FredR

Marty,

An interesting photo for sure. What I see are a cam sprocket with the coating gone and a belt with some teeth missing. The $64 dollar question being why are they missing?

You state that the belt failed due to the "wear pattern" on the cog and that may be correct but do you know this for an established fact or is it something that was reported as "being the case" in an article or whatever?

If nothing else I think our dialogue to date has established that such a wear pattern on the 928 cogs has not resulted in a known failure to someone like Sean with vast experience but as he states [quite rightly I believe] an incorrectly tightened belt most certainly could.

If you can shed a bit more light on this particular failure in terms of what you know to be fact much appreciated.

Regards

Fred

M. Requin

I do sometimes forget the word "probably", Fred - it should have been in there somewhere... Here's a link to the Supra forum where I found this pic. http://www.supraforums.com/forum/sho...ng-belt-Eater!

hacker-pschorr

Thread cleanup time.... Yet another topic on the list that can jump the shark and go viral.

The "missing" posts have been moved to the master Kibort vs the World thread for posterity.

Carry on.....

FredR

Marty,

The interesting bit in the thread you linked was that the cam wheel was some after market item that was withdrawn because it was no good- too soft. They state that the belt failed because of the wear profile but there is no analysis of the failure to support such- just a statement of "supposed fact" that may be true but just as easily may not be. For instance it may well be that the sprocket had worn but the belt tension was incorrect leading to more susceptability to failure.

Thus in the case of my postulation- to date we have no evidence of cam sprocket wear having caused a belt failure. Indeed one has to look at why belts became all the rage rather than cam chains. Less cost, no lube bath, less noise- but not more reliable than a chain working correctly.

I remain intrigued as to why Porsche changed the material spec a couple of years ago to a more robust material- what can we read into this? Has any other part of the 928 been upgraded so long after the car's demise from production?

My mindset tells me there is more to this than meets the eye.

Regards

Fred

the flyin' scotsman

Perhaps of interest is the long term expectancy of the 'new' cam drive system should we all run our cars long enough.

What I mean by new; all new gears (cam, crank, OP) with a new water pump, new Gates belt properly tensioned. The 'new' system uses the latest cam gear and belt technology.

My GTS has a 'new' system that I'll be glad to report on many years and miles from now.

For comparo: is there any current manufacturer with a similar drive system stating maintenance of the belt and gears?

FredR

Malcolm,

What I find worrysome is the roger barely got past the official timing belt interval before he had to change the job lot. 80k miles and he had to change them out... outrageous I would politely suggest unless work was done by some PO and an unknown but probably official dealer mechanic who may or more likely did not get tension correct.

Why else would a set "wear out" at this rate even if well caned?

Rgds

Fred

ROG100

Fred - In my case the belt was over tensioned by a Porsche shop who did the TB & WP about two years earlier. They also did not replace the cam gears when they were already showing signs of wear. I remember them telling me this and saying they "should be good for another 5 years". One of the many reasons I started doing my own wrenching. They also put on a Porsche rebuilt pump - that also went in the trash when I replaced everything.

There were a vast number of PTechs issued to Porsche from Porsche shops regarding the rapid wear of the cam gears and oil pump gears. A lot also resulted in warranty claims. Once it starts costing Porsche money they do eventually wake up and do something about it.

Porsche is up issuing lots of parts and many within the last two years. What comes to mind is Transmission mounts, idler rollers, all rear axle bushes and suspension springs to name a few.

FredR

Roger,

May be a stupid question but if a Porsche dealer screwed you over with poor quality control on the tensioning could you not claim compo? Perhaps it is one of those difficult to prove but "without any doubt" type of things.

On the one hand it is kind of scary to know they are still modifying parts after all this time but on the other perhaps they have not given up entirely on the 928 as was looking to be the case. Perhaps what we need is a "major surge in values"-

Regards

Fred

mark kibort

Its amazing how many "professional" shops still dont know how to tension the 928 timing belt. i think this is the main reason the pulleys start to wear. i also think that if you look at the forces of the belt as was shown earlier here, you can see that if the coating is worn on the top, it doesn't mean there is not a lot of life left on the pulley. Also, in my mind, there is a lot of questions regarding the belt stiffness and teeth profile. are the Gates vs Conti exactly the same? maybe the conti experience ive had is due to the differences of the belts. after all, ive only used conti and never had any wear issues. and you saw the shape of my pulleys.
as far as the rice burner link and the pictures of the belt getting chewed up, from that discussion it seemed that the tooth profile was the cause and the manufacture changed it eventually, while the group seemed to just use another brand and not have the issues.

mark kibort

has anyone measured the difference in timing with a very old belt vs a new one?..... with the replacement i saw on two engines, about 1-2 degrees of cam pully change to the advance vs the replaced belt. (the cams advanced 1/4 tooth , on both pulleys.)

FredR

Mark,

Do correct me if I am wrong but even the WSM says somewhere that a new belt should not be used for doing the cam timing. On the 32VR tool I seem to remember that Ken recommends a correction if the timing is done with a new belt which I seem to remember I did given I had to remove the cam wheels. Might be interesting to check my cam timing now to see where the cams are.

I seem to remember the 1/4 bank is a bit more impacted because they are second in the train and the correction factor is either 1 or 2 degrees. On my motor I have experimented with cam timing by up to 2 degrees either side of the stock point and my assometer could not really tell any difference. Mentally one expects the difference to be top end for retard and low to mid range torque for advance of the cams but the difference would need a dyno to resolve.

If you have not seen it the definitive thread on this matter was posted by Ken [Porken] a couple of years ago- a must for those newer to the list who may not have followed this and one of my all time favourite threads that I revisit every so often. From this I concluded that having your cam timing off by a degree or two, although it makes a small difference, to all intents and purposes it is not too relevant. no idea how this translates when timing by measured cam lift but even the factory procedure gives a tolerance range or so I seem to remember.

Rgds

Fred

the flyin' scotsman

Fully agree Fred.....we're talking about street driven cars

kkgmgcooke

Hi. My timing belt if off and I am removing my cam gears and recoating them...but this is not the source of the question.

I rotated the right cam and it jumped clockwise about 1/8 turn. So, not wanting to go backwards (and not thinking about it), I turned the timing gear all the way around (i.e. another 7/8 turn clockwise) to re-align the marks I made on the cam gear housing with the gear. Now, I'm thinking that perhaps I created an issue with valves being mis-aligned (?) since turned it clock-wise a full turn?!?!

Is that OK or do I go counter-clockwise 1 full turn to the original location before it jumped. Or, do I need to rotate it clockwise more to re-align things?

Please help this newbie

Ken

the flyin' scotsman

is the crank at 45?........if so turning the cam clockwise to your marks is fine.

bureau13

I think if your crank is at 45 degrees BTDC, you can rotate the cam however you want without worries, just end up with it in the same spot it was originally. That is this newbie's understanding (I'm in the midst of my own timing belt/water pump job).