M103

Introduced in 1984, the M103 replaced the twin-cam M110 and the M123 with something deliberately simpler: one overhead camshaft on a roller chain, two valves per cylinder in a 15° crossflow head, CIS-E injection, a wet sump. Two sizes shared a single 80.25 mm stroke — 2.6 liters on an 82.9 mm bore, 3.0 on 88.5 mm — and the mechanical differences between them came down to bore, intake valves and airbox, which is why one drops into the other almost unaltered.

A catalytic converter cost the three-liter eleven PS, 190 falling to 179, and cars of the period carried a jumper plug in the engine bay so the ignition could be set for 95 or 91 RON depending on what the local pumps sold. The spread of applications was enormous: 190 E 2.6, 260 E and 300 E, 260 SE and 300 SE, the R 107 and R 129 300 SL, the 300 GE. The 24-valve M104 arrived above it in 1989, but the single-cam six stayed in production alongside for years.