Rattle rattle bang! The spine-chilling case of tensioner failure

Pictures: Don Kimberley
Words: Rob Bell

Expect the unexpected. But what is more unexpected than a complete engine failure following a routine 100,000 mile service and second cam-belt replacement? What indeed - other than complete failure of the cam belt tensioner. This was the nightmare faced by Don Kimberley one evening, driving home without a care in the world - but then, a sudden terrific noise and complete loss of engine power... GAME OVER.

Unfortunately, the K-series is not a 'fail safe' engine when the cam shaft timing fails: pistons WILL come into contact with valves - and huge financial expense is the usual consequence.

Cam belt tensioner has completely broken up. Centre of picture is the central hub of the tensioner: the outer ring has fallen out! Note the quantities of metal swarf present, and arrowed, an escaped ball bearing!

Here's that missing outer bearing ring. This runs on the smooth part of the cam belt. Loose it, and the cam belt goes loose, and the valve timing goes to pot, leading to...

... impact between the inlet/exhaust valves the top of the pistons. An unhappy marriage. In this picture, you can see that the exhaust valves appear slightly open. They shouldn't be...

... but they have bent! Note darker edge where the valves have struck pistons. Ouch!!!

Very similar damage to this can occur if the cam belt itself snaps through neglected servicing (the cam belts HAVE to be replaced every 60,000 miles or 5 years, which ever comes sooner).

Unfortunately, the cam belt tensioner is not part of the normal cam belt servicing schedule, although the tensioner should be closely inspected. It is now suggested by many owners, not least by Don himself, that the tension perhaps ought to be replaced at each second cam belt service. On the basis of this kind of experience, it is hard to argue against that logic. Certainly, this is what I plan to do!

What happened next to Don's engine? The most economical approach was to replace the cylinder head, complete with valve train. Fortunately, the bottom end of the engine was felt to be serviceable - thankfully because the failure occurred at a low engine speed!