The main differences are the crank and the barrels/heads. The cranks ARE NOT interchangeable, the barrels are, however.

The complete motors are interchangeable in the frame, so any model NSR motor will fit any other model's frame (i.e. MC16 complete motor will fit into an MC28 frame).

To use the MC18 motor in the MC21, make sure to fit the MC21's G.P.S. (if you are using the PGM-III, that is!), and also use the either the complete MC21 carb assembly, or the at least the MC21's T.P.S. as I still can't confirm whether the T.P.S. readings differ on each model - I think they do.

Andy.
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