OM312

Stuttgart had wanted a small, light, fast-turning diesel since 1938, because Opel's cheap petrol Blitz owned the three-tonne class while the company's own L 3000 was too heavy and too dear. The war stopped the tooling, then in 1942 obliged Mercedes-Benz to build the Blitz under license — and in 1945 that Blitz machinery was the production equipment still standing. So the pre-war OM 302 six was reworked to fit it: seven crankshaft main bearings retained, cylinder spacing made deliberately uneven to suit the transfer lines. Proper testing was only possible because the lorry ran a year late.

The L 3250 that carried it took its bow in May 1949 at the Hannover export exhibition as the group's first post-war commercial vehicle, and was renamed L 3500 and then L 311. The gamble that had looked reckless in 1938 — a 3,000 rpm rated speed for a diesel — delivered nearly the specific output of contemporary petrol engines at a fraction of the thirst, and gave the lorry an unusually low curb weight. A turbocharged OM 312 A of 115 PS followed in 1954, but early turbochargers were unreliable, buyers stayed away, and it was confined largely to fire appliances. Detuned to 65 PS, the engine also opened the Unimog 406's career.