Hi,

I recently changed the timing belt on my 1987 S4. The belt now has about 1500 miles on it. Now, while doing some other maintenance work on the car, I happened to notice that the belt tooth shape is different from the cam pulley tooth shape. The teeth on the pulley have a round shape while the teeth on the belt have more like a square shape. Looking at the old belt I took out, its teeth have a round shape. I did not pay attention to this detail when changing the belt.

Is this an issue? The workshop manual says that the pulley tooth profile was changed in 1983 and that you should only use matching pulleys and belts. The workshop manual calls the round tooth shape HTD (High Torque Design).

I also looked in a timing belt databook from Optibelt and they say that at least their belts can be used in HTD pulleys even though the tooth shape of their belts look pretty similar to my belt (more square).

The new timing belt I put on is manufactured by Federal Mogul. Could it be that the belt I put on was intended for an earlier model with the non-HTD tooth shape? Do I have to go back and change it again? What are the symptoms of using incompatible pulleys and belts, accelerated belt wear?

Best regards
Niklas Kampe
1987 S4 5-speed manual
Finland, Europe

 

Niklas,

Please do not start your car until the correct rounded belt is installed. You are looking at major motor damage if the belt slips!
Spend a few more $$ and get the Porsche belt.

 

Niklas,
The belt you got installed is the old model - Porsche changed the belt shape in (I think)1984 from the squared to the round.
As John said change the belt ASAP

 

Yep, as John & Eric pointed out, you installed the old-style square toothed belt.

Replace it with the correct newer round tooth belt type.

If when you ordered the belt you specified the model year, then the parts supplier may be liable for some of the replacement cost, including labour.

 

I posted on a similar problem some time ago, my local Porsche dealer put the old square tooth design on my 82 auto, in spite of the fact that the PO had updated all the gears with round tooth HTD gears. The dealer is going to replace the Tbelt with a round tooth belt at their cost. Strong concensus from the board was 1-mismatch is a very bad idea, and 2-the dealer should eat the replacement costs, which they've now offered to do. BTW, the mismatched belt was showing trailing edge tooth wear at 15K miles, although not sure if this was a function of the mismatch or not.
Good luck.
RAS

 

Not to mention Russell, that using the wrong type belt will wear down the gears as well!

 

From the Manuals.

<img src="http://home.attbi.com/~87928s4/cambeltteeth.jpg" alt=" - " />

 

Thanks for the replies, everyone. Looks like I have to do the job again. Damn it! <img border="0" alt="[crying]" title="" src="graemlins/crying.gif" />

The dealer I bought the belt from should be liable for the extra effort this is going to cause. I'll give them a call and ask about it.

I need to do some other work in that area in the near future anyway, so it's not that bad.

How quickly should the cam pulleys wear? I can see that there is noticeable wear on my cam pulleys. The car has 110 kmiles on it. The crank pulley and oil pump pulley did not show too much signs of wearing, though. What can cause this?

Best regards
Niklas Kampe
1987 S4 5-speed manual
Finland, Europe

 

The sprocket gears are aluminum, which is susceptible to the erosion you would expect at high speeds/temperatures between rubber and metal. The gear and belt design work together: It stands to reason that wear would be accelerated by incompatible shapes rubbing against one another, even with the proper cambelt tension. The concern you need to have, as noted above, is the extremely costly damage you WILL experience if the belt slips - and the astronomically costly penalty you will pay if the belt actually breaks (I know - I have had it happen - just from a timing belt tensioner roller that seized without warning, snapping the belt): All S-4's are interference motors, meaning that the valve and piston synchronization is precise to the extent, that if it changes, valves will be impacted by pistons, and $1000's of dollars in replacement parts and labor will result - more damage with a break than a slip of the belt...
This may be useful in your argument that the co. selling you that belt may find it very well worth their additional investment in supplying both the proper belt, and whatever you may estimate your time cost you in replacing it - but, they alreay know that - they just think that you may not....