S4 T-belt failure & cam sprocket wear

My thinkings on the subject:

At our recent Sharktoberfest in sunny So Cal, one of the tech sessions focused on timing belt issues. And one of the issues was expected life of the belt. Presentation was by the owners of Precision Motorworks in Anaheim, by the way.

They talked about belt life expectancy this way: You get sixty thousand miles on a new belt in an an engine with all new parts touching the belt.
That would be as it is delivered from the factory. As the belt stretches, it wears the cam gears and the other sprockets along the way. You decide to
replace the belt on schedule, and see that the anodizing is worn in a few spots on the faces of the teeth. Mmmmm, not too bad, and those gears are
expen$ive, so I'll replace a roller or two, maybe the water pump, and of course the new belt. So now I'm good for another how many miles? The worn faces on the cam sprockets rub on the faces of the teeth on the new belt. Face it, if the wear was nice and even on the whole gear, it would be even
on the belt. But the wear is never that even on the gears, and the reason is that the old belt didn't pull evenly on all the faces of the cam sprockets. Install a new belt, and those worn spots won't initially pull on the belt, at least not until either the belt or the sprocket wears or stretches enough to make that contact pressure even again. Then you are back where you started before the belt replacement-- with a worn belt!

As Marc Thomas points out, the life expectancy of the cam sprocket drops off quickly once the hard anodized surface is breached. After that, belt and
sprocket wear are both accelerated, and the life expectancy of the whole thing is diminished.

Consider the common failure mode of the belt. Seems tome that they break because there's excess tension put on them. What causes that? A lazy mechanic or owner who cranks an extr half-turn on the tensioner to keep the light out? Once in a while that happens. Consider that bits of the belt
and the worn flakes of aluminum both build up in the recesses of the various sprockets, making the belt ride up slightly higher in the cogs. That puts
additional strain on the belt, as it's driving on less of the face of the gear. The belt and the gear wear faster with that smaller contact area, more rubber from the belt at the edges means the tooth isn't square any more, more debris to pack the recesses in the belt and gears... and soon you have a drive path that's longer than the belt. Belt jumps a tooth, just kinda walks over one, or maybe two or three teeth. Maybe a tooth breaks off on the belt. More common is that the added diameter of the belt path puts more strain on the other rolling components, and the belt starts to track off-center. Starting to see a pattern here?

So maybe some guidelines are needed:

1) If you have worn through the anodizing at any place on a cam sprocket, you need to replace them. Period. If you are replacing the cam sprockets,
replace the other worn sprockets at the same time. A stretched/worn belt wears everything it comes in contact with.

2) If you have visibly worn the anodizing on the cam sprockets, you might consider reusing them, but you get to derate the new belt life based on the
amount of wear. In this group we often discuss 45k as a good second-belt lifespan. So 25% of the belt life goes away when you install on used sprockets? Someone mentioned $75 per cam sprocket, so add $150 to your parts cost to gain 15K (33%) more belt life and comfort. Even my pea-sized brain sees that as a good investment.

3) Rebuild the rollers or replace them. Period.

4) Rebuild or replace the tensioner. Period.

4) Check the belt tension regularly. Period. Record your readings! The belt will stretch slowly during it's life, with almost all of that in the first several thousand miles after installation. If you see the belt stretching much after the first 10k, it's a good idea to find out why.
Similarly, if you see the belt getting tighter, the only thing that can cause that is accumulated stuff in the cogs.

5) Keep the timing belt area of the engine clean and covered. It's a bit of a chore, say fifteen minutes, to pull the top belt cover and peek inside to see how things are wearing. Got any rubber debris flying around in there? Coolant or oil in there? Belt tracking off center? Back of the belt really getting polished by the tensioner? Might be time to set up temporary camp, get the parts shipped in, and get your hands dirty again. Hey, you're already part of the way there with the covers off, right?

6) Offer your used belt and cam sprockets on E-bay. Use the Kuhleast928 seller name so folks will know exactly what they are getting.

I'm really looking forward to putting all this stuff to the test in a few months when I get to haul out the hammers and pipe wrenches for a TB swap on
my own car. Video camera will be running the whole time. In the meanwhile I'm socking away milk money to cover a set of cam sprockets. Just in case!

dr bob
'89 S4 Auto, 58K

Last t-belt was at 22K in 1997, dealer in Denver when I bought the car. I'm definitely due now!