When I'm laying out the engine mounting holes on the rails, I set the engine in and cock it about 2° to the right, then mark the holes by reaching through the holes in the engine lugs with a scribe. Then I remove the engine and using the scribe, poke a punch mark in the center of each inscribed circle. Then I drill my pilot holes with a pin vise and a 1/16" bit. The final holes are the next drill size larger than whatever screw size I'm using.