OM603 3.5 questions
 

I've found a good running OM603 out of a 93 300SD. The car has 288K miles, no tag on the block to indicate it is a replacement engine/block, #18 head. Car drove like crap (bad vibrations about 30mph, probably driveshaft related) and the turbo didn't spool thanks to most of the vacuum lines being shot (later 602s and 603s used a ECU and a vacuum system for boost control) but even without the turbo, I could feel this engine has some grunt down low compared to an OM602.

How would I tell if it is still a 3.5L and not a 3.0L? I know some of the 3.5s didn't seem to have the rod bending problems. I plan to swap it into my 1990 300D 2.5 that needs some significant cylinder head work.

Is there a difference in size between the turbo on a 3.0L and the 3.5L? I've got an 87 OM603 with the #14 head that seems to be fine but could pull the turbo off of that if needed because it doesn't have external boost control.

Is there any reason the 722.3 out of the W140 wouldn't work in a W124? I know it uses a different mount, but don't know how that's accomplished. It shifted great and I expect it would hold up better against the OM603 than the 722.4 that's already in the W124.

Transmission lengths are different between the 722.4 and 722.3 so you will need a driveshaft from a 87 300d or similar. Your current diff yoke will need to be replaced with one that will accept the larger flex disk. 722.3 can handle the torque the 722.4 would be marginal. Transmission linkages and wiring harness will also need changing to the 722.3 style. Under the hood go with the larger oil cooler and radiator. Exhaust diameters are different and plumbing changes also.

The 3.0L pretty well ended production in the US in 1987 until the introduction of the 3.0L OM606 in 1995. If it's a 603, it'll be a 603.97x which is 3.4L. There is a very noticeable improvement in performance and torque compared to the 3.0L and probably night and day compared to a 602. Convert the turbo to a pressure operated wastegate or find a turbo from a W126 350SD/L. The 3.4L turbo is different than the 3.0L, different geometry. Keep the W140 style exhaust manifold, it doesn't have that stupid bypass pipe on it like the W126 did.

The 3.0 603 engines ended in 87, IIRC. The 606 came along in the early-mid 90s.

For the 3.0 vs 3.5L earlier 603 engines, the turbo is different.

It seems that not all 3.5L engines had issues, and if they did, the failure come in the low 100k miles.

I bought one with just about 100k miles, with the intent to do the head. There are theories out there about what caused bending, I think mine was on the cusp of a HG leak, fortunately it was in a different direction, not into the cylinder.