The advantages of pushrod engines: CHEAP

There are no other advantages to speak of, since regular maintenance will eliminate timing changes due to chain stretch.

Overhead cams eliminate most of the slop in the valve train, reducing valve float and permiting higher rpm. They also permit steeper cam lobes, and an opening/closing valve train (where the cam actively closes the valve) is much simpler.

While higher rpm isn't necessarily a problem with diesels, Renault makes a double overhead cam diesel that runs about 6000 rpm (they also make a gas racing engine that turns 12,000, or did!).

Dual overhead cams also allow a better hemispherical combustion chamber for gas engines, and better injector placement for diesels.

Incidentally, pushrod motors are more difficult to machine properly -- those lifter bores are a critical as the piston bores -- get them out of alignment and you have different valve timing on each cylinder!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!