The pistons, cylinders, rods and heads of the C & G21 series were interchangeable.
So you could put together a stunt or combat version with or without bearings. The CRC setup was the winning engine for Profile Carrier.
The baffled (stunt) setup had far less power than the combat (flat top).
But when you wore out all the combat piston & sleeves you could set it back to stunt for something to do with the leftover parts.