ROG100

We have been selling the new gears for about two years now - cam gears 85 to 95 32v are $390 each and the new steel oil pump gear is $118.

ROG100

Currently the new cam gears are still alloy - much harder and hard anodized. Only the new oil pump gear is steel. The crank gear has always been steel.

New crank gear is $182

MainePorsche

My bad on the crank gear.
Thanks Roger, see you soon.

Mongo

Those gear prices = Ouch.

ROG100

Here are some pictures of the severe wear on my GTS cam gears. 80k Miles back in 2005.

Mongo

Wow those were way worse than mine and my car is an '87. I have no documented date of replacement on mine, but given the wear I found back in 2013, they had to have been replaced sometime as my car has close to 170k on the clock.

FredR

Sean,

Some very valid points there. When analysing my sprocket wear 2 years ago my [logical?] reasoning told me that if the wear was excessive, the belt would be slack and the alarm would go off - this did not happen.

I looked very carefully at the crank sprocket and the oil pump sprocket- got the impression both were steel- should have checked with a magnet!- no signs of wear on either. This left me puzzled as to what I saw on the cam sprockets and I wanted to understand what was going on.

My interest at the moment [specific to the cam sprockets] is to understand whether the wear pattern accelerates out of control once the coating is breached or whether it wears until the teeth engage as perhaps they should and then the [accelerated] wear stops. Given you understandably change the cam sprockets once the coating has gone I would not expect you to see this advanced wear pattern unless of course someone brings one in.

If the belt dust is down to wear on the cam sprockets it might answer my query but if such were down to the oil pump sprocket it would not albeit replacing the oil pump sprocket would be appropriate.

That Porsche changed to the HTD design in 1983 is interesting -3 years before the 32V motors came into service so one has to wonder why they made the change.

Similarly, if you have seen a 32 valve belt fail clearly due to cam sprocket wear that would be an interesting data point.

Having thought about this for some time I am left wondering if Porsche got the sprocket design slightly wrong. If they did, from Porsche's point of view they would not want to admit to this as they might get a deluge of claims for wrongful design. Given age and thus mileage piling up on most examples, failures of the sprockets become more and more evident and if they changed the design geometry slightly someone would notice this- thus when Roger posted that they "changed the spec to a harder material a couple of years ago" this might be the clue I was looking for. They know something is not right and solve it by applying a harder material to make the original design geometry last longer [edit- last as long as it should]. That Roger's only lasted 80k miles is telling- surely not adequate for a top end machine like the 928.

Just a theory -

Rgds

Fred

SeanR

I've not seen a single belt failure that wasn't due to improper belt/tensioner installation or pump seizure. That's where the factory warning system plays an important part on the factory set up. I've seen belt teeth shorn off, belts so old you could see through them and belts that were only 1/2 there due to idler arm bushings or bent arm bolts. The belts don't fail unless something else in the system fails.

The oil pump gear seems to be made of a softer alloy than the cam gears as once the coating goes, they wear faster. I change these out at the first sign of wear because I KNOW what happens after time to them with out the coating. All replacements are steel so if yours was, it was changed after production.

Crank gears generally don't wear down but most people want all new parts so I change them in sets. I don't scare people in to spending money on parts and thank God most of my customers are not dumpster diving for parts. Those that do I generally turn loose to another shop or just be honest and tell them I can't work on their car as I will not let something go that is not quality.

I don't think Porsche made a mistake on the design, I think the quality of the parts used was most likely pretty state of the art back when they first sent their specs out to the providers. It takes time and use to find out what the weak points are and that's when revisions come out. I've had to change gears on cars with sub 50k miles due to improper belt tension, and most times it is because the belt was to LOOSE, not due to over tightening. Over tightening will cause the pump bearings to crap out first.

Hence what I said above about using good parts to start with when using a PKT, you aren't going to have either issue.

FredR

Sean,

Some more interesting discussion points there- my apologies but have still not got the hang of multi part editing concept.

I hoped you might have seen a belt fail due to wear on the cam sprocket so we still have a zero on that front as it were.

For the oil pump sprocket given it is pulling the two cam sprockets, one might have thought they would have made it harder than the rest so clearly they got that one wrong. As regards design of the cam sprocket I suspect they would have got that right just that I wondered if they might have done something silly like forgetting that the coating adds an incremental thickness- you might be surprised at just how many errors of that kind happen in design and engineering. One only has to look at some of the brilliant things they did with the 928 and then wonder how the same design team came up with the flex plate clamp design that looks like something that holds the kick starter lever onto the splined starter shaft on my 1957 BSA Rocket 650cc motor cycle.

I have wondered about the wisdom of fitting a fixed tension device on a system that presumably has variable tension on different parts of each revolution. If the PKT alone eliminates cam sprocket wear then it is worth its initial cost. The PKT is in my project wish list just not as high a priority as some other items. I guess it all depends on whether one thinks the original tensioner is up to scratch. I have no axe to grind with the stock tensioner that I know of but that may be due to ignorance on my part.

Never had a belt problem in 16 years but then I usually change them out for other reasons like WP failure or every 5 years [or when I wreck!].

Regards

Fred

M. Requin

Perhaps not on a 928 sprocket, but as I have posted before, in this pic you can clearly see the top and ramp wear on the teeth that result in a scraper-like profile that did in this belt:



And on further thought I submit this idea for the math mavens to chew on:

The useful life of a cam sprocket can be calculated by: T=∫{Ytm*(sc)}

where

T=service life of cam sprocket
Y= year of manufacture, according to part #
h- hours of use
t=average tension of belt
m= belt manufacturer
sc= service coefficient. 1= Kibort 2= race. 3= normal driving 4= lol (little old lady [me])

Add any variable you think of...

SeanR

Amsoil?

MainePorsche

That's good Martin.

I just don't understand the contention of some regarding a worn gear.
It is almost as straight forward as telling if your tires are shot. Whether to coat or get new post a pic and let the knowledgable help guide you, but not to do something about worn out tires is...not wise.

M. Requin

That's an interesting mixed bag of wear levels: some show valley wear, some not.

Now that's a dangerous wear level, IMHO.

Actually, I'm a skeptic here, like Kibort, and I think at least to some degree like FredR, Ricardo, and others (whom I am NOT speaking for, of course). I do not think wear of the tops of the sprocket teeth by itself indicates the necessity to replace the sprocket, but when wear reaches the flanks and valleys of the sprocket teeth, it is definitely replacement time. This begs a question if you think about it: how much wear on the tops of the teeth indicates imminent wear of the areas just mentioned?

bureau13

So is there any consensus on the positives or even negatives of re-coating gears, such as with Finish Line Coatings? I know many have done it...I'm just curious about the impact. My guess would be it should lengthen the life of the gears, but does it do so by a significant amount?

My cam gears can be seen in my ongoing timing belt/water pump thread. They look pretty good, but there is a little bit of aluminum peeking through on the tops of the gears in the middle...more so on the driver's side. There was some advice to recoat now, and some said they look OK...my (not very expert) opinion is that they'll easily go until the next belt change, but will re-coating now or not be the difference in needing new gears next time or not?

There's probably no definitive answer to this, but I'm wondering about it...

MainePorsche

Jeff,
Just took a peak at some of your pics. Yes you have what I would call minimal wear in the center of some teeth. Your 'valleys' look untouched. You have life left in these.