Back in May, I predrilled the engine frame airframe bolt holes just to get a step ahead while waiting for my canopy sika to dry. I had these plastic bushings that perfectly into the engine mount's bolt tubes perfectly, and they were center-drilled with a hole just right for #10 screws which fit the fuselage's reference holes. Image 1 shows the engine mount screwed to the firewall with #10 screws held centered by the plastic bushings. I then drilled the bolt holes to right size through each tube by removing that screw/bushing, and installing a proper bolt as I drilled each hole.
Installing and torquing the bolts was straight forward. Image 3 shows the engine mount in place, and the front end is up on a table to raise it up for landing gear installation.
