An '86 connecting rod is 5mm shorter than an '87 through'89 maybe if you have the wrong year piston it might bottom out the skirt on the crankshaft.I have an '86 piston in my '88 motor and use a cylinder spacer with 2 base gaskets,if you have the opposite it probably would bottom out.Also I'm pretty sure the ATC and the TRX motor were the same in '86 my Pro-X says it will work in both.

'88 250R many extras

 

THanks....now that you mention it....i think the piston was for an 87....but not sure....that wouldn't make sense though, for the box to say 87 atc..since they didn't make them then.

 

Part number 526P* is for '85-'86 ATC and TRX
Part number 562P* is for '87 on TRX's

Those are Wiseco part numbers...The part number will be stamped/engraved on top of the piston crown...