Timing chain - 420SEL vs. 560SEL

Conventional wisdom holds that timing chain breakage is much more common on the (W126) 420SEL than the (W126) 560SEL. I'm very curious about this as I assumed that both engines belonged to the same family and were identical but for differences in bore and/or stroke. If I'm right, why are timing chains breaking more often on the 420? If I'm wrong:

1. Does the 560 have different and more substantial timing-related parts?

2. Are these parts interchangeable on the 420?

3. Is it simply a case that since many more 420's were sold in the U.S. than 560's, the incidence of timing chain failure is statistically identical in both models, but the raw numbers work against the 420?

[mbdoc]

As the M117 engine in the 560SEL a "wider" engine block than the M116 engine in the 420SEL, the chain is longer. Exact same chain BUT different number of links.
The M117 engine does use LONGER chain rails with different part numbers.
Usually chain failure is a result of TENSIONER failure and then the rails(plastic) break & jam themselves between the chain & gears, that breaks the chain. THE chain should last 200,000 miles. IT IS the rails & tensioners that really cause the problems!!!

As you have supposed only one(1) 560(SEL, SL, & SEC) engine for every five(5) 420's.

[Richard Seaman]

MB.Doc,

Does the problem with the plastic guides in the 420 engine also exist in the 1993 400SEL W140 engine??