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?