There are some maintenance tips to be found in the buyers guide for the 560SEC that I co-wrote back in '98, and this specifically is mentioned in there:

www.mbcoupes.com

It's not a bad idea to have the timing chain, tensioner and guide rails done, at 100,000 miles, as preventative maintenance, and maybe the cam oiler tubes and injectors if they need it.

The guid rails are white plastic. If you take off a valve cover, and look and you see they are root beer brown, probably they are past their "sell by" date, and should be replaced, they get hairline cracks, and will crumble soon, and you know the rest. On tyhe 560 they let go more often on the left side.

You can gamble and wait, but without records, on your car, proving this has been done, yours is probably way past due, and if you hear timing chain rattle on start up, you could be seconds away from a major catastrophe in that scenario.

I have heard the 420 is less critical than the 560 with regard to this, but in any case I wouldn't wait on this one.

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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...

1995 E 420, 170k "The Red Plum" (sold)

2015 BMW 535i xdrive awd Stage 1 DINAN, 6k, <----364 hp

1967 Mercury Cougar, 49k

2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive (sold)