Hi, I have a '71 W108 280se that's got a diesel OM615 engine in it, with 4 speed manual (originally auto) gearbox. The conversion has been done nicely (started by the PO, finished by me) with custom mounts, exhaust, auto timed glow system with pencil plugs and I've even wired the glow light to the low fuel bulb to keep everything looking stock! I attach photos.

Its been a reliable daily driver for 5 years now, running on 90-100% WVO single tank all of the time. (has to be done in the UK where diesel is now £5 or around $8.10/gallon!)

The trouble is its sloooooooow to the point of being dangerous in some situations.
Now I know I could put a N/A 617 in it, but here in the UK they are getting hard to find in good condition (probably why the PO put a 615 in it!), and the power improvement isn't what I'm looking for. The turbo 617 is near impossible to find here - I know, I've been looking for over a year and haven't seen a single good one!

Does anyone have experience of putting one of the later 60* series diesels in a W108 or even a 123? They are much easier to get hold of and generally in better shape.
I'm guessing I'll have to change the gearbox at the same time but I want to stay with manual (stick shift). Any idea of which engine would be best? It must be a prechamber engine without electronics.

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you could try this, search around the yards there, and find some 617 flywheels, and manual 5 speed transmissions. get them together, and crate them up. we will find you some nice 617 turbo motors, and do a nice swap with you.

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Nice idea. Turbo 617's are fairly common on this side of the pond, but 617 flywheels and 5 speed manual transmissions are quite rare. Why they never offered a factory manual with the 617 turbo is something I don't understand.

OTOH, I have seen pictures of a 606 in a W123, so I guess anything's possible.

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83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
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88 Jaguar XJS V12 94,xxx miles. Work in progress.

I'd say that a nice turbo OM603 or OM606 would be a fine install (with their transmissions), but I think the exhaust might be problematic in a RHD situation. I believe the UK didn't get these engines because the exhaust ran into the steering box.