I see another difference, In the pics above, it looks like the crankshaft is straight, mine is tapered, and the flywheel has a cone shaped hole in it. AV8TOR1977, I think you mounted your prop directly on the crankshaft, with no "conversion" parts needed, in my case, with the taper, that isn't possible, strange how the engines are so similar, but so different. Maybe this is another diff between the 16 and 21 cc versions ?