General Motors M32 / M20

General Motors' European six-speed manual — in the Astra K, the Cruze and the Chevrolet Aveo, and across Opel, Vauxhall, Fiat and Alfa Romeo.

It has a specific, well-documented and widely misdiagnosed failure: the end-case bearings. The bearings supporting the input and output shafts at the end cover wear early, and the symptom is a rumble or growl that rises with ROAD SPEED and is present in every gear including with the clutch pressed while rolling. It is loudest in fifth and sixth because that is where the shafts turn fastest.

That combination is close to diagnostic, and it is what separates the fault from the two things it is usually mistaken for: a wheel bearing changes with cornering load, and a worn synchro shows on the change rather than at a steady speed.

The reason to catch it early is that the failure is progressive and destructive. A collapsing bearing lets the shaft move, which damages the gears and eventually the casing — a bearing job while it is still a noise is a fraction of the cost of the same job after. An uprated bearing kit is the standard remedy and is widely available.

Do not confuse it with the F40, the earlier six-speed in some of the same cars; the two share an application list and no parts.