I had the same mount and engine on my Hobbico trainer. It never came lose or let the engine move even when I crashed and totalled the plane, but it just never felt right either. Even though it worked perfectly I won't use a mount like that again. Just a mental thing with me I guess.

OS says 11x7 prop for breakin so I went with that one.

BTW, the wing joiner on that plane isn't strong enough and broke on mine on a hard landing. Repaired it with fiberglass. If I build anouther I'd glass it duing construction.