How do you replace the propeller on a mercruiser alpha one outdrive?

I replaced my cast alum prop with the same pitch stainless prop last nite and everything went fine.

However when i tightened down it it seemed like it wouldnt tighten up all the way it seems like something was stripped but i looked at the treads and every thing was fine and we took the boat out sunday with the old prop and everything was fine.

Is there a torque setting or something i am missing?

Any ideas?