You are correct... cutting part of the bolt away i asking it to break. Instead... if the plane size weight and preformance doesn't require a 1/4-20 use 10-24... on down to 2-56. (yes they do make 2-56 nylon bolts.)

if you want to promote bolt snapping on wingtip mpact.. you have the bottom of the wing (high wing plane) met by a 1/32 ply plate, (drilled as if it were a washer for the bolt) over a balsa block which extends down to the hardwood or plywood which is threaded for the bolt. The thin ply acts as a knife if you have a wingtip impact. It won't bother the nylon bolt in normal flight.