This is a replacement Novarossi connecting rod for .21 engines. This includes the P5, P5X, 421B, 421BX and all Plus 21-4. If you are replacing the piston/sleeve in your engine, you should always replace the connecting rod and wrist pins clips at the same time to avoid premature failure.This connecting rod replaces both the FMS7007 and FMS7006 connecting rods.