420 SEL, no compression on left bank
 

A friend has this car and suspects the timing chain is broken. He wants to know if there are any special tools / procedures to replace the chain or camshaft. I think it's an 86 model year.

Only if the chain has fallen down into the chain case!

If both ends are still accessable, pull them up with some wire, remove the rocker arms (keep them in the same location going back in or the cam will die VERY quickly!), then roll a new chain through, reset the timing correctly, and re-install the rockers.

You can check the condition of valves by removing the valve cover -- bent valves will have loose rocker arms when closed.

If you have cranked the engine post "disaster", check the chain case on the lower left -- the chain tends to run down there a bunch up, and attempting to start the engine will drive it through the chain cover, requiring a new cover. If this is the case, it's best to remove the heads, it's VERY difficult to get the front cover back on without leaks. With no compression on the left side, I'd be surprised if you don't need a valve job anyway (check for a broken left camshaft, too -- if so, definitely a valve job).

Peter

No comp. on left bank only? Right bank has comp?

The chain guide rail on the inside of the left bank has broken, gotten wedged between the cam gear and chain, jumping time. This invaribly bends several valves requiring a valve job to repair. The chain hasn't broken b/c the right cam is still in time allowing compression on the right bank. A ball end 8 mm allen socket is real damn helpful gettting the headbolts out. Gonna need 6mm allen hardware on several bolts along the way as well. A guide rail pin puller is real nice but not absolutely necessary. Other than that it's a lot of nuts and bolts. About 25-30 hrs worth of labor to have repaired at a shop.

Wow!! Dead on diagnosis!! It's exactly as you said!! All left bank exhaust valves appear to be bent, one rocker is broken and the cam is scared badly, actually chipped on one exhaust lobe.

So there are two timing chains on this engine? He hasn't pulled the right valve cover yet.

No, just one timing chain. You just had an unforgettable experience with arguably the most common catastrophic fault associated with Mercedes (God love 'em for the plastic chain rails). It's a preventitive maintenance procedure in my shop, and is a considerable weakness in an otherwise decent engine design (it is still a common fault in later-model engines as well).

As a coworker of mine found out, Toyota 22RE engines are worse -- they use cheapo plastic rails that break off and the timing chain then proceeds to chew through the front cover into the water pump passage.

First sign of trouble is typical chain knock at low speeds, but this gets ignored as it never gets worse and the engine runs fine (dizzy is driven off the cam). Next thing you know, there is a whole lot of coolant in the oil and the engine is locked up.

Single row chain, plastic guides, just liek the 380. Cure is the same, too -- get a chain, sprokets, and cover from a 22R engine, that one is the "old" two row chain, runs forever on steel guides with rubber faces.

Sometimes changes aren't improvements, just changes!

Peter