I installed the engine mount, mostly to get it off the floor so i wouldn't be tripping over it. The firewall has pre drilled holes at four corners, but they dont necessarily match the frame exactly. You kind of need to find a best fit, and final drill the holes to 3/8" (final size) with the frame in place.
I was able to drill the first three holes to 3/8" in their original location but the fourth hole was off towards the side of the hole in the bracket. So, with the other three torqued down, i drilled 3/8" through the frame and firewall toegether. There are also two holes at the bottom of the engine mount near the landing gear support that are not pre-drilled, so i drilled these to 3/8" through the frame as well.
Correct torque for an AN6 bolt is 160-190 In-lbs. I torqued these to 175, then continued to tighten the rotation until the next castle nut flute aligned with the cotter pin hole.
Had to use two washers under each castle nut to engage cotter pin