Chain and rails
Those seem to be the major maintenance items - a loose chain smacks the rails on start up, they break, chain jumps a tooth or two and you need a new top end.
Replace the upper chain and rails and put in a new tensioner while you are in there. BTW, I believe this is an M117 motor (420, 560), not M116 (380, 500).
I did these on our 380 (M116 - rails and chain) and 560 (M117 - only did the rails as the chain was OK) and illustrate both on my site.
The tranny, as has been wisely pointed out, is the other major area of concern - you are looking at $1500 for a rebuild/overhaul. Not cheap, but may be worth it if the car is priced right and has excellent paint and upholstery - these are very expensive to fix - and is rust free.
Steve Brotherton once mentioned that he has a customer whose 420SEL has over 500k on it, and the customer has finally learned to have the chain and rails changed every 100k after two top end jobs.... Steve has also stated many times that the bottom ends on these engines are pretty much bullet proof. No reason why you shouldn't get this car up to 500k with proper maintenance. Mr. Brotherton has seen more of these engines than most of us have seen hot dinners.
BTW, no nitrogen cells on the 420SEL rear - it uses regular Bilstein shocks. The 560SEL has the cells. Also, I suspect many of the valve cover leaks are caused by overtigtening the bolts (these need very little torque), reusing old gaskets and not using new crush washers every time. Check your valve cover on a really flat surface (glass) and see if its warped.
All M117 engines have the double row chain - pre-1985 M116 380 engines have the dreaded single row. The 500 always had double row.
Last edited by Thomaspin; 07-09-2003 at 11:23 AM.