The MGB's water pump, crankshaft pulley, and fan pulley work together as a belt-driven system that circulates coolant through the engine and, on cars up to September 1976, drives the mechanical cooling fan. The pulley sizes and belt specification changed during production, and selecting the correct combination is important both for standard replacement and when upgrading the cooling system.
Pulleys & Belt Drive
The crankshaft pulley drives the fan belt, which in turn runs the water pump and the alternator, or dynamo on very early cars. On chrome bumper 18V-series engines the crankshaft pulley was a 5?-inch diameter item, and from the start of rubber bumper production in September 1974 the diameter was increased to 6 inches, with a correspondingly smaller fan pulley, new fan belt, and revised alternator adjusting link.
These changes affect the drive ratio of the water pump and alternator, and the components from the two configurations should not be mixed without ensuring the correct belt length and tension can be achieved.
Water Pump Types
Four different water pump types were fitted across production, each with its own mounting bolt pattern and pulley configuration. Three main bearing 18G and 18GA engines from 1962 to 1964 used a unique pump, five main bearing engines from 1964 to 1971 used a different type interchangeable within the range assuming the correct pulleys are used, and from 1971 the 18V-series pump was commonised with a wider BMC application, this type with revisions in 1974 and 1975 continuing to the end of production.
The later 18V pump supersedes the original 18V chrome bumper type but requires different mounting screws when fitted in place of the earlier item.
Maintenance & Electric Fan Conversion
On modified engines, particularly those producing more power or generating more heat than standard, ensuring adequate coolant flow is essential, and the water pump, pulley ratio, and belt condition all contribute to the cooling system's effectiveness. A worn or slipping belt will reduce both coolant circulation and alternator charging simultaneously, with overheating and a flat battery as the potential consequences, so belt tension should be checked regularly and any belt showing cracking, glazing, or fraying replaced immediately. When converting from a mechanical fan to an electric fan on pre-1976 cars, the fan blade is removed from the water pump spindle but the pulley and belt must remain in place, as they continue to drive the water pump and alternator, and the removal of the fan blade itself has no effect on the belt specification or the water pump's operation. The range covers water pumps, crankshaft pulleys, fan pulleys, fan belts, alternator adjusting links, and all associated gaskets and fixings for every MGB engine type.