The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: v8ivor on 06.11. 2015 21:46
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Hi again,
I got a new set of rings today, and this is what I found ;
An 8 thou gap at the front of the piston and a 12 thou gap at the side of the piston, at the top of the bore.
A 4 thou gap at the front of the piston and a 10 thou gap at the side of the piston, at the bottom of the bore.
When I put the new top ring in the bore I had a 35 thou gap at the top and a 22 thou gap at the bottom.
So, the pistons are slightly oval and the bore a little worn.
Does all this sound good or bad ??? What do you think ???
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How was the bike running and why is it in pieces now?
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Sounds like it is about time for a rebore.
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Hi,
35 - 22 =13 divide roughly by 3 = 4 so theres approx 4 thou taper on the bore
Pistons are made with ovality inbuilt so ignore side to side and top measurements
The place to measure a piston is near the bottom of the skirt at right angle to the pin
Place the piston near the bottom of the bore and measure between the piston and bore with feeler gauges
If theres a lip near the top of the bore then it usually means a rebore is needed
If there are rub marks on the piston above the top ring then it means the piston is rocking in the bore and is another sign of lots of wear
HTH
John
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Thanks John, The measurement at the bottom is 4 thou. A tiny ridge at the top of one cylinder, but on one side ? No marks on the piston ( photo on earlier post ) And the pistons are a reduced in diameter at the top from new,
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Hi Triton, I bought allot of parts that were from a 1970's chopper, and so I stripped the motor because I have no history. It all looks in good shape, apart from finding both top rings broken and that may have been done on the last rebuild.
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G'day Ivor.
A 4 thou tapper would be more than enough ring flutter to break the top rings.
But I've also seen both top rings broken by pinging.
Cheers
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G'day Jim, By pinging do you mean when it's too advanced or low octane fuel ? So, do you think that I would be silly to put it back together with new rings ?
Cheers.
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Jim, I just remembered, the rings that the last builder used had a lip on the top rings and they were 17 thou smaller than the grooves and when I offered the second ring in the bore I measured something like a 6o-65 thou ring gap, so I'm thinking that the wrong rings were fitted. I have re-measured this-morning (This time about an inch up from the bottom of the barrels ) And found only a 8 thou ring gap difference, and divided by 3.142 gives a taper of just under 2.55 thou.
It is so difficult to figure things out when an idiot gives you the information ! SORRY :!
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It is so difficult to figure things out when an idiot gives you the information ! SORRY :!
Relax your among friends and other idiots *whistle*
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G'day Ivor. Yes that's what is meant by pinging.
So the old rings were not right for the job.
Give it a light hone and throw it back together. Put note on wall "rebore next overhaul"
Cheers
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Now I'm worried, know I've seen these stepped rings somewhere, hoping it wasn't while putting the RGF together *sad2* as it had a pinging period until I fitted Pazon.
Anyway might lift the barrels over the winter for a look, are there any outward signs or symptoms of broken top rings
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Now I'm worried, know I've seen these stepped rings somewhere, hoping it wasn't while putting the RGF together *sad2* as it had a pinging period until I fitted Pazon.
Anyway might lift the barrels over the winter for a look, are there any outward signs or symptoms of broken top rings
There's nothing wrong with the stepped ring design.
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By stepped rings are we taking about ridge dodgers, designed to be fitted to a part worn bore to avoid hitting the wear ridge and hence breaking? Fitting these was common practice as a cheap way to pep up a worn engine.
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(http://i257.photobucket.com/albums/hh236/tritonthrasher/image_zpswubiycp6.jpeg) (http://s257.photobucket.com/user/tritonthrasher/media/image_zpswubiycp6.jpeg.html)
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are we taking about ridge dodgers, designed to be fitted to a part worn bore to avoid hitting the wear ridge
so how does that work then if as has been said the step goes uppermsot as in TT pics (thanks TT)
I feel a Google coming on
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http://i257.photobucket.com/albums/hh236/tritonthrasher/image_zpstgk6n9nm.jpeg
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Cheers TT, I get it now, earlier pics show the step on the inside, depends which bit you call the step I suppose
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Oh, the second photo makes sense to me, but the first diagram looked like it was doing the exact opposite to me. The rings that I took out were not like those and so I took a photo and I still can't attach it because it is telling me it's oversize,,, Grrrr ! (Jim Musky tried to help me post a photo but that don't work either )
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The old rings
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some ring info here http://www.andover-norton.co.uk/Norton%20GPM%20Gandini%20Piston%20Rings-%20fitting%20instructions.pdf (http://www.andover-norton.co.uk/Norton%20GPM%20Gandini%20Piston%20Rings-%20fitting%20instructions.pdf)
and this pic maybe suggest your rings are either just worn or wrong and upside down
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The old rings
Can you do one with the end in focus?
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Sorry TT, I had a hell of a job to post any photo at all. The rings are not like anything on your post Bill and because I can't think of any reason for a lip facing up or down + the lip on the outer edge is the same size as the groove, I'm thinking it's down to wear.