Autopsy on a Timing Belt

Here at we have a low-miles 1987 924S that came to us with a “broken timing belt.”  The engine had been partially disassembled – head removed – and all eight valves were bent in a rather dramatic fashion.  The pistons showed some remarkable damage, too, indicating that the valves and the pistons tried to occupy the same space at the same time.  High School Physics Class says that bad things are bound to happen.  And they did.

There is observable damage and there is implied damage.  Observable damage is something that you can see.  Implied damage is something that you assume, but cannot actually see or measure without a lot more work.  Implied damage can also be assumed from experience – an expensive and cruel teacher.

These are not supposed to stick up like this…

In this case, the valves weren’t just bent, they were disfigured in a horrid way – like Steven King horrid.  The tops of the pistons were also beaten badly by the poor misguided valves.  Three of the cam followers were also collapsed.  These were all in the category of observable damage.  From that, what were we NOT seeing?

Well, the beating that the pistons took could have transmitted that pounding to the wrist pins, the connecting rods and the crankshaft.  That implied damage may not be readily visible, but could point to a future failure.  Were these parts weakened by this event?  We could get the parts magnafluxed to check for cracks, but this process will not readily identify weaknesses that may have been introduced into the part.

In this case, we decided that the crank, rods, pins and pistons were not to be reused.  The block was in good shape, although it can be argued that the pistons could induce weaknesses into the cylinders.  The camshaft could have also taken a beating.

How did this happen?

As is the case all too often, something strange happened here.  A full belt service was recently performed with the belts readjusted at 2,000 miles.  For the service, the only roller that was replaced was the balance shaft adjustment roller that was making noises.  The other rollers were left alone.  The car had less than 45,000 miles on it at the time, but 30+ years old.  Porsche and others recommend changing the rollers when you do the timing belt…ever wonder why?

A typical timing belt failure occurs when the belt just breaks.  This was NOT the case here.  Even with all the wonton destruction in this engine, the timing belt did not break – in fact, it was in pretty good shape.  From the invoice history, it looks like the belt was one of those kevlar super-strong belts.  Instead of a broken belt, the tensioner gear broke – the plastic toothed part of the roller disappeared, leaving only the bare metal center part remaining.  This of course loosened the timing belt, took it off its task of keeping things in time, and the rest is 924S history.

These photos show the tensioner and the faulty tensioner gear.  Included is a photo of a new gear for reference.

In checking around in our circles, this is a new type of failure, although replacement of rollers has been recommended for a long time.  In this case, the failure of a thirty-year-old $70 Genuine Porsche Part #944 105 631 06 basically destroyed a good-running, low miles engine.  What a shame.

The Moral to the Story

When doing a belt service, make sure you also change the rollers.  We have questioned this recommendation in the past, but now see the why and where.  It is necessary and needed, and is a great insurance policy against possibly catastrophic damage.

Has anyone seen this kind of failure and determined the cause?  We would like to know and pass the information to our folks!


Author: Kevin Duffy, LLC, DeLand, FL

After retiring from a career in Law Enforcement, Kevin Duffy turned his attention to one of his passions, Porsche 944's and 924S's. He owns LLC in DeLand, FL, rescuing and restoring forgotten Porsches, bringing them back to a useful life. He is especially interested in the rare-but-beautiful 924S Special Edition. He can be found at Porsche Club events, including track days, tours and shows, as well as other car-focused events around the southeastern United States.

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