Replacement Piston for Pinasco 177cc Cylinder

Hello, I have a 2-port Pinasco 177cc aluminium cylinder with Nicasil lining. My kit has done 7,000kms and there is a lot of burnt carbon below the rings yet the ring gap is within specs. 

Pistons for the cast iron version of the Pinasco kit are available as standard 63mm diameter and 63.4mm or 63.8mm oversizes. However pistons for the Nicasil cylinder all have a diameter of 63mm yet have variations on the SIP catalogue listed as 'A', 'B' and 'C'. What do these letters mean? Is it anything to do with the wear of the lining? 

With thanks, David

Hi there ! The A, B, C meant is the size diameter. The difference between them most probably 0.0000001 i think. C is bigger than B and B bigger than A.

 

Just two cent. correct me i am wrong.