All good ideas. I clamp the engine but mark only one hole with the centering tool, drill and tap, secure the engine through the one hole, mark and drill a second hole, use it to secure the engine along with the first hole then mark the last two and drill them. If I try to mark all four then drill, I always end up with one or two that are off enough so that they can't be tapped and used. I guess I'm just not good enough at it.