I've searched that very thing many times. What exactly are the differences between the 797s, Super 797s, CP125, and the SP125c? I know about the antivibe on the 125s and the different covers, plus McCulloch seemed to play wheel-of-carburetors... What's the real difference in terms of port, piston, and crank design? Surely the MC101s enter the discussion here, too? I would think for the ultimate feel, one would want a 797 chassis with an MC101M/C, not the 101b?
I turn this over to you, hot rod Mac pros... Educate us.
I got a little time while my lasagna is cooking, so here goes:
The 797 engine was birthed from the 101b yes, but the timing is not quite the same but close to a 101b. If you look at the piston, you'll see it's identical to that in the 101b. Later pistons from other 101 motors can't be easily interchanged without changing the rod as well, since there were minor variations in the crown height of the piston(s). There's a 101 piston with windows that's timed for the boost port blocks that works well in the 797 with the corresponding rod. There's a rod that's proprietary to the 101b, 101d, and 101AA that can be used in the 797 if you have the matching piston.
The 797 is part of the family of "Super Series" engines which include some of the kart engines and saw engines. The 797 blocks have the removable boost port "window" which later became boost ports actually cast into the block, seen on later kart engines and 125 saw motors. The 797 block is not a third-port block like the Mc20 kart engine or D44 saw motor.
Main 797 features: removable boost port window, bridged exhaust, hemispherical combustion chamber, horizontal-mounted coil, DSP-style compression release, right-hand start, moderately aggressive intake and transfer timing, same stuffer as stock 101b motor (w/cut-out for pulse oiling), crank w/bolt-on weights, and kart-mount style exhaust. The 797 Super would have the updated main bearings, different carb (usually SDC?), thin-ring piston.
The CP-125 block has the cast boost ports, slightly smaller combustion chamber, bridged exhaust, less aggressive intake and transfer timing, horizontal-mount coil, kart-mount style exhaust, single-reed intake manifold, and straight compression release. The piston in the CP motors should have cut-outs for the boost ports and came in both thin and thick-ring. The CP rod would use the 1/2" wrist pin with needle rollers in the piston rather than in the rod. The CP blocks are all steel sleeve blocks as are the 797s.
The SP-125 block has the large oval-shaped exhaust port (longest blow-down time of any of the saw blocks) w/different exhaust mounting, larger combustion chamber, lower intake and transfer timing, dual-reed intake manifold, diagonal-mounted ignition, and "hockey-puck" piston w/no cut-outs for boost ports. The rod in the SP motors uses a 9/16" wrist pin with needle bearings in the rod rather than in the piston. The SP pistons were usually thin-ring. The SP block came in both steel sleeve and chrome-lined bore. The SPs used the straight compression release and the funky squared off muffler with its own proprietary cover. These are not compatible on the Cp and 797 blocks. Cranks on the SPs had the welded-on weights.
