Another broken timing chain!
The two MB engines with the most "timing chain problems" are the 3.8 in the early 80s 107s and 126s and the 4.2 in the 420SEL (i.e., 126 chassis) from 1986-on.
The early 3.8s had a single row chain in the *US* only, I believe and, by 1984 (I think this was the year), the factory went back to the double chain for US models because of breakage of the single row chains. Important note: if you are considering a 107 (SL) purchase (now or in the future), please check to make sure that the potential buy has a double chain. If it doesn't the cost to replace is a cool $3000 or so on this model, so make sure that this is factored in to the purchase price. If I owned a single row chain 3.8-equipped MB, I would *park* it until it was converted to a double-row chain.
In contrast, the SELs are, relatively speaking, much easier to replace. Also note that, even though the 103 engine (pre-93? 300Es, and 88-89 300SELS) has a single chain, it is *not* a problem. Again, the 103 engine's single chain is not a problem. It is an inline six and has a shorter chain.
On the 420s, the "timing chain problem" is actually a timing "rail" problem -- specifically, on the driver's side. If you find a 420SEL with jumped time, 99.999% of the time, it will be the driver's side. The problem is a timing chain rail that gets brittle over time, breaks, interferes with the chain b/t it and the cam gear, and the chain jumps time and bends valves. I have a friend on the left coast who has been in the business for years as a service manager and a technician and his "fix" is to cut (yes, cut) the offending rail's end that breaks when he puts in a new rail.
I have not owned a 420SEL, but probably will buy one when the right deal comes along (read: jumped time, owner doesn't want to reapir, will sell cheap). Almost bought one this past winter, but the owner decided to rebuild the engine.
If you are a reader of this site, and are looking for a luxurious, yet *reasonably* inexpensive MB *and* you are an accomplished DIYer, I would highly recommend the 420SEL as a potential bargain -- there are plenty of them out there -- now and there will be in the future - with jumped time.
Personally, if I owned (or bought) a 420SEL, the *first* thing that I would do -- regardless of the miles on the car or engine -- is replace the offending timing chain rail with a fresh one that has a fresh "cut" in it . . .