Another thing is to make sure the bolts are straight - if the holes are drilled at an angle and you crank the head up against the mounting lug, you are placing tremendous bending forces on the junction between the head and the bolt shank and could induce a shear failure under the heavy vibration. I'd be more worried when using a tapped metal mount though. On a profile I would be tempted to fit some aluminum plates under the engine lugs. What do the lugs sit on, plywood? That's a big shaky engine to rest itself on two littl metal lugs on wood.
A single cheap grade #2 6-32 bolt has a maximum clamping strength of about 375 pounds. For grade 8 up around 800lb.
Stay away from SS. It has no value as a motor mounting fastener except for corrosion resistance, bling, or for a weak link. Jett uses SS bolts on their mufflers so they shear off in a crash.
All good points, thanks a bunch. The engine does not sit directly onto the hardwood but rather, onto 2 degree nylon right thrust plates that go between the engine lugs and the hardwood. So the holes are drilled perpendicular to the 2 degree plates. These plates could be a source of vibration but that is the common way of mounting to a profile fuselage. The bolt heads sit correctly onto the engine this way, but on the back side of the profile plane the lock nuts do sit at an angle. The washers there seem to dig into the wood just fine. It is the head of the bolts that are occasionally breaking, not the lock nut side anyway. Hopefully I'll be okay by not over tightening them.