Is it possible to retrofit an OM606 head onto the 603 block?

I am looking for a W124, I'd love to get either the 603 or 602, but everything I have found is pretty dang ragged. I've resigned myself to I'll take a car with bad paint and acceptable interior with a good working mechanicals, or vice versa a poor mechanical car with good paint and interior. Well, everything I've found is poor all around. Except for the 94/95 E300Ds I've been able to find. I don't like the prospects of a normally aspirated diesel, but the cars are much cleaner and in good running condition.

This leads me to my next step. I want turbo diesel, and I want it to be mechanically injected. Searching yielded nothing, seems like nobody has asked this question. Looking at the heads/blocks they seem to have the same bolt pattern, but obviously this isn't something that can be discerned from a photo.

Eventually I want to stuff a turbo OM606 into the W124 with a manual transmission. When I do this, I'll have the NA 606 to play around with. Since getting a 603 block is relatively inexpensive I started wondering, can the 606 head just bolt on? Certainly I will need a new timing chain to accommodate DOHC. But will the head and head gasket bolt right up to the block?

Not without custom pistons. The combustion chamber is matched to the prechamber projection in the head. 603's are in a different spot than the 606. There are likely other differences preventing a simple swap, but that's the biggest showstopper.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

This is a well walked path, go with the 95 E300D and then swap in a turbo 606 engine. You only need to swap the IP. It's about as easy and straightforward as an engine swap can be. You get the benefit of the newer '95 body with r134a A/C and all the other minor updates the w124 received over the years. Replacing just the cylinder head is actually more work than swapping the entire engine.

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Current fleet:
1998 E300 turbodiesel
2008 E320 bluetec - leaky oil cooler seals replaced
2012 S350 bluetec
2014 GL350 bluetec

Sold:
1996 Passat TDI
1997 E300 diesel

There I go being dumb... I didn't think about the pistons. Yes, the engine swap should be easy as dirt. I've done plenty of R&R of engines and that's about what swapping a turbo om606 would be.

I was asking because I was thinking about doing an experiment with the spare engine after swapping. To be honest though, I just might use it to get other cars smog exempted in California 🤣. Considering they are about as easy as it comes for swapping an engine is probably a good candidate.

Its well documented the NA engine can handle 300hp regularly, even towing in pickup installations, with just a pump and HY35 turbo.

That's true, however the turbo 606 engine handles 600 hp, so clearly the internals are a substantial upgrade over the NA parts. Just a data point to help with the decision making.

__________________
Current fleet:
1998 E300 turbodiesel
2008 E320 bluetec - leaky oil cooler seals replaced
2012 S350 bluetec
2014 GL350 bluetec

Sold:
1996 Passat TDI
1997 E300 diesel

500+ engines are not stock, or don't survive long when they are. Forged rods are minimum in 500hp+ builds and run in excess of 6000rpm.

Correct, the high rpms introduce new requirements. Even a 300 hp build, if you want reliable 6000+ rpm operation, you'll want forged rods, ARP bolts, etc.

__________________
Current fleet:
1998 E300 turbodiesel
2008 E320 bluetec - leaky oil cooler seals replaced
2012 S350 bluetec
2014 GL350 bluetec

Sold:
1996 Passat TDI
1997 E300 diesel