Hey Bahnse,
Just a note mate, Do your homework, I recently scored a DG head for my RM125a, this bike had a b Pipe on it. Once i put the DG head on, the B pipe didnt fit over the head and i went back to an A pipe. Now i'm looking for a down pipe. As i have went one step forward and probably two steps back. Keep in mind the Bore of the S and A is 56mm std and the B is 54 I don't know what the C is.
Brett