The mighty Ford Corporation had a very costly time in the early 70's and then the 80's with the ring gears of many Capri 1600 and 2litre engines ring gears failures, and as Lu has already said it was the heat treatment of the teeth making them not hard enough, the secret in starting equipment is to make the smallest and least costly piece of kit the one most likley to fail, Ie the starter pinion should be the weakest item, ring gear next and then engine last.
Vfr