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Yz 125 rebuild help

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Hi i need some advice on top end rebuild, whats best way to pick the correct size piston, do i go standard or do i need to mesure the cylinder somehow and pick from there, is there within a 1mm tolerance or somthing, also is squish a big issue, how does it work, i no its probally covered a million times but i cant seem to find the infomation, i just dont wana buy a random piston and gasket set, put it in and bam locks up, thanks for the help 

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If you really want to do things right and avoid a lock-up, get a factory service manual, read it over and over.

It will have all the necessary infos, tolerances, fastener torque values etc etc.

Yes the piston is matched to the actual measured size of the cylinder, pending the plating is still serviceable.

 

Squish band height is less of an issue on YZ125's (more common on the YZ250)

but yes it can be optimized, making for a crisper running engine.

(it also reduces octane requirements and, can allow for a tad leaner jetting)

 

I have on mine, with still room for future improvement I reduced it from 1.37mm to 1.17mm which also gained a modest 5psi of compression.

Well built 125's can run as little as 0.7 - 0.8mm but the compression dome will need correction to maintain a reasonable compression ratio.

Edited by mlatour

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Does the bike run wel now with no unwanted noises for example piston slap. The easyest way is to look at the top of the old piston it will have a size marked on it and order the same one. You will have to clean the carbon off lightly with some sand paper but if you are unsure take it to a shop to be 100%.

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