ZF S5-31 (S5D 310Z / 320Z)

ZF's five-speed manual for longitudinal BMWs, in the range from 1990 until 2006 — the 3, 5 and 7 Series, the E53 X5 3.0i and the larger-engined Z3.

BMW does not call it the S5-31, and knowing how to read BMW's own name for it saves a wrong order. The label says S5D 310Z, and every character is data: S manual, 5 five forward gears, D direct top gear, 310 the maximum input torque in Nm, Z for ZF. So the number on a BMW gearbox tag is its torque rating, not a model number — which is why the reinforced version built from September 1995 is the S5D 320Z, twenty newton-metres stronger and otherwise the same gearbox.

Its ratios make it a five-speed with a direct top, not an overdrive box: 4.21, 2.49, 1.67, 1.24 and 1.00, with 3.85 reverse. Fifth is 1:1. Anyone expecting a tall cruising gear from it is expecting the wrong gearbox — the final drive does that work on these cars.

It is the small-and-light one of the pair BMW used, about 39 kg, against the Getrag units fitted to other engines in the same models. The trade's reading is consistent and worth stating plainly: the ZF changes more notchily and the Getrag has the better synchros, while the ZF is the stronger of the two and is what BMW put behind the more powerful engines. Neither of those is a fault to be chased.

It takes about 1.3 litres. On a car this old that is small enough that a slow leak empties it well before anybody notices a level.