ZF S6-650 / S6-750

ZF's heavy-duty six-speed manual for American trucks — in the Ford F-250 and Super Duty, and the Chevrolet Silverado and GMC Sierra HD.

It is the manual behind the 7.3 Power Stroke and the Duramax, which is most of its standing: a very strong gearbox, in vehicles that tow at their limit, and a large number still working.

Its recurring failure is the input shaft bearing and the fifth and sixth gear synchros. The bearing announces itself as a whine present in neutral with the clutch out that goes quiet when the pedal is pressed — the same tell as on every manual in this catalogue, because pressing the pedal unloads that bearing.

The synchro wear shows only in the tall gears, which is where a towing truck spends its time, and presents as a grind on a fast change into fifth or sixth alone.

It takes a synthetic manual-transmission fluid to ZF's own specification. A conventional gear oil makes it notchy when cold and hard into second — the fifth instance of that mistake in this catalogue, and as always it reads as synchro wear in a gearbox that is fine.