Piston A, B or C?

[wence]

G'day,
I have just pulled the head and piston out of the bike and will be doing a big end up rebuild now I have more time.
Just wondering whether it states anywhere on the head or piston what size I need ( A,Bor C),as I can't see it anywhere?
Thanks in advance,
Mark

[gasser]

Back of the cylinder, at the bottom an A,B, or C will be stamped in the middle.

[wence]

Beautiful...
Thanks Gasser, I had a quick look but couldn't see anything.I will check again.
Cheers Mark

[GMP]

You can safely go up a size as well for a first top end, so if your an "A" cyl like most are, shop for a "B" piston.

[wence]

Thanks Glenn,
I am about to measure it tonight as a second check.Does anyone know what the factory clearances are piston to bore or ring to bore?
Cheers Mark.

[GMP]

Unless you have a very good bore mic and/or a shop that has one is doing it, don't bother. The margin of error with lesser instruments is greater than what you are trying to determine. I have measured this years ago on a '00 XC250 with a good 3 point mic and verified its fine, but don't have the numbers handy. The "micro" size difference between A.B,C.D is only 0.01mm. Clearance spec is pretty much industry standard for a liquid cooled 250 2-stroke, I just don't remember off hand. Its related to the piston type (forged vs cast) for the same size. You will be fine with a "B" in an "A" cylinder, many of us here have done it as standard practice. I've even gone to "C" in an older "A" bore on a friends bike that was in good shape after deglazing.