I bought a 1991 Mercedes 420 sel with 82k miles. I understand a weak point in the motor may be the plastic guides that the timing chain rides on. They can crack or break. Does anyone have an experience with this .?..does Mercedes recommend changing the rails at a certain mileage point.

 

I have plenty experience with these chain rails. A few years ago one of the rails broke in my 380SLC causing the timing chain to jump and the valves were bent. This is a problem in all Mercedes V8's. I have heard every 60,000 miles the chain, rails, and tensioner should be replaced. If you ever hear a loud knocking sound the first few seconds of startup, the chain is loose and banging against the rails, eventually they will break. If you have heard that banging you should have the engine serviced immediately.

Good Luck
Nick