Here is an idea that will help some of you keep that engine snug against the firewall. My sketch is very explanatory; however, a few extra words are necessary. Set screws come in all sizes, lengths, and most have allen sockets broached in one end. Choose a size and length to fit your application. The nut may be soldered in place or you might try some CA glue. There is really not much torque, if any, on it. This idea does provide an effective way of fastening your engine in place and eliminates reaching inside a fuselage hunting for a hex nut to tighten.