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Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: v8ivor on 06.11. 2015 21:46

Title: Oval pistons and worn bores ??
Post by: v8ivor on 06.11. 2015 21:46
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 ???
Title: Re: Oval pistons and worn bores ??
Post by: Triton Thrasher on 06.11. 2015 22:10
How was the bike running and why is it in pieces now?
Title: Re: Oval pistons and worn bores ??
Post by: trevinoz on 06.11. 2015 23:16
Sounds like it is about time for a rebore.
Title: Re: Oval pistons and worn bores ??
Post by: chaterlea25 on 06.11. 2015 23:53
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

Title: Re: Oval pistons and worn bores ??
Post by: v8ivor on 07.11. 2015 00:15
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,
Title: Re: Oval pistons and worn bores ??
Post by: v8ivor on 07.11. 2015 08:01
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.
Title: Re: Oval pistons and worn bores ??
Post by: muskrat on 07.11. 2015 08:58
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
Title: Re: Oval pistons and worn bores ??
Post by: v8ivor on 07.11. 2015 09:21
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.
Title: Re: Oval pistons and worn bores ??
Post by: v8ivor on 07.11. 2015 10:46
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  :!
Title: Re: Oval pistons and worn bores ??
Post by: bsa-bill on 07.11. 2015 13:07
Quote
It is so difficult to figure things out when an idiot gives you the information !   SORRY  :!

Relax your among friends and other idiots *whistle*
Title: Re: Oval pistons and worn bores ??
Post by: muskrat on 07.11. 2015 19:01
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
Title: Re: Oval pistons and worn bores ??
Post by: bsa-bill on 07.11. 2015 19:23
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
Title: Re: Oval pistons and worn bores ??
Post by: Triton Thrasher on 07.11. 2015 19:52
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.
Title: Re: Oval pistons and worn bores ??
Post by: u28909z on 07.11. 2015 21:24
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.     
Title: Re: Oval pistons and worn bores ??
Post by: Triton Thrasher on 07.11. 2015 22:40
(http://i257.photobucket.com/albums/hh236/tritonthrasher/image_zpswubiycp6.jpeg) (http://s257.photobucket.com/user/tritonthrasher/media/image_zpswubiycp6.jpeg.html)
Title: Re: Oval pistons and worn bores ??
Post by: bsa-bill on 08.11. 2015 09:02
Quote
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
Title: Re: Oval pistons and worn bores ??
Post by: Triton Thrasher on 08.11. 2015 12:53
http://i257.photobucket.com/albums/hh236/tritonthrasher/image_zpstgk6n9nm.jpeg
Title: Re: Oval pistons and worn bores ??
Post by: bsa-bill on 08.11. 2015 14:50
Cheers TT, I get it now, earlier pics show the step on the inside, depends which bit you call the step I suppose
Title: Re: Oval pistons and worn bores ??
Post by: v8ivor on 08.11. 2015 15:16
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 )
Title: Re: Oval pistons and worn bores ??
Post by: v8ivor on 08.11. 2015 16:13
The old rings
Title: Re: Oval pistons and worn bores ??
Post by: bsa-bill on 08.11. 2015 17:31
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
Title: Re: Oval pistons and worn bores ??
Post by: Triton Thrasher on 08.11. 2015 20:18
The old rings

Can you do one with the end in focus?
Title: Re: Oval pistons and worn bores ??
Post by: v8ivor on 08.11. 2015 23:09
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.