2006 yz 250 oem piston question

It depends on what color powerband you have.

For my YZ125 one of the guys at the local shop who used to be involved with Yamaha pro racing back in the 2 stroke days, recommended using a D piston regardless what the cylinder is marked once the cylinder has some wear on it.

That's what their pro team used.

To me that doesn't make sense because the Nikasil plating is so hard, that my understanding is that the cylinder should actually get smaller over time as the ring material wears onto the cylinder. When you're cleaning up a cylinder with scotch brite or super fine sand paper, you're actually taking the ring material off the cylinder, not the nikasil.

I'm currently running a D piston in I believe a B cylinder without issue, but next time I'll probably order a B since it'll have the right clearance. When I can buy an inside micrometer I'll measure the cylinder, then order a piston to match the actual size. For now, I'm going to match the marking on the cylinder.

I'd love to hear other opinions (or facts).

On the back of your cylinder (the reed side) there is a letter stamped. A, B, C, D. Type B cylinders are in my opinion the most common on both YZ125's and YZ250's. Ive owned three YZ's, and they all had type B's.

RED = A

ORANGE = B

GREEN = C

PURPLE = D

The differences in clearance between the pistons are so small, its negligible. I run type D pistons in all my yamahas with zero issue's regardless of cylinder type. My friends dad is a machinist, he measured all four piston types one time from yamaha and the difference is in the thousands of micron's. I regularly get 40 hours on a YZ125 top end, and I average 75 hours on a YZ250. When I tear them down they're practically spotless.

I cannot say the same for aftermarket pistons/rings. I use only OEM everything. Ive seen what happens when people build internals with JE/vertex/hot rods/eagle/LA sleeve stuff. OEM costs more for a reason!