I clamped the engine mount to the fuselage for drilling. Starting with upper right hole I sighted down the mounting tubes to check position with pre-punched holes in firewall and also checked measurements left and right, up an down, to get it exactly right. The pre-punched holes were close, but would not have worked if I just drilled them. Used tubing to step down the mounting tubes and used them as a drill guide to start the holes. Drilled the first hole then upsized using the tube as a guide. Used the step drill to open the hole in the firewall stainless, then continued increasing drill sizes up to 11/32 inch. For the final hole I ran a 3/8 inch reamer through the engine mount and firewall. Inserted first bolt and snugged it down. Re-checked position then repeated this process for the upper left mounting hole and the two lower holes in the center.
