Yetti,

I understand what your saying it happened today to my son. He was putting an old OS 40 back into service on an airboat he is building using a dome type prop nut that is tighted by inserting a pin as a wrench. Upon starting it backfired loosening the nut.

When we say tighten the nut firmly, it needs to be very tight. Otherwise you risk the danger of throwing a prop either on the ground or inflight which happened to me some years ago. Compressing the prop to some degree is normal. As mentioned above once the prop seats, I hold it with the non wrench hand close to the hub and torque it down to where the nut stops not over forcing it beyond tight.

Hope that helps!

Steve