The OM621 is an inline-four diesel engine produced by Mercedes-Benz, from 1956 to 1968.
It was succeeded by the OM615 engine.
The OM621 is based on the petrol M121 engine, but features revised camshafts, cylinder head, pistons, and fuel injection system.
[1] In 1961, the engine capacity was increased to 2.0 L (1,988 cc) and power was uprated to 55 PS (40 kW) for use in the newly introduced W110 models.
After being replaced by the OM615, a 2.2 L (2,197 cc) updated version of the OM621 engine was used solely in Unimog vehicles until 1988.