4 different pistons for '95 YZ250???

[FNG]

So I need a new top-end for the old dog and Yamaha lists 4 color coded pistons for my bike. According to the dealer I need to look at my cylinder to see which piston I need to match it. He also said the piston has the same letter on it to signify which piston I need.

The aftermarket world, or at least Wiseco and Pro ex, offer 1 68 MM piston and no other options.

Anybody have experience with this or ideas? Is this common in the motocross 2 stroke world?

My only thought is as the '95 racing season went along, Yamaha made changes to the cylinder/head/piston.

Thanks for any info.

 

[hot125mod]

the piston needs to match the bore size, you cant go wrong with a wiseco. some times there is an A and B piston the B piston is usually about .010 larger if the cylinder needs to be honed a good amount. measure the bore to make sure it is in spec and check for roundness and ovality use an inside mic and a micrometer to get the size a bore gauge is a handy tool too. some of the manufactures played with the bore size some what throughout the years your 95 will differ some from a newer 250 2 stroke.

 

[Ol'89r]

some times there is an A and B piston the B piston is usually about .010 larger if the cylinder needs to be honed a good amount. .

That is incorrect.

Your dealer is correct. Find the letter on your cylinder and order the piston with the corresponding letter. Pro-X sells the B, C and D piston. There is less than 1/3 of a thousands between the B and C pistons. Aprox, .00025" not .010". Your cylinder is plated and cannot be bored or honed oversize without installing a cast iron sleeve.

The sizes for the standard pistons are 68.00mm, 68.01 and 68.02mm.

The Pro-X piston kit is available through LASleeve.com.
Buy the correct piston.

 

[FNG]

That is incorrect.

Your dealer is correct. Find the letter on your cylinder and order the piston with the corresponding letter. Pro-X sells the B, C and D piston. There is less than 1/3 of a thousands between the B and C pistons. Aprox, .00025" not .010". Your cylinder is plated and cannot be bored or honed oversize without installing a cast iron sleeve.

The sizes for the standard pistons are 68.00mm, 68.01 and 68.02mm.

The Pro-X piston kit is available through LASleeve.com.
Buy the correct piston.

That is the EXACT answer I was looking for!! THANK YOU!

I found the differences on DENNIS KIRK website.

b=.0004
c=.0008
d=.0012

 

[Ol'89r]

According to the Pro-X catalog there are three standard sizes available in the Pro-X piston. 67.95mm part # 7080B. 67.96mm part # 7080C. And 67.97mm part # 7080D.

The Pro-X piston is the same as the OEM piston.