PeachParts Mercedes-Benz Forum - View Single Post - Who has the most miles on their original timing chain?

phidauex · #13 12-22-2005, 05:28 PM

Quick chain concept filler:

Chains do not 'stretch'. We use the term 'stretch' because they get longer, but its not because the links are actually physically elongating.

A chain is formed of links, flat pieces of metal with holes in them, and pins, which slide through these holes. There is usually a bushing of some sort in between, but not always (the link is often doing double duty as a bushing). The pin is constantly rotating within the hole on the link, and with time, it begins to wear the hole out, and the hole gets larger. But it doesn't just get overall larger, it becomes oval shaped, since the pin is always putting pressure on one part of the link (the driving edge). As this happens, the pins get farther apart, and the chain gets longer.

Now, assuming that this stretch, or elongation, doesn't cause the chain to straight up break (which it can do), then it causes two problems:

It changes your timing, because more chain is now taken up by the tensioner. This is OK, because you can adjust the timing to compensate.

The worst problem, really, is that the increased pitch of the chain (the distance between the centers of the pins) is increasing, which wears out the sprockets.

Imagine a brand new chain and sprocket. As the sprocket turns, each pin falls right in between two teeth. But as the chain ages and elongates, the pins fall farther and farther forward, and begin to eat away at the tooth in front of it. This creates a 'shark tooth' appearance on the teeth. The result is that the sprocket's pitch will increase (the effective distance between teeth) as well.

So far, this is OK. You've got a longer-pitch chain, and a longer pitch sprocket, but they are matched, and you can adjust out the timing difference.

But here is the problem! When you replace the chain, you now have a chain with a shorter pitch than the sprocket, and they are mismatched! Now the chain will wear out very quickly, because the sprocket will force additional wear on it. Additionally, the fresh chain will scrape away the other edge of the teeth, making them smaller and smaller. Soon, one of a few things will happen, you'll break a tooth, or you'll break a chain.

Here is a lesson from the cycling world (and motorcycling world too).

In cycling, we use a 1/2" pitch chain and sprockets. You always replace a chain when the length of 12" worth of links has increased by 1/16" of an inch. The chain isn't in danger of breaking at that level of elongation, but it means that you haven't started destroying your sprockets yet. If you wait until the chain elongates to 1/8" or more over 12", then you'll need to replace your sprockets AND your chain at the same time, which is more difficult, and more expensive. 1/16" of wear over 12 inches corresponds to about .5% elongation. 4 degrees of elongation in a timing chain corresponds to about 1.1% elongation. And finally, let me tell you, I'd rather replace 100 bicycle cassette sprockets, than have to replace one main crank sprocket on a 617.

So anyway, I know that is really long, but its important information to know, when talking about chain/sprocket systems. You replace the chain sooner than you might think, in order to prevent damaging the sprockets. Timing chains are cheap compared to the sprockets they ride on.

peace,
sam