With the ST it sounds like you've stripped the thread in the brass nut.

With the GMS, I gather you mean that with no prop fitted the crankshaft turns easily (at least while the exhaust port is open) but locks up when you fit a prop. If this is the case then tightening the prop is forcing the prop driver back too far on the tapered collet and it's hard up against the crankcase. This may be because (if the engine is new) the crankshaft isn't seated in the bearings fully forward or it's been in a crash that's pushed the crankshaft back. Another possibility is that the prop driver/collet is too loose a fit and seats too far back. BTW, this locking against the crankcase is fairly common on the ST's because their prop driver is cast, not machined from bar stock, and a crash or overtightening the prop can crack them.