Didn't mean for it to be taken that way... Anyhow; while not claiming to be any sort of expert, yes I have flown all models of 421 and maintained them (As an A&P) but that experience is relatively ancient. The reason I stated don't run them past TBO is that they have a relatively low TBO for a reason and tend to have issues like reliability... especially if run hard with high time. Basically you don't want to be stretching a GTSIO much over TBO like you might get away with operating say an O-470 or such. Sure you want to save the boss money, though with the high time GTSIO I'd just tell, not ask, the boss to get out their checkbook and order a factory Reman... The bad raps are normally from ham handed pilots or cheap owners not maintaining the engines properly or trying to run past TBO. The reduction gears (prop shaft) don't last as long as one might hope either, without becoming out of tolerance. The 421C especially the later w/trailing link mains is a great airplane. Just be nice to the engines; maintain them and don't run them over TBO, at least not by much...