I am going to be replacing the piston and rings in my 2006 yz450f this winter. It will be the 4th piston/rings in the bike (including the stock piston) and I usually just replace with OEM parts. I have recently become aware of different piston sizing, such as A, B, C, and D. What do I need?? I am guessing they are slightly different bore sizes to accommodate for possible wear that may have been caused to the cylinder. I used to just go to my local bike shop and order a new piston, and they have never mentioned anything about these different sizes. Please help, I am confused.