TRX250r cylinder questions...

[sirfallsalot243]

My buddy has an '88 trx250r with an '87 motor that recently suffered a cracked piston. The piston didnt break, so the bottom end of the motor is fine, but the cylinder walls are scored, and that fin on the intake is damaged. He's planning on buying a new cylinder and piston for it, but the problem is that the bottom end is (supposedly) a stroker crank. That is all the information we were given by the previous owner, so we have no idea if its even a stroker crank or not, much less what size it is. Im also aware that there are two different pistons available for the 250r? I read something about a long rod style and short rod style? How de we know what piston to use? And if the cylinder he has was an overbore (supposedly 290cc), would a stock cylinder still work on the stroker crank?

 

[Matt Fisher]

This is from memory, so research it a bit more, but here goes-

The 86 (maybe the 87 too) motor had a longer rod and taller cylinder than the later motors, but it was common to run the newer cylinder with a spacer plate to make up the difference. Frequently if a quad has a stroker crank, there will be a spacer plate.

As for your cylinder size, you'll need to measure it. An easy way to get pretty close is to measure the piston. If it's a big-bore it'll be much larger than a stocker. Same with the stroke, you'll need to measure the distance from BTC to TDC and compare that to what's stock.

If you find yourself in over your head, I'll bet that Eric Gorr has had loads of practice with these motors.

 

[sirfallsalot243]

I measured the piston, and it measures 68.5mm... 2.5mm larger than OEM. To honest- we really have no idea what year this motor is, but Wiseco's largest piston for the 87+ motors is 68mm. The only make a 68.5mm for the 86 motor.

but I'm not sure what company made the piston we have.