On the MB? Complete engine swap was the most difficult so far. Not all that hard, just time consuming (marking all the lines, swapping things between the motors to make the new one work, etc.).

Most difficult of all jobs I've done? In recent years, probably converting my 86 F250 from a T-19 4-speed to a ZF 5-speed. Did it without removing the engine - or even the turbo. Had to jack the cab up 3"-4".

I'm in the middle of a gasoline to diesel engine conversion in an 85 Jeep Cherokee (not an OM617 though). That may end up topping the F250 tranny on the difficulty scale - we'll see...

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown