Well, I have to admit this has been one of the most challenging pieces of the puzzle to solve so far. When you scratch-build an engine mount, you line up the engine with the fuselage and fill in the gap with motor mount. Any variations are compensated for at this point. I received an Avipro mount for my IO-540 and as a result I have to put the engine alignment process one place back and a number of steps earlier in the building process, and that's at the firewall. The first thing I did was to fit the bushing to the apex of the firewall station. That was the easy part. You can see the location that Bob Barrows specifies for the fifth bushing for the O-540 in the Beartracks publication. I've seen some builders that tuck the bushing under the vee, but it really doesn't do any good there. I asked Bob about this and he said it needs to be in the center of that cluster to have the proper strength.
