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new rings on old pistion
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Posted by: speedracin

i have a 1987 honda 250 R bottom end with a 1986 topend. i was wanting to know how to tell if i can just get away with just rering it or should i just buy a new pistion?

Posted by: wilkin250r

The answer is in your piston-to-cylinder clearance.

A stock setup, brand new (or fresh rebuild) is around 0.0025"-0.0031", but a forged piston like Wiseco will require slightly larger clearance because the forging process changes the thermal expansion properties.

Obviously, as you run the engine, things will wear down and that clearance gets larger. 0.006" is the upper limit, anything larger than that is unnacceptable. Anything under that and you can re-use the same piston.

Often it's the piston that wears down most, because it's aluminum and thus softer than the iron sleeve. So if your piston-to-cylinder clearance is larger than 0.006", a new piston might bring it back in spec, and you don't have to overbore. If maintained properly, every bore can usually go through multiple pistons before requiring to go to the next overbore size.

However, actually finding the answers for yourself requires some very precise (and thus expensive) measurement equipment. You might be best off going to a shop and have them do the measurements for you, if you trust them to give you the actual honest numbers.

Posted by: woodsracer144

to add to walkin250r....

shouldnt you get a hone job done? if oyu have have to bore it out?

Posted by: wilkin250r

Yep, always get a hone when replacing the rings.

The hone itself will also remove a small amount of material, so you would never re-use a piston with the max 0.006" clearance, because the hone will make that gap just a tad bit larger.

Posted by: woodsracer144

hey i got a question of my own... i have a piston but the jug that is for is a little rusty so could i just hone it or do i have to bore it out...

also know what size piston is a 256 with pro-x? thanks

Posted by: speedracin

if your cylinders rusted have a hone ran in it and see if it cleans up well and that its not pited out. if its pited real bad and a hone dont clean it up then u will have to bore it. and about the piston size dont know sorry

Posted by: speedracin

thanks guys for the info on the new rings helped me out a lot

Posted by: All250R

Cylinders go out of spec pretty easily with only .001" allowed for out of round and taper. A hone should be able to clean this up, but your new piston needs to be large enough to accomodate the extra material lost. Often cylinders with some hours on them or particularly if compression is high and ports are large can accumulate wear larger than .001" relatively easily. By the way, stock setups run for an amazingly long time...

Anyway, tools to measure this accurately can seem expensive, but if you have the time to disassmble the engine you might want to also take the time evaluate it as well. Parts and labor are also expensive. Knowing is better than not.