Try using what motorcycles and snowmobiles have been using for years, a compression spring under the head of the bolt.

I use Associated clutch springs found in the RC Car section, good and stout, fit perfect under 3mm bolts, a 4-40 would need a washer. There's about maybe four winds to them, really heavy gauge wire diameter. They will exert as much force as a tightened bolt but will allow expansion and contraction and soak up the vibration.

Not cheap though, about four bucks a crack but I think it will cure your problem to your liking. Still use removable loctite though on the bolts, but at least they can be taken out if needed OR make studs out of a bolts for the engine, which take the wear, not the block threads, and use a locknut and then the spring. The studs can be replaced. Easier on the block threads if you find yourself removing the muffler or whatever frequently. You'll see that in race engines that get taken down constantly.

Looks like I got here a little late, but thought I would throw it in, might help someone along the way.