RE: DLE 20 engine mount?
Yes, drill a bit small and then run a tap through the nylon threads. The screws go in with incredible strength and vibration does not tend to loosen them. I assemble the two piece nylon Great Planes mounts with a thin film of CA or epoxy between the two mating halves once you get the width you want. Snug them up with bolts through them on the bench until that sets. Then they are a joy to work with during the rest of the assembly process.