Piston Size Question

I need some help on ordering a piston.

 

I have a 2002 YZ 250.  I don't know what piston size (A, B, C, or D) the bike has. The piston size is not marked on the outside rear of the cylinder.  I have someone that is going to help me change the piston and ring and I would like to order the parts without taking the cylinder off.

 

My questions are: How big of a deal is it to get the exact size right?  And if I guess, which would be the best one to order (A, B, C,or D)?  Thank you in advance.

 

 

 

 

 

 

Just buy a "D sized piston if you are positive you cylinder is still in spec

D because ur cylinder will be worn after 11 years of use.

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