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.

[wence]

No worries Jeff,
I had a bloke measure it tonight( does it for a living), and he measured 72.01 basically everywhere in there .
It is a B cylinder by the way.
Piston had about 5 thou difference(tolrance), is that about right?
I cannot get a workshop manual from the gasmoto link(only the handbook).
Cheers Mark

[jostby]

tolerance depends on size per GG workshop manual:
200cc STD: 0.3mm, limit 0.6mm
250cc STD: 0.4mm, limit 0.7mm
300cc STD: 0.5mm, limit 0.8mm

But differant pistons require differant clearances based on whether they are forged or cast. My Vetex and Athena pistons both came with a small slip of paper stating what clearances were required for each. Go with whatever your piston calls for.
If you can find a supplier that knows what A, B, C etc sizes are then you can just ask them for a B or whatever you need without worrying about it.

I should add my '00 was a stock "A" piston and it still sits on my desk at work. It is marked 66.33mm.

[jostby]

I will also add that based on your cyclinder size you have a 300cc.
cylinder diameters:
200: std, 62.50mm limit 62.60
250: std, 66.38mm limit 66.50
300: std, 71.99mm limit 72.10

[wence]

Thanks Jotsby,
I was talking to Scotty of Scotties dirtbikes today and he said the same.
By the way where is the workshop manual found these days?
I can find the user manual but not the workshop manual.
Cheers Mark