In a fastener driving tool, a retention system for a piston comprises a buffer and a set of bearings in a cage, the bearings being abutted against the buffer. The head is molded over an integrated camming surface, which is provided with one or more preferably four raised camming portions The camming surface is arranged for engagement with a corresponding camming surface of another cam component to operate the clamping mechanism. Each arm includes an operation portion suspended above the heat sink and a hook connecting with the operation portion.