Porsche M64 (air-cooled flat-six)

Porsche's air-cooled flat-six — the engine of every 911 up to the 993, and the reason those cars share almost nothing with the water-cooled 996 that followed.

⚑ There is no coolant, no water pump and no head gasket, so three of the commonest fault paths on a modern flat-six do not exist here. What replaces them is oil: it cools as well as lubricates, the system holds a great deal of it, and an oil leak on one of these is a cooling problem as well as a mess.

The 3.2 Carrera and the 964 share a bottom end and almost nothing else. The 964's M64 moved to a single distributor belt and twin-plug ignition, and the 993 added a hydraulic valvetrain — so a service interval or a part number quoted for the air-cooled six is worth nothing without the year.

Its known failures are era-specific. The 964 is the one with the dual-mass flywheel and the leaking cylinder-head studs; the 993's are the secondary air system and the exhaust manifolds. The 3.3 turbo before them, the M30/69, is a different engine again and belongs to the 930.

It runs on 98 and it does not like short journeys. Oil that never reaches temperature is what kills these, far more often than mileage.