M129

Mercedes-Benz sold the M108 and the M129 side by side, and mechanically they were one engine: 2,496 cc from 82 × 78.8 mm, seven main bearings, gray-iron block, aluminum head, a single chain-driven overhead camshaft and two valves per cylinder. The difference was how fuel arrived. Where the M108 breathed through carburetors, the M129 had a mechanical Bosch six-plunger pump with three-dimensional cam control, which let compression rise from 9:1 to 9.3:1, and to 9.5:1 from 1966. The reward was 150 PS at 5,500 rpm.

It went into the 250 SE saloon of the new W108, the W111 250 SE coupé and cabriolet, and from 1966 the 250 SL. Then it stopped, because the casting had run out of bore. The cylinders were grouped in pairs with water between them and could not be opened up further, and the fix — cutting those jackets back so the bore could reach 86.5 mm — produced a different engine, the 2.8-liter M130. The M129 lasted from 1965 to 1968, and is remembered mostly as the bridge between the two.