The exhaust valves** on both the R and Niagara are equally unreliable, however the increased lubrication and 'sealing on the later engines did give them a better service life. **It seems that 40 hours were considered to be 'life'. My 'Valve man' in Surrey was quite dismissive of material quality and the lack of a decent tulip shape, but in all fairness they were over 50 years old with little history. PP