We have a 81' 300SD W124 with a problem motor, but a fine shifting transmission. I'm wondering what else the trans might fit, without any fancy mods?

Might it bolt right into an 82' 300D?

or, might an 80' 300D trans bolt into an 82' 300D?

An 81 SD is a 126, not a 124.

No it will not bolt right in and work on a 123. You would need to swap the tail shaft as your trans has an electric speedometer out put and the 123 is mechanical.

To answer your question, 81-83 SDs will take the trans without much modification. 84 and possibly 85 if you don't mind the tach not working.

An 80 300D trans is different than an 82 300D trans. You would need to swap some things to make it work.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

The tach has nothing to do the transmission. On 84/85 the tach pickup is on the engine side of the bellhousing, so what trans is there does not matter.