G'day Jim...I gather you're using a plain bearing 40? In general, plain bearing cranks are smaller in diameter than the ball raced cranks so they've got less resistance to the bending loads from ignition. Maybe that's why they keep breaking. I doubt the Higley hub contributed to it because it mainly loads up the front end of the bush, especially in hard turns, from gyroscopic precession plus of course the extra weight.

Unlike a glow engine where the flame propogates from one location and gives a reasonably smooth rise in combustion pressures, in theory with a diesel it's possible for the complete mixture to start burning at the same time. This would give an incredibly fast rise in pressure and to a much higher maximum than a glow could ever do. This won't happen in practise of course because no mixture is completely perfect but it can still ignite at any point in the combustion chamber and maybe at several points which still gives a very fast rise in pressure. I know a lot of guys have had success with conversions but maybe the Tower is a bit on the weak side mechanically.