I moved forward with drilling the engine mount so that I can get a better idea of how things will fit on the firewall.
The plans have you drill one hole to 3/8″ and then match drill the remaining holes.
I chose to clean up the firewall pilot holes at 3/16″ first so that I could use long AN3 bolts to check the fit. This allowed me to ensure that all pilot holes would be captured by the holes in the engine mount.
There was some variance in the hole positions from those on the firewall, but I was able to position the mount such that all of them would be captured at 3/8″. I also checked the alignment of the nose gear attachment area for square with the fuselage.
I drilled the top left hole first. The plans don’t call for close tolerance holes, but I felt most comfortable drilling them slightly undersize with a T bit (.3580″), then reaming them to .3725″. That left a very clean hole that fit the AN6 hardware with no extra play.
After getting the first hole drilled, I attached the mount and adjusted the position.
It took some creative clamping to pull all of the attachment points against the firewall and to align the remaining holes. Once everything was aligned, I match-drilled the remaining holes, inserting bolts as I went.
I used Mouse Milk as a cutting lubricant which worked better than the solid Boelube that I have. The stainless steel of the firewall was the slowest to get through (and most likely to grab).
The reamer difference was on the high side of the recommended pilot hole size so it cleaned up the firewall hole edges nicely.
Success! One more task completed.