Inherent design flaw?

[fz500sel]

I'm a little confused about the tensioner rail for the timing chain in my '84 500SEL. My mechanic said that it is plastic and that it becomes brittle with age. Mine broke and thru the timing out and consequently bent all the valves on one side of my engine. Why would MB make something so cheap in a position that is so crucial to the engine that when it goes it could cause such a major problem? If so, have they changed it so that it isn't so fragile?

Thanks,

[mbdoc]

Most all MB & BMW motors use plastic timing chain guide rails. Do a search & see how many posts there are on these subjects.

[cth350]

There are no shortage of 2$ items that when they go, they go big time. In this case, the rail didn't fail, it was killed by the stretched timing chain (didn't you pay your mechanic a lot of money to check it often?).

This is one big difference between a benz and a chevy. The mercedes engineers of yore that designed your engine assumed that mechanics are "properly" trained and that owners do what they're told to in the owner's manual. Chevy engineers have assumed that the car will have an uneducated owner and/or mechanic inflict their will upon the car at least once, if not always. Something akin to my mother driving and my dad doing the tuneups.

-CTH

[Benzmac]

My $.02.

I have customers with 150k miles plus with no problems with the timing chain or the rails. The REASON for this is they always do their service. I mean EVERY oil change has been done and the engine was never over heated.

If a customer ever over heats the engine or misses oil changes, the rails will become brittle and the chain and internal parts will be worn.

Benzmac,

OUTSTANDING reply.

Larry

[Larry Delor]

That's the trouble with buying someone else's used stuff...Like Forrest Gumps Mama used to say: Life is like a box of chocolates, you never know what you're gonna get. Sometimes you can look at the outside, and have a good idea what's inside, but you never know for sure until you open it up.
My Mama always said Life is like a roll of toilet paper, the closer you get to the end, the faster it goes.

-Larry

[Jim H]

Sorry to hear of your malfunction.

My experience is that plastic isn't always cheap OR fragile. Plastic/nylon compounds work extremely well under design conditions. As pointed out, outside design conditions they can become brittle, etc.

Would metal be a WORSE material for a tensioner rail? My thinking is that it would introduce small metal particles in the oil during the life of the rail, and being as hard as the chain, would shorten its life by wearing it down.

The above wondering comes from observing a plastic/nylon, not metal, guide block on the swingarm of my motorcycle where the chain can rub at certain angles. This block will outlast several chains...

Maybe I've got too much free time on my hands, and had to put in this $0.02. Nah, I'm just addicted to this place.

BCingU, Jim