The M123 Engine was a straight six cylinder single over head camshaft engine, and was built solely for the W123 range and appeared in the 250 saloon 250T estate model and also the long wheel base versions. the engine never found its way into any other MB model.

the engine ran the large Solex 4A1 Carburettor. As with the M115 engine both the inlet and the outlet are on the right hand side of the engine. The engine output was 129bhp which was raised to 140bhp during production.

The engine was not a popular one due to high fuel consumption and carburettor tuning being quite problematic and after 1982 its use in the w123 market was seldom.

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