YZ250 Piston/Cylinder Letters (A,B,C,D)

[almon]

The YZ manual indicates that one should match piston letters and cylinder letters when doing a top end. I.e. order a 'C' piston if you have a 'C' cylinder. My dealer said that doesn't really matter and that yamaha supplies them with piston kits that are 'B' to be used for all bikes.

Anyone know if this is true or not and whether it matters?

Thanks,
Al

 

[jmics19067]

well yes and no, Manufactures use this type of system (a,b,c,d, pistons in plated cylinders) to compensate for slight tooling wear and discrepancies in production to match up for the best possible fit in assembly lines.The differences are very small but are there.
Study the actual measured differences in your manual and the service limits and that will tell you exactly what you need to know.Most of the piston/cylinder combinations will fall within specifications but will not be the best fit.
I am looking at my 94 yz manual and it stating that the smallest "d" piston would barely squeeze into spec with the 'b' cylinder and the "a" piston is well within worn out spec of the "d" cylinder. So for dealership to stock the middle of the road (b) piston and say it will be fine is not an actual lie.
I would use it if I didn't want to wait for them to order it,knowing that it is not best and slight accelerated wear could be an issue(probably negligible after all if it is in spec it's in spec). If I something had to be ordered though I would get the right piston

 

[bedell99]

Whats the big deal if they order you a C piston.

Erik