Cluch and engine casings plus a few other internals are significantly different, but other bits do interchange. Power about the same @50bhp cb500 cluch has a bit of a propensity to slip, especially if giving it the beans. Cb500 clutch pushrod a problem to source. The cb550 gearbox is also stronger I would say, having never had a 550 gearbox die but a couple of 500 ones wear out 1st - at very high milage, 200,000 or so out of my sidecar bike.