ROG100

Well I am back and we have plenty in stock - office was incredibly busy last week in my absence but both Laura and Mercedes were answering the phones and shipping parts like no tomorrow.
Let me know if I can help?

Bmw635

Thanks all for the help in locating a cam gear- 928Int'l found one that they have so I got it since I need to get some other parts from them locally.

As for the damaged cam gear, here's a pic and my analogy of why it failed. You can see the cupping more on the left as the lose belt from a bad/ lose tensioner flapping as it ride on to the pulley, seems like the belt teeth tried to ride on to the gear profile which cause the wear. The belt is new so I guess the previous owner didn't adjust the tensioner after 1000 miles?

As for tensioner / cause of the this lose belt, I don't know the exact design of the Porsche but the PKtensioner is definitely a better design as the logic makes sense and I've seen these tensioners used on Toyota and Lexus also. I know there are many opinions on this topic of OEM Porsche vs aftermarket so I am not debating this.

Even thou I am new on this forum but I've done lots of maintenance , restore, build on my personal vehicles including rebuilt the 95 Land Cruiser engine with 275k miles 4mos ago, restore and built my own custom stroker engine on my 635 that was sold recently, and built, modify my 740 with SC and lots of other mods. I have a BMW computer to achieve these task so I hope some of you think about these PKtensioners for your trouble free 928 next time you do the TB.

Disclaimer, I have no gain or interest in these PKtensioners.

GregBBRD

Actually, that is just normal cam gear wear. They all look like this, after 80,000-90,000 miles.

You can install one gear and put off replacing the others for awhile. Many people do this, to save money. I generally replace all 4 gears and the belt.

Bmw635

The driver side cam gear, oil pump pulley, and crank gear still have coating on it, just like the spare cam gear I picked up from 928 Int'l today. I'd pick up your gear if 928Int'l didn't have it since I had to pick up some other seals from 928Int'l also- sorry.

Mark said you would have stick with Porsche tensioner instead of the PKtensioner. Would you share with me/ us your valuable experience? TIA.

Peter

GregBBRD

Since the coating is applied, how long it lasts is totally dependent on how thick they sprayed it on. The "normal" sequence of events is for the oil pump gear to show some wear first. And then either cam gear.....sometimes the left and sometimes the right.

I always thought that the oil pump gear supercession to steel was because it wore more and they made it steel to compensate for this wear. I've not re-used an aluminum oil pump gear, for many years....even if the coating looks good, I replace it with a steel gear.

Turns out that they may have had another reason to change this gear to steel.....

Yes, that line, near the center of that oil pump gear, is a crack....into every "window" except one! Betting an engine on a used oil pump gear makes little sense, to me.

I personally prefer the stock tensioner system. Very robust and reliable, for me.

Mark Anderson and I are on the "same page" about the Porken system....we both asked the same question at the same time, when Ken built the first one:

"What is this guy trying to fix? The stock tensioner system has a zillion miles on it and I've never seen a failure."

j.kenzie@sbcglobal.net

Hi Greg,
The Porkension'r also eliminates some of the plastic bushings and poorly supported rollers. When the belt is tracking off-center because of a bent shoulder bolt, that is failure of the stock system. All those pictures of 1 cup of black rubber shreds are failures of the stock system. Broken timing belts may be failure of the stock system, depending on age of belt, tension checks, etc.
For sure the stock system has the miles advantage, but it doesn't really have a perfect record.
Dave

GregBBRD

I agree that hacks can screw up anything. While a bent shoulder bolt would be a real problem....I've never actually seen one. Plastic bushings are wear items and should be automatically replaced, along with tensioner boots and gaskets.

Again, keep in mind that I've prefaced all of this with my personal experiences, which may be meager....

However, in my entire history of working on these cars, I've never seen an engine failure from a tensioner related failure. Never.

And I can count, on one hand, the number of belt failures I've seen!

I believe that "created fear" drives sales, in this instance.

Case in point....the op has taken his theory about why his one timing gear was worn out....on the only 928 engine he has ever worked on, and stated, as fact, that the problem was the tensioner, in another thread.

This is the stuff that drives rumors and inaccurate conclusions, feeds sales of useless pieces.

j.kenzie@sbcglobal.net

Interesting and good to know. With all the threads about toothed belt warning lights, and bent valves, it's good to know that failures are so rare.
Thanks,
Dave

Bmw635

Thanks all for the technical discussions.

I understand the Porsche use thermal expansion disks with the oil acting as thermal transfer. If oil leaks out (my case with new wp and belt after less than 10,000 miles), the discs can not get true thermal transfer as designed, therefore the tensioning is lost- am I correct?

I went with the PK tensioner because it's the exact one used in my wife's old Lexus when I did that timing belt. The BMW s with timing belt also use the big spring tensioner with no other stuff. Also, it's saves me the hassle of tightening 1000 miles later.

Thanks for posting the oil pulley cracks. I've read it but didn't see it till now. Luckily, my pulley is solid with no hairline crack so far. Does this fail catastrophically or do I have time when cracks appear? I bought the car as project but didn't anticipate these hidden cost so I am having second thought about replacing everything I "like". TIA.

Peter

Hilton

No you're not correct.

The belleville washers de-tension the belt as they warm up, compensating for the alloy engine expansion by reducing the tension slightly.

They still warm up without the oil - just not as quickly.

One of the locals with a broad engineering background has opined that the majority of wear to the cam gear surfaces is dust and dirt falling in from the vents on the top of the belt covers abrading the surface by getting under the belt.

MjRocket

How important are those vents? Would it be wise/ OK to cap them off?

ROG100

Just for interest ---

The original square tooth gears (cam, oil & crank) are NLA and now supersede to the round tooth style.

Both the round tooth iterations of cam gears are now made from a much harder alloy and the natural oxide layer on the surface anodized to further enhance its wear properties. This means that the new gears will (should - time will tell) last much longer than the original softer alloy gears with the sprayed on wear coating. More expensive - yes but in the long run cheaper.

DKWalser

Because the washers do not warm up without the oil as quickly, a leaking tensioner causes the belt to be over-tight. This could cause premature wear of the belt, gears, and water pump. There's nothing wrong with the stock tensioner if it's kept in good working order. Too many of us tend to ignore a leaking or dry tensioner (it's just a small amount of oil, right?).

Bmw635

OK, this is weird. There was no record of the belt done by PO but from the looks of the belts (looks new), WP (fairly clean and new) , it looks like within 10k miles 3-5 yrs old. The belt was VERY loose, even the belt tension tool didn't move. How could the tensioner become so lose? I can't imagine they installed the belt very lose to begin with since heat supposed to tighten up the belt.

Chalkboss

Odd indeed. Is the warning light connected and working?