MB bottom ends last forever for three reasons: High nickel cast iron
cylinder liners -- harder, more wear resistant; Chrome-molly rings
without sharp edges, unlike many of the standard american style
compression rings with a tapered face; and very close tolerance piston
fit.
The combination of the above, along with the generaous abount of babbit
material on the bearings, pretty much eliminataes wear so long as the oil
is good and the engine isn't overheated. The hard cylinder liner is MUCH
harder than material that can successfully be cast as an entire block,
and it can be hardened further after casting, unlike the whole block.
The crhome moly rings have micro-cracks in the face, so more oil is
carried between the ring face and the cylinder wall without going in the
the combustion area, and the very tight piston fit greatly reduces blowby
and associated acidic compounds.
This is what you pay for when you buy a Benz, or at least used to. Techs
in California, where people actually put 500,000 miles on cars, say that
the lower end in an M117 should alst 500,000 miles between overhauls with
no problem. Upper end usually goes 250,000.
Peter Frederick