on whether there signs of gas leakage".
He goes on to quote the "service department" of "one of our most knowledgeable manufacturers" . . . whose practice, "in the case of an 82mm bore is to replace when the original gap of 15 thou has increased by 30 thou, making 45 thou in all".
I don't know really, but I certainly don't replace rings just because they have larger gaps than when new. If their springiness has dwindled a lot compared to a new or known good one, then I'll replace them, or if there are marks of blow-by, or if there is a step on the top one from whacking a carbon ridge, or if there is smoke or an obvious lack of compression not due to valve trouble. Otherwise, I tend to leave well alone.
When Torrens was writing, people were using their machines as ride to work and family transport, on tight budgets, so he was probably trying to discourage people from spending money they hadn't got on parts they didn't really need. Advice I quite like, being mean, and lazy as well when it comes to not looking for problems to solve when I could be riding the thing.
It's a good book, and Torrens was widely respected in the day. (Shame one of only two commercial advertisements in the rear of the tome shouts - 'The man who knows motorcycles . . . chooses Triumph'. Hmm! I'll happily remain ignorant then.)