"B" piston in an "A" bore?


New member
Hello I just got my 06 DE 300 and am selling my 01 xc 300. I am wondering if the Athena "B" piston kit that I got for my 01 will fit in my 06 with an "A" cylinder on it? Thanks Keith
All the 300 pistons are the same from 98 to current I believe so as far as fitment goes, yes it will work. How much time does your new A cylinder have on it? Stock size is a 71.94MM and the B size is a 71.95MM, a very small bit larger. I would venture to say that the B would be fine. Just be careful to doublecheck piston to wall and ring end gap when you get the bike apart. :D
You should have no problem with the next size up piston. I always run a "B" Vertex in my "A" cylinder. I never used an Athena. Verify the size of the Athena "B" piston to be .01mm over the stock "A" piston, don't just go by the letter marking.

Also, unless you have a very good bore micrometer(expensive), don't worry about measuring the cylinder yourself. If you suspect wear then take it to a machine shop. The errors that will occur from improper measurement are greater than the differences in bore dia. that you'll be looking for.
I was thinking that I might start shopping for my Top end rebuild. I have a 2005 Gas gas EC 250. I went to GoFasters to look up the parts, and there are several pistons available: an "A" , "B", "C", and so on.

Is it possable to know ahead of time which piston I will need ? This is a new bike, with about 50 or 60 hours so far. All easy fun miles, but I like to do a rebuild every season.

I have read on this forum and others that I can just swap rings, reassemble everything re-using gaskets, and GO. I would really rather do it the RIGHT way and install all new good quality parts.

Any advice... ??
For the amount of work involved, I always do a complete top end. I do this every winter, and I'm good for the next season on my 250 with good oil (Amsoil) and always a clean filter. I run a "B" size Vertex in my "A" cylinder, and it works fine. Holds performance all year and looks decent when it comes out. Vertex are relatively inexpensive. Remember there is more to time on a top end than wear, the piston fatigues as well.
There should be a letter stamping in the casting near the rear base area.