ROG100

"This too casual usage seems to perpetuate the wide-spread-outside-of-928-cognoscenti-world the false impression that the timing belt is the only thing that needs to be changed during a belt service when in fact it is usually the least critical item that needs to be serviced, replaced or at least inspected."

I could not agree more - it always worries me when a customer orders "A" timing belt. How many cars for sale with the statement "timing Belt changed" and then you see the receipt for the work, and guess what, that is the only part they changed. If a belt snaps it is always another part that causes the breakage.
I cannot ever remember hearing of a belt breaking with no other part involved.

worf928

ROG100

Dave - I doubt I can count them on one hand - extremely rare. More cases where the belt strips teeth because the WP seizes or bad maintenance practices - belt too tight or too slack.
The more I dig into the gray matter, the less I remember of an actual case of the belt snapping. Everything I can remember, that was catastrophic, the belt did not break. Maybe Sean, GB, DR or Mark A have a story to tell.

docmirror

Brings back the horror story from Porsche club meet a few years ago. Guy rolls up in a 928, just getting dark, we look down in the air tube well on the pax side and 'holy-mother-of-god!' all we saw were a few threads left in there of what used to be a timing belt. I was - gobsmacked. Towed home, survived another day.

Christopher Zach

I have had one belt *break* on a 944S (it was a type 00 belt installed by Porsche) and the symptom was engine running at low RPMs then stopping. No smoke, no noise, no muss, fine water pump. It sheared several teeth and broke, requiring several new valves and a refresh.Porsche redesigned the belt after 87, my guess is they ported the fixes over to the 32v belt as well.

Never had an .02 or better belt break since. I did have the tensioner pad crack, destroying the head, but that's another problem. Never had a balance shaft belt break (not on 928 of course) but have heard of one breaking then taking out the timing belt.

Edit: Even when the tensioner pad cracked, broke off the head, took a chunk of head with it, and the chain ripped off most of the teeth on one camshaft the .02 belt did not break or shear. Granted I *did* replace it when I spent a year fixing the whole resulting mess myself but it was not broken.

Yours for "30 years of Porsche ownership data" on the 4v engine types.

Bigfoot928

I've seen 3 928's where the crank peeled off some teeth, 2 of them were early cars where the belts appeared to be original and the cars were parked for decades and then attempted to start. On one of them the teeth were semi intact, and just stripped off the belt.

worf928

A first hand account! Excellent. Tell us more: Do you know of the outcome of a root cause analysis? Age? Under-rotating pump?

Interesting info, but we know that the 944 has certain common failure modes that are very uncommon in the 928. So, I can't say that anecdote counts.

This I have seen on several occasions. There are many potential root causes. Extreme age. Sharp sprocket teeth from normal wear or excessive wear due to poor belt tension. Others?

Right. So, like the above: belt didn't actually break. Some teeth went MIA. In my most extreme case ~12 teeth (as I recall) in a row were missing and still no valve crashing. In that case, I recall counting 26 teeth on the crank sprocket with ~50% wrap so the valves were hanging on one belt tooth.

The one case I clearly remember was Heirich's GTS where the 'technician' somehow managed to wrap the belt the wrong way around the idler roller and the wrong way around the tension roller such that eventually belt teeth met belt rubber and 'gathered-up' the belt in the area of the rollers until it just gave up. There are (used to be) pictures on an ancient thread 'round here. 2007 I think. As I recall the collective analysis was that it would have been very, very difficult to string the belt the way it was strung.

I also remember seeing Rennlist pictures of a severely dry-rotted ~30-year-old belt taken at one of your gatherings. Might be the same one DocM's writing about.

So.... *extreme* age, or accelerated aging due to storage conditions, or someone trying really really hard to get the belt to actually snap...

Maybe Mark can tell us about a not-too-old belt just 'snapping' for no apparent reason.

ROG100

Pretty sure the same car at the OCIC in Dallas way back. One owner car with IIRC 47k miles. The belt had not been changed since the car was new. The belt was badly cracked but the radial fibers were still very much in place and still holding the belt together. How long it would have lasted if peer pressure had not made the owner flat bed it to Sean's place is anyone's guess. It never snapped!!! Same car was totaled a few years later and the owner nearly killed - the car saved him. If he had been driving one of his other cars it would have resulted in his death - very lucky guy.

Mark Anderson

Count me as never having seen one break. Just lots of sheared teeth

polecat702

The Gates HTD belt is the same belt used in Harley-Davidson motorcycles. That's a fact, ask Gates and they will tell you it's the same belt, construction wise. The only difference is the width and length. Even the teeth are the same size. Detergent oil will cause the belt to deteriorate, they generally don't break, the teeth will shred as the belt delaminates. They're really almost indestructible.

Back in the late 60's the early belts were square toothed, and came off a lot, very unreliable. When the tooth was changed to the rounded bottom, the belts became very reliable, didn't stretch like a chain would.

Bigfoot928

A shop here locally kept some of the timing belt teeth in a baby food jar on the counter. The owner would pick it up, take off the lid and dump out the the teeth onto the counter to show people who would not ante up for a timing belt service. He would also take a new timing belt out of a box and show it to customers and explain that they would want to buy a new car when the teeth came off and they saw the repair bill from the damage. He would simply tell them that he will make more money if the belts break but he would rather keep his customers long term.

Rob Edwards

I was struck by this enough to take a picture of it, burn car at Mark's. Those belts are tough:

Christopher Zach

I wouldn't quite dismiss it that quickly: The 944S was a 16 valve car that used the same distributor, camshafts, chains, tensioner (87+ models), idler pullies, and head (it's a 928 part number head :-) as the 85 and up 928S'. The belt is shorter, and the engine does run at a higher RPM/MPH (3,000 at 60mph as opposed to 2,000 at 60mph for a 928S) but the same basic design with the same potential for hilarity (well, divided by two). The first belt did snap, the third belt (the one that was on the car when the head self destructed) did *not*.

I'm scheduled to replace the 928S belt in another 15k miles or so, I could just run it till it breaks :-)

docmirror

Yeah, that was the one, at OCIC. He was planning to drive it home. I basically told him to give up the keys, and take the batt cable off so it wouldn't move again until the belt was swapped. I swear I looked down that hole with a flash light and saw light go right through the 'belt'. It was nas-tay.

Shawn Stanford

Hey Doc: I mentioned your Bonanza purchase to a couple of aviation-minded friends on FB and they're quite jealous. They also said something to the effect that until it was upgraded with new electronics you wouldn't be able to fly IFR. They thought cost on this upgrade was in the $5k range (which still seems cheap, all up). Is that your plan?