Timing Chain Replacement Risks
 

My 300 and 420 has 200,000 miles a piece with original timing chain on each. Both run like a charm, no noise. (Except the 300 needs power steering fluid).

What is the risk of changing the timing chains when they simply give rather than changing them now when everything now is fine?

I know with some 4 cylinder cars you can bend the valves because some pistons can hits the valves (my understanding) when the timing belt breaks, but if you have clearance on the 6 cyl and 8 cyl I don't see the benefit of pre changing your timing chain before any symptoms?

What are some symptoms of when you chain is giving? I heard that the Tach will flucuate at idle. So far the needles are stready.

I believe the valve on both engines are interference fit with the piston crowns, so you may suffer damage if the chain breaks. The M103 has a single roller chain and rarely breaks. The biggest issue with the 8 cyl is the chain rails breaking. The tach will tell you nothing, other than the average rpm for the engine

I would replace the chain and upper rails every 100k to 150k on an M117 V8 because the chain is so long. It will make a, "racka-ta-racka-ta" noise at startup. Also, the guides get brittle. If I am not mistaken, the M117 IS an interference engine and the valves WILL get bent on chain failure. I got mine changed for $600--cheap insurance! Hope this helps, Andrew Seidel.

bent valves
 

Oh YES!
the Valves WILL get bent on the V-8 engines!
I would change chain,tensioners,and plastic "rails",unless you have records indicating this has been done recently.

DO IT NOW!!! The results of broken rails can be massive. Look up "420sel timing chain" in SEARCH and get ready for lots of reading. WITH THIS ENGINE - PREVENTATIVE MAINTENANCE IS THE WAY TO GO.

If the chain guides break, the timing chain will likely break. If either break you can expect to have bent valves on the left head, plus be prepared to see up to 16 broken rocker arms on the left head too.

Long story short - get the chain and upper guides changed ASAP.

Maybe you should buy a couple lottery tickets. You've been lucky.

I take it then when this tensioners and guides break there are not many symptoms when they start to go. You just have to assume that they are on their way out.


So with the 420, you recommend to keep the chain, but get new guide rails?

mattsuzie - change the guides, tensioner and chain.
There won't be any symptoms that you will be able to detect - one day you will be driving, next day, after it happens, you will be crying :).

I forgot the tensioner - Thanks Jim.

As stated, your only symptom will be the clatter and racket when any of the components let go. Don't reuse the chain. It's 198 links, that's a looong chain, lots of turns and changing directions!!!

Good God!!!:eek: Don't even think about NOT changing the chain. It is cheap insurance and the chain will most likely eventually give way. You will not get a warning. You will be cruising along one moment and then it will be like the engine just turned off and you will coast a longggg ways. How do I know? It happened to me.

Change the guides and rails. They are cheap parts. This would be the minimum.

I don't know that you've necessarily been 'lucky' mattsuzie. MB doesn't list a change interval for the timing chain, it's deisgned to be very durable with (surprise!) regular maintenance (regular oil changes).

I've always understood (hearing from two well-experienced Indies) that only if the chain stretch is over 5 degrees should one replace the guides and tensioner - if the chain isn't worn, don't change it!

Now personally at 200K, I'd feel pretty comfortable chaning it all out, but I hear a lot of people here changing the whole shebang at 100K (or a little over) for 'peace of mind'. Unecessary imho, but then we all have different comfort levels of preventative maintenance...

Regards,
- Ryan

I have a similar concern with a 1988 300TE I just purchased. It has 190k on the clock and it is as quiet as my new VW. I have scattered service records and one states that the waterpump was replaced. Shouldnt a timing chain be done in conjunction with a waterpump. I know on some Audis that is the case.