1979 RM125 need advice on piston & cylinder assembly

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8/30/2018 8:09 AM

I have owned this bike since 1980 and this is the first time I had to repair the engine.
The kick starter gear broke so I had to take it apart. I have got it together to the point of mounting the cylinder.
My question is should I have it bore out. I had it measured at work and it is only .002 out of round and the book says it can be .004 out of round before boring. Or can I just use the old rings and assemble it?
What should i do?
Thank you

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8/30/2018 9:31 AM

you also need to check the bore for taper and check the piston to cylinder clearance.

suggestions can be made depending on these results and the type of riding you are going to do. if it was running decent before and you just occasionally ride it but don't race it, you could just put it back together as is.

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8/30/2018 12:51 PM

It was running perfect before. I am only going to ride it occasional and it was never raced. So i well put it back the way it is. I will not do anything to the cylinder.
And thank you vert much.

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8/30/2018 8:38 PM
Edited Date/Time: 8/30/2018 8:39 PM

TBrag wrote:

It was running perfect before. I am only going to ride it occasional and it was never raced. So i well put it back the way it ...more

no problem, but i forgot to mention that you could check your ring end gap and replace it if it is near the limit of the spec of you want. if you do this you need to hone the cylinder lightly, however, even if it is worn beyond spec, if it was running acceptably to you before you took it apart, you could still run it like that but it will have a tiny bit less power and probably smoke a little bit more than it otherwise would, but that's not a big deal. my 1980 yz250 smokes more than the marlboro man and still hauls ass.

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