The Mercedes-Benz M121 engine was a 1.9 liter single overhead camshaft inline four-cylinder engine introduced by Mercedes in 1955 and used in various model lines during the 1950s and 1960s. Originally rated at at 5500 rpm, it replaced the 1.8 liter M136 introduced in 1935, offering improved performance over the M136's side camshaft design. Applications included mid-size sedans, such as the 190, the 190SL roadster, and trucks such as the Unimog and L319 models. History The M121 engine was developed just after the end of World War II. It was built in the Sindelfingen Works factory, which assembled the Mercedes Ponton Model series to which the engine belonged to. The M121 engine made its debut in 1955. It was the first generation of 4-cylinder engines from Mercedes Benz. The M121 replaced the less efficient M136 1.8-litre engine, a pre World War II engine that was introduced 2 years before in 1953. The M121 benefited from innovations and technology from Mercedes' larger engines such as the M186 3.0-litre and M180 2.2 liter engines. One of these innovations was a new single over head camshaft design which allowed for more power and efficiency. It was developed by a team led by Hans Scherenberg. Within this team, Karl-Heinz Goschel was one of the key engineers who developed the engine design. The M121 series was in production until 1967, when the M115 series engine took its place. M121 variants M121 The M121 engine featured a single chain driven overhead camshaft layout with staggered valves operated by long and short rocker arms. The cylinder block was made from cast iron. The cylinder bore diameter for the M121 was . The three ring crankshaft was forged. The stroke of the engine was . This gave the M121 a displacement of . The engine had a compression of 8.5:1. Through this configuration, the M121 BII engine had an initial rating of at 5500 RPM and of torque at 4000 rpm. After testing of the final vehicle, the power rating was lowered to . The crankshaft was carried by three main bearings. In the 190 SL, the engine had two parallel carburetors. The M121 engine was distinctive during its time due to the fact that it stood upright in the engine compartment, and not at an angle like the 300SL's. Looking under the hood, one will notice a series of cast aluminum plenums. A circular air cleaner is