Chain guide/rail(?) replacement on 87 420SEL
 

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!

Buy a lotto ticket.

Then do a search on chain guide and chain replacement. Easy job for a good DIYer, should be no problem to do over a weekend. If not a good DIYer, then the job is best left to a shop, preferably an independant as the cost will be less. Once the guides become brittle they often break, usually on start up, but can let go at speed. Once that happens expect the chain to jump and the result will be a destroyed left head (valves and cam for sure).

The topic has been covered several time with pictures and how tos. Memeber Thomaspin has the procedure well covered on his web site.

Be sure to do, or have done, the chain tensioner at the same time.

I just had this procedure done on my 88 420SEL. I was told that the chain was still good, and that the rails still looked ok too. The other thing that I had done, was to have the valve stem seals replaced, as I was using so much oil, that it hardly had a chance to get dirty. Seems my engine isn't the only one to need new seals, so you might want to have this done too. I was not so lucky when the cam was inspected...it was groovy, and not in a good way. I ended up needing a new cam and rockers, as well as lifters. Ended up spending twice the amount I thought I would. Oh... and somehow, one of the pins that holds in one of the rails broke off, and had to be extracted - a process that was rather time consuming.

In essence: Replace the chain and rails, and do the seals too while you have the thing apart - it will save $ on labor later.

Good luck,

-Larry

Re: Chain guide/rail(?) replacement on 87 420SEL
 

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:

www.pindelski.com

This expensive failure is detailed with pictures at: http://www.import-car.com/ic/ic40024.htm These could have been pictures of my engine except one of my valves broke and went through a piston.