So a new hub arrived and now it has a M4 hex bolt as a locking pin. Great, except the gearbox has jammed up yet again.

I know what the issue is - the gearbox is a 2 stage reduction gearbox and that just cannot handle the shock through the gears (and the bearing case by the looks of things). I can get 3 or 4 hits at the full distance the axe can reach of it before it packs up.