Plastic timing chain guides
 

I just bought a 1991 Mercedes 420 sel with 82K miles. I understand a weak point may be the plastic rails that the timing chain rides on. I have been told they crack and break causing the pistons to smack the valves. Does anyone have an experience with this?.. and when should the rails be replaced?..Thanks in advance Ted.

Yes,

The V8 mercedes engines need rail replacements. Use the search feature on this site to get some information.
You should also avoid the aftermarket parts on something this critical. Spend the extra $20. Ussually you do chain/guides at the same time. I think you should pull the valve covers and do an inspection. If you have any chain slap on cold starting, that would be a sign to get it to the shop ASAP.

Michael

The plastic parts do get old and brittle and are prone to breakage. Seems that 100K miles and/or 10 years is the replacement guideline. As long as you are doing the guides you probably should replace the chain too. There are not too many reports of the chains actually breaking on their own. Usually the problem is the plastic parts break and lodge in the chain causing the chain to jump time and cause major engine damage. But since the chain will be easy to replace at the time you are doing the guildes you might as well do the chain too....

for peace of mind, replace them now (top 3 guides). @ 80K, the chain should be ok but I would also replace the chain tensioner.