When you say "jam nut" do you mean the second nut like on a Saito or the one and only nut like a 2C? If it is the one and only nut you don't have enough. I had a St 51 that was the same way. The crank was just to short. Some props are thinner than others you might look at that. Or just use a spinner nut. I have great luck with those.

David