KTM TOP END KIT-SUITS 300 EXC 2018 TO CURRENT

The cylinders produced by the manufacturer are verified according to the final measurement, they are stamped as "A", "B", "C" usually "A" is the smallest (nominal measurement)

"B" is 0.01mm larger and "C" is 0.02mm larger than "A".

For this reason pistons of different sizes are also manufactured by "A", "B", "C" to be able to assemble the piston with the most suitable clearance in each cylinder.

Best practice is to measure your bore with a dial gauge to work out the correct piston sizing. If you cannot do this then it is best use an A size under 100 hours of engine run time and use a B size over 100 hours of engine run time. It’s advisable to have a fraction too much clearance than run a piston that is too tight, if in doubt use an A size.
If you can measure your bore with a proper bore dial gauge, select the piston that offers the appropriate piston to bore clearance. For example most 250/300 pistons require 0.06mm clearance (0.07mm clearance for 300 TPI pistons)