Can you fit a 603 from an 87 into a 123? Not just the engine but the trans also? I have a line on another coupe with a crappy engine and since I already own the 87 I thought it might be an interesting project.

I remember seeing someone who tried and I think the engine was too tall

Ya, I was wondering that, also might be too long... Nice thought regardless.

looks like it fits after all...

2 examples...

W123 COUPE 300D (OM603) 1977 - YouTube

W123 COUPE 300D (OM603) 1977 -

I thought it was the other way around. ~ a 617 wont fit in a w124 as its too tall. The 60X are over at an angle to keep them short.

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1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving