YZ GP #808 Wiseco Piston in 08 RM250

I can look back in my notes to confirm, but I am almost positive that piston is the same height. I have used them many times in 06-08 RM250's... they work great! We were seeing issues with the center power valve wearing slightly and contacting the upper piston ring on a stock RM piston, was never an issue with the 808 due to the position of the single ring. Plus ring life seemed to be longer even with it being a single ring.

The piston will last longer than an OEM rm piston, and as mentioned above I found ring life to be longer as well.

In my YZ250s, I run the Pro-Lite pistons. As long as you set the ring gap correctly, your cylinder is in spec with a proper crosshatch and you run good oil, you should get 40 to 60 hours out of each piston riding at the A or B skill level.

I run 32:1 with Yamalube 2R or Maxima Super-M and switch pistons & rings around 40 - 50 hours in the YZs racing the 30+ and 25+ expert classes.

With the forged piston, you don't just kick start the bike and go ride. You need to wait until the tanks on the radiator feel warm before you take off. You may already have that habit... Otherwise, I've been running the forged pistons for many years.

When you assemble the engine, check the squish with a piece of soft lead solder. If you don't know how to do that, let me know and I'll explain. A safe reliable 250cc will run extremely crisp with .040" to .050" of clearance between the piston and head. You won't be bumping the compression enough to cause detonation if your jetting is right and your fuel is good. I shoot for .040" - .045" of squish clearance on the YZ with a 19.5cc head.

I have these numbers below saved from an old thread on Thumper Talk YZ forum. (Probably about 5 years ago) ProLite is the standard Wiseco 2-ring forged replacement piston while Racers Choice is the upgrade piston. The wristpin to dome measurement is what you want. I did not take the measurements myself, just someone else' posted data.

Racer's Choice:
TiN ring free gap - 5.0mm
ring groove to crown - 3.55mm
diameter - 66.08mm
length - 79.45mm
wristpin to skirt - 5.90mm
wirstpin to dome - 32.13mm

ProLite:
chrome ring free gap - 6.9mm
ring groove to crown - 3.07mm
diameter - 66.0mm
length - 78.19
wristpin to skirt - 5.45mm
wristpin to dome - 31.49mm

As a note, these are rough numbers. To get precise measurements I would need much better equipment than a simple digital caliper. Yet you can still see some noticeable differences.

I saw this thread and got nervous about the GP Racer's choice piston that I am about to put in my 06 KX250.

So I called Wiseco tech support this morning and they assured me that the GP Racers Choice piston is designed to be dropped into an otherwise stock engine.

^^^To the measurements above, I tried to replicate doing them with my digital calipers last night, there is no way that I would trust anything I measured, too much human input and variability.

I agree and only posted that to give an idea of the differences. Those aren't my measurements and I included the "disclaimer" from the source.

The only accurate way to measure clearance is to put the piston in your engine and use solder, plasti-gauge, putty or clay.

Personally I run the standard pro lite. I measure using soft lead solder and cut the cylinder-head on my lathe.
The taller piston is going to change port timing as well as compression and squish... I don't want the timing change.

Measurement from the top of the wrist pin hole to the top of the piston crown is the same between stock RM250 piston and 808m06640 YZ250 racers choice piston ( I did not write the measurement down, sorry).

Some additional reassurance, every engine I rebuild whether it has all oem parts or not I check squish clearance... I've installed that piston on many RM250's with no cylinder or cylinder head modifications. It will work without modification on your bike.

The squish gap on late rm250s is pretty good at 1.2mm, I ran wiseco gp piston and the hight was the same as stock and didn't change anything.

Many rm250s had a slight mismatch on the cases, meaning sometimes you could get a messed up measurement depending on how the cylinder was bolted down. I had an air leak on the base gasket on mine once.

What if I had the cylinder and head done could I run that piston in my rm 250?morgan is working on my rm cylinder so I am thinking about what piston to use