On the other hand when I had my shop I found that the biggest cause of belt failure was belt tensioner bearing seizing up usually after the first belt was replaced(even though the tensioner felt free the sealed bearings had dried up) In my opinion the tensioner(s) should be replaced at the same tine as the belt(s) . One that sticks to mind was a Peugeot MY16 twin cam, the customer didn't go for the $100.00 + tensioner , 4000km later had $3000.00 rebuild (valve to piston contact at high rpm). It's pretty hard to have redundancy on timing belts, I like the odds with a rotary!! Georges B.