The pre-combustion chamber engine reaches its limits

In the OM 322, changes such as a new pre-combustion chamber, new valve seat rings, a specific microsection for the pistons and an improved honing pattern for the cylinders paid testimony to the efforts and ingenuity of the developers. Nevertheless, there were some teething troubles with the engine: despite modifications that were promptly implemented, its reputation was damaged by cracks in the cylinder head, piston scuffing, and fractures in the connecting rods and crankshafts. The unit was one of a kind, and this was also true of its appearance: for in order to avoid confusion with the OM 321 produced in parallel, the OM 322 was painted blue rather than the traditional green. It became clear that the pre-combustion chamber engine had finally reached its limits in terms of thermal capability. In 1964 the damage frequency was 9.7 percent; by way of comparison, the corresponding value for its successor, the OM 352 (which now operated with direct injection), was just 0.8 percent. Tests with direct-injection systems acquired top priority as early as 1956.

This kept operations in Mannheim very busy from 1959 to 1964: the OM 312, OM 321 and OM 322 – three generations of the 300 series – were all produced in parallel. In addition, the first four-cylinder engine in the 300 series was built just a year after the premiere of the OM 322. Derived from the OM 321, it was given the model designation OM 324. With a bore of 95 millimetres and a stroke of 120 millimetres, it offered a displacement of 3.4 litres and had a power rating of 75 hp. However, the engine was to play only a minor role, finding use in Toyota offroaders but not in the company’s own vehicles. The engine had not yet found the appropriate light truck or large van, but this situation would change a few years later.

Further developments with durable direct injection engines

But first the 300 series reached a watershed with conversion to direct injection. “New engines with a special focus on economy” was the title of a Mercedes-Benz brochure in 1964. Following testing of direct-injection systems over a period of more than twelve years, Mercedes-Benz finally presented the OM 352 in 1964 – a little late, perhaps, but not too late. The company announced: “After thorough testing over an unprecedented period … all Mercedes-Benz trucks with a payload over 3.5 t will now be fitted with new engines featuring the Daimler-Benz direct injection system.”

Contrary to its predecessor, straightaway the OM 352 proved an especially durable unit.
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The direct injection engines of 1964 continued to rely on a 97-millimetre bore and a 128-millimetre stroke.
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