Here is a link to my chain guide replacement thread.
http://www.peachparts.com/shopforum/showthread.php?t=185820
Contrary to popular opinion, the chain does not break. It wears causing it to elongate. This causes the chain to flop around at the inner guide on the drivers side. The guide breaks off and gets drawn by the chain into the space between the sprocket and the chain. The sprocket slips a tooth or two and the valves on that side contact the piston and bend. The engine stops and needs a top end rebuild. The root cause is the long chain. My Ford 4.6 has two short chains so this is not a problem although I do need to do chains and guides on that engine too.
http://www.import-car.com/ic/ic40024.htm
Contrary to some other popular opinions, the M103 I6 is a very stout engine and smooth as silk at highway speeds. It does rev high though esp. in the 300SE due to a large car with a smallish engine. The engine is really quite good above 60 mph and the acceleration to 90 is very good. I cannot opine on the M104. Nercedes has lots of experience making I6 engines. The car does need a 1st gear start though, something I'm working on for both my cars.
The chain on the M103 is short and doesn't have the wear problems of the V8's. Take it from me, I have both engines in the W126 cars.