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.
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