many yrs ago I had both alternators on Seneca "fighting each other" - eventually one failed so decided to replace them both at the same time - about £400 each i think........ " do you want the rubber drives as well ?" said the man from Piper dealer near oxford..... " just as well I say" thinking might add £30 to bill................ £ 1100 + VAT each !!!!!!!!! these are little cotton reel size toothed drive with rubber ( cushion) inserts ---- now that was 18 yrs ago darent think what they are nowadays