Austin,

Your first post is interesting in that one can clearly see some wear of the "coating" on the leading face of the "valley". This is precisely what one might expect to see but rarely [if ever] do we see such. What we typically see, as happened to my original GTS motor sprockets, is the coating on the top flats starts to wear through and then everyone "knows" it is time to replace them before the belt gets "cut to ribbons" on the alleged "sharp edges". Although this is not a bad practice it has always struck me that there is something not quite right here. If such were likely to happen then logic says we should see plenty of examples of belts being shredded and 32 valve carnage but in some 25 years of 32 valve ownership not once have I seen such happen. Does this not sound a bit "strange"? Just as there is and remains over blown paranoia about timing belt warning lights [remember the inital tensioner design did not have a warning light capability] one wonders whether the consequences of belt failure have caused an understandable but possibly unwarranted cause for concern?

One of my "pet theories" is that when the sprocket/belt system was designed [possibly by Porsche engineers but more likely Gates Engineers], they had two bites at the cherry. The original design was abandoned and replaced by the HTD tooth design. Whether that was specifically to accomodate the 32 valve motors not sure but the time line is consistent with such - sort or more or less. My puzzlement was that these HTD cam sprockets invariably wear on the top flats that do not take the drive and theory [to my mind at least] suggests that the belt should have little to no contact with the flats as the torque thrust loading should be taken by the teeth of the belt as they sit in the valley.

Now in your case as photographed, the top flats are well "polished" and [at last?] we see some wear [of the coating] on the leading edge of the valley. Does this mean the sprockets are toast or that they have worn more to where they should be [aka "run in"] and thus more like how they should perhaps look?

The logical conclusion this suggests [to me at least] is that maybe there was a design flaw in that the designers [possibly?] did not allow for the coating thickness in the dimensioning. With the coating gone on the top flats the teeth are now [it seems] taking load as intended by design?

Getting these sprockets coated has been available for quite some time. Colin [Lizard928] sent me a pair of used sprockets he had coated quite some time ago after I noted my original sprockets were showing signs of wear of the coating on the top flats. The coated sprockets have now done about 30k km over some 8 years or so and no signs of coating wear at the last inspection a year or so ago.

To be clear, i have no firm position on this matter but I do find it a technically interesting point for discussion purposes.

 

Couldn't agree more. After all, this is a technical discussion forum.

In the case of this particular car, I have not so much as rolled over the starter in over 7 years. I was a bit surprised when I took the timing belt covers off the other night since I had done a TB/WP job just 5k miles ago. The coating had only been penetrated by roughly 3/8" spans on the tallest points of the sprockets. My thought process at the time was that a properly tensioned belt would bring the wear to a crawl. This was not the case & I did evaluate the belt tension prior to its removal. It checked out fine with the Kempf tool.

I had actually totally forgotten that Colin was restoring these parts.

 

Interesting that your friend uses this on steel gears. On the early 928s with steel gears there's no coating like there is to harden the surface of the soft aluminum gears, and the steel gears go hundreds of thousands of miles with essentially no wear.

I would think that a lubricant on a cam gear would contribute to belt slip and jumping teeth, but maybe not. The hardened anodized coating on the aluminum gears isn't slippery like a lubricant would do, or maybe that's not really what this product is.

When I redid the engine in my 79 with an 83, I wanted the steel cam gears to look nicer but purposely used an old belt in the belt run location to prevent any paint or coating from messing with the belt tracking.

 

Pete,

Have you seen any signs of the belt polishing the flats on those steel sprockets?

 

Pete I would give this advice to you regarding the painted gear sets ,
I would suggest that you remove any coatings where the gears are clamped together,
as they will eventually start to wear,
the coating will also wear out and this will reduce the clamped force imparted by the bolt and washer.

NOTE this can cause the gear to start walking back and forth on the cam shaft.
Worst case you start to hear a clacking noise followed by the gear possibly failing.

What you need is to have the clamped surfaces with no paint on them,
as the metal clamped areas will begin to work harden over time.
this can be seen with old pulleys that have been dressed with 400 grit paper on the clamped areas.
the area that was clamped will be below the surrounding surface.

This is why you should always resurface with 400 paper these clamped areas on the pulleys and washers.

NOTE also your cam belt drive gear should also have its coating removed as its got a bigger clamped surface area .

NOTE there is also a chance that the coatings you have used could be acting like glue,
and sticking the parts together,
but this wont be visible until you have performed an inspection of the clamped areas.

NOTE I have had 2 Euros with 16 V fouled up cams,
due to loosened cam pulley bolts,
in both cases the driver side snout snaped off since its longer.

 

I'm not sure if something that is "anti-friction" is necessarily a lubricant. Somewhere else I saw it described as a "dry film lubricant", so I defer. What I can tell you is the gears feel no slipperier than before the application. I spent a fair amount of time last night tinkering with the coating. It doesn't "reverse" with heat or soft abrasives. A dry cloth or tack rag does transfer it though. At one point, I put on a latex glove & just scrubbed a single tooth with my index finger & moderate pressure. While there is a transfer on to the glove, I could never expose the bare aluminum with any combination of the above tactics.

Additionally, compressed air will not touch it. I also chucked a sprocket into a drill and spun it over a dry cloth held to about a 1/4" away. No transfer.

it's obvious that there will be a transfer to the belt. Now I think I understand how my friend was able to report that the "dry film remained intact" after 900 hours of use in plastic mold injection application.. This product must have adhered to all of the parts (belt included) well enough in that application that the continual transfers of material kept all of its moving parts coated. In other words, the product is adhering well enough to not blow away or otherwise be measurably consumed.

To the point about the steel gears in use in the mold injection application.. There is no telling what components they were trying to protect in their application. Like you, I doubt it was done with protection of the gears in mind.

 

My understanding of the process being discussed is that the Moly is being used as a release agent for the timing belt to reduce the friction caused by the moving of the timing belt on and off the cam gear.
It does seem to work well for cam gears that are not completely worn out.

 

I think that's a good way to put it. I was completely unfamiliar with the product until a few days ago, but it seems some of you are not only familiar.. At least a couple of you have already seen it at work inside a 928!

 

Here are pics of the aluminum square tooth gears I pulled off the motor I used where you can see the anofixing worn away and the metal work out. Then a few pics of the steel gears with essentially no wear on the belt run area.


Worn aluminum gear from donor motor. I didn't even know they made aluminum versions of the square tooth gears.

The used steel tooth gears I cleaned up and painted with high temp ceramic paint. I have a 2nd set of these that look just the same. I have a brand new pair of round tooth anodized gears for my 84 Euro S

Better view of the steel gear running surface. Polished a bit but teeth still perfectly formed. Thesw gears have over 175k miles on them