Ref: '91 W126 350SDL
When is it necessary to replace the timing chain Slide/Guide Rail? Mine is worn/grooved to about 3mm or 1/8". The head will have to come off to replace it.

 

Thanks much for the reply and the cautionary note on the 350SDL. So far so good--260K miles and low oil consumption between changes with synthetic oil between 3K and 4K miles. I've owned the car since 72K miles and had the head off once at 150K miles in fear of hydraulic lock when I saw excessive white smoke.