8 cylinder front engine iconic vehicle
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By ChrisZ
#159527
So talking to people at SITM about belt changes and the consensus was that no one has seen a 928 belt fail.

What apparently fails is the items touching the belt. Idler pulleys, water pump, tensioner pulley, gears. All resulting in the item jamming, smoking the belt, then causing a failure either immediately or upon a restart once the molten belt is welded to the item in question.

So why replace the belt? Instead just pull and replace all the pulleys+pump and be done with it. One can set the car to 45 degree time, mark the belt at the engine and each gear, then put it back on.
  • No more underneath to remove the starter and lock the flywheel.
    No more 300 ft pounds of torque on the crankshaft pulley
    No more futzing with the resonator
    Etc.
Thoughts?
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By maddog2020
#159532
teeth on the belt do fail. I've personally seen teeth stripped off belts when cars are left non running for periods of time. Newer belts are probably not as susceptible to this failure but who wants to find out on a 32 valve motor?
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By icsamerica
#159541
I think the belts dont fail because they are changed so often, As stated... sometime a pulley, roller, tensioner, or water pump can fail and damage the belt. Changing the belt is just one part of the over all system that needs to be checked and have components replaced now and then so changing the belt is just part of the whole process.
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By smiffypr
#159581
Belts do wear, and deteriorate over time and with heat. They don't suddenly go from being perfect to falling apart at 5 years or 60,000 miles, but with enough time and heat they will get to the point where they snap independently of another item failing. Also remember the cost of a failure.
Are you seriously suggesting that anyone should change worn pulleys (which do wear the belt) and not put a new belt on? If I take a belt off for any reason I would put a new one on.
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By worf
#159588
ChrisZ wrote: Thu Jul 07, 2022 7:57 pm Thoughts?
You’ve provided a lot to unpack. A half-dozen paragraphs are required. Maybe this evening. Short story now, so that this thread isn’t an ear worm all day:

It was me, Howard and Sean at the center of this conversation and agreeing that the belt is not the *initial* failure mode.

You replace the belt because it is literally the cheapest part(*) in the system and it has to be falling-off loose to change any other thing that touches it.

(*) If you are paying Porsche prices for new bearings with rollers.

Deciding that you are not going to replace the belt doesn’t result in any of the ‘savings’ you list in bullet form.
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By Gary Knox
#159605
I'm aware of at least one 32V motor in which the teeth sheared at about 80K miles on a replacement belt (not sure, but was probably Conti). I also have experience with the first 928 I owned in which the original Porsche belt was first changed at 90K miles. An '87 that I'd driven since ~55K miles, and there was very little knowledge in the 928 community about belt changes. no internet forum, and no official Porsche service recommendation that I was aware of. I also had a '91 Audi V8 with dual overhead cams and a similar timing belt that had an official 90K mile recommendation - so I used that as guidance for the '87 S4. When that belt was changed at a Porsche dealer, the mechanic told me the belt was still in 'good' condition. The '95 I currently own had the belt changed (Gates) by Precision Motors in CA, and the service records recommend replacement after 50K miles with no statement on years of service.
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By worf
#159710
ChrisZ wrote: Thu Jul 07, 2022 7:57 pm So talking to people at SITM about belt changes and the consensus was that no one has seen a 928 belt fail.
It is most correct to say that no one has had, seen, or heard of a case where belt failure was the primary failure mode.

ChrisZ wrote: Thu Jul 07, 2022 7:57 pm What apparently fails is the items touching the belt.
Or the person that last touched the system.

ChrisZ wrote: Thu Jul 07, 2022 7:57 pm Idler pulleys, water pump, tensioner pulley, gears. All resulting in the item jamming, smoking the belt, then causing a failure either immediately or upon a restart once the molten belt is welded to the item in question.
Not quite. Water pump bearing failure is the typical failure that results in the belt melting to the pulley. Theoretically the main roller could also seize and cause the belt to melt to it. But, I've never seen or heard of that mode. The idler pulleys usually just make noise when they seize because they don't actually touch the belt if it isn't loose.

Worn gears will eat teeth from the timing belt.

A bad tensioner - several ways it can be bad - may enable the belt to jump on the gears, may stretch the belt leading to a loose belt (and then jumping) and may "assist" the gears in eating teeth from the belt.

ChrisZ wrote: Thu Jul 07, 2022 7:57 pm So why replace the belt?
Because in order to replace any of the other components that it touches you have to release tension from the belt and then once the component is removed the belt's going to more-or-less fall off anyway.

The belts do have a life time that varies by age, use, installation, etc.

I have seen 17-year old belts that looked new and I've seen 20-year old belts that looked like alligator skin.

Porsche's last recommendation was - IIRC - 10 years or 60k-miles.

ChrisZ wrote: Thu Jul 07, 2022 7:57 pm Instead just pull and replace all the pulleys+pump and be done with it. One can set the car to 45 degree time, mark the belt at the engine and each gear, then put it back on.
  • No more underneath to remove the starter and lock the flywheel.
First, in 25 years of doing 928 timing belts I've never had to remove the starter.

Second, there is no way to remove the tensioner roller carrier with the pulley stack and harmonic balancer on the crank.

Third, it would be rather difficult to remove, rebuild and reinstall the tensioner with the carrier and balancer installed.

ChrisZ wrote: Thu Jul 07, 2022 7:57 pm No more 300 ft pounds of torque on the crankshaft pulley
218 foot-pounds.

ChrisZ wrote: Thu Jul 07, 2022 7:57 pm No more futzing with the resonator
Resonator? I don't know what that is in relation to doing a timing belt job.
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By worf
#159711
Gary Knox wrote: Fri Jul 08, 2022 9:15 am I'm aware of at least one 32V motor in which the teeth sheared at about 80K miles on a replacement belt (not sure, but was probably Conti).
Are the missing teeth due to the *belt* just shedding them?

I doubt it.

By 80k miles most cam gears (and definitely any aluminum oil pump gear) are worn to the point that the 'teeth' are sharp.

So, my Razor (made by Occam) tells me that an 80k motor that sheds timing belt teeth needed at least one new gear when the belt was last done.


So... this one:

Image

did the belt just shed teeth? Or maybe, was it the last "technician" that screwed the tensioner bolt ALL THE WAY IN?

I did a belt on an 18k-mile '95 GTS that needed all four gears 'cause the previous belt job done at 9k miles was done by a tech from the screw-it-all-the-way-in school.

The above-pictured belt, BTW, has 12 teeth in a row missing. The crank gear has 50% belt wrap on 26 teeth.

So, ONE belt tooth is strong enough to turn the cam gears until it gets eaten by a gear "sharpened" by a belt set to "piano wire" tension.

That belt's 928, once the cam timing was reset correctly on a new belt, ran fine.
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By ChrisZ
#159923
worf wrote: It is most correct to say that no one has had, seen, or heard of a case where belt failure was the primary failure mode.
Ok. I have had two timing belt failures and one timing chain tensioner failure on the 240,000+ miles I have on my 944S engine. First belt (factory 00 type) shredded and snapped at 35k, replaced under factory warranty. Second belt (type .01, installed by Porsche) failed with stripped teeth at 70k miles (40k in) replaced NOT under warranty (that cost me). Third onward are .02 or later (.03) and have not had problems. Timing chain tensioner failed at 180k miles, mess beyond words.

So I have seen belts like these fail. That's why I was wondering.
Not quite. Water pump bearing failure is the typical failure that results in the belt melting to the pulley. Theoretically the main roller could also seize and cause the belt to melt to it. But, I've never seen or heard of that mode. The idler pulleys usually just make noise when they seize because they don't actually touch the belt if it isn't loose.
Makes sense. However the pulley on the tensioner does ride the belt.
Because in order to replace any of the other components that it touches you have to release tension from the belt and then once the component is removed the belt's going to more-or-less fall off anyway.
Ok, so you don't have to remove the front crankshaft pulley and the harmonic balancer (what I called the resonator, my boo) to get the belt off?

Because if you have to remove that don't you have to lock the crankshaft so you can apply the 218 ft pounds of torque to get the pulley off? And to lock the crankshaft don't you have to remove the starter and put in the flywheel locking plate with the teeth? That's the pain in the rear that I would prefer not to do; on the 944S you can sneak the belt off and on the crankshaft pulley with care.
First, in 25 years of doing 928 timing belts I've never had to remove the starter.
Does the flywheel lock go somewhere other than the starter?
Second, there is no way to remove the tensioner roller carrier with the pulley stack and harmonic balancer on the crank.
Ah, that's the problem then.
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By worf
#160086
ChrisZ wrote: Sun Jul 10, 2022 2:17 pm Ok. I have had two timing belt failures and one timing chain tensioner failure on the 240,000+ miles I have on my 944S engine.
This is a discussion about the 928 timing belt system. I will not claim to know anything definitive about 944s. All I know about them is hearsay and for all I know their timing belts are made of Paper Mâché and will break on the first humid day.

worf wrote: ... Theoretically the main roller could also seize and cause the belt to melt to it.
ChrisZ wrote: Sun Jul 10, 2022 2:17 pm Makes sense. However the pulley on the tensioner does ride the belt.
However, that's what I wrote. See excerpt above from my original post.
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By ChrisZ
#160174
Rod: Thanks for the info. Good to know, makes the removal of the front pulley a good bit simpler.

Worf: Understood, most of my belt experience has been on the 944S. It's a similar belt type, smaller length, same basic rollers and gears, runs about 30% faster but they had serious problems with early revisions in the 80's before they figured it out. Not sure how many of the 928 owners have owned since new, maybe the spate of similar failures on 928's in the mid to late 80's are just lost in time.

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