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Old 06-24-2003, 01:56 PM
suginami suginami is offline
Join Date: Dec 2001
Location: Southern California, U.S.A.
Posts: 8,503
Re: Chain guide/rail(?) replacement on 87 420SEL

Originally posted by drumrman2002
Hi everyone. This is my first visit to your site. I'm hoping to get some insight on whether this repair can be tackled myself. I own a 1987 420SEL with approx. 142K on it. While I was having my car serviced at the local Benz dealer, the shop manager suggested that the chain guides/rails should be replaced. He said that on high mileage engines such as the 420SEL and 560,these guides become worn and can cause the timing chain to jump,causing a major mechanical disaster. They quoted the replacement at around $1,000. Since then,I've put quite afew miles on the car without any trouble and she still runs well. I'd like to do this repair myself..just not too sure how involved it will get. Any suggestions would be greatly appreciated. Thanks!
Your dealer is right. You are running on borrowed time.

These engines are very notorious for timing chain stretch. When the chain stretches, it bangs and slaps against the plastic chain guides / rails. The plastic is white when new, but becomes very dark brown in color when it gets old. The plastic also gets very brittle.

Soooo, on start up, the loose chain slaps against the plastic, it breaks in little pieces, and gets caught between the chain and cam sprocket, and BAM! the chain binds up, and you have engine damage: bent valves, broken camshaft, etc.

This board is full of examples of this happening.

The chains can get loose enough to skip a tooth on the sprocket, but the broken chain guides / rails is more common.

Thomaspin performed this repair on his car at his website:
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".
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