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.