The water pump and thermostat are critical components of the MGB cooling system, circulating coolant through the engine and regulating the operating temperature. Four different water pump types were fitted across production, and ordering the wrong pump is one of the most common mistakes made when servicing the cooling system, particularly on cars where the engine has been changed or rebuilt with non-standard components.
Water Pumps
Three main bearing 18G and 18GA engines from 1962 to 1964 used a unique water pump. Five main bearing engines, 18GB through to 18GG, produced from 1964 to 1971, used a different pump type, though within this range the three pump variations are interchangeable assuming the correct pulleys are used, and a new water pump with a pressure balance seal was introduced in October 1968.
From August 1971, the pump was commonised with that found on the BMC Austin and Morris 1800 models for the 18V-series engines, and this type, with further revisions in 1974 and 1975, continued to the end of production. The 18V water pump supersedes the original 18V chrome bumper pump, and when fitting the later pump in place of the earlier type, different mounting screws and bolts are required, three short screws and one long bolt in place of the original two short screws and two long bolts, a detail easily overlooked that will prevent correct fitment if the original hardware is reused.
Where there is any doubt about which pump is correct, owing to components having been altered from standard during a previous rebuild, it may be necessary to measure the depth of the water pump from the engine block face to the rear of the pulley to confirm compatibility.
Thermostat & Housing
The thermostat housing sits at the front of the cylinder head and directs coolant flow to the radiator once the engine reaches operating temperature. Three different thermostat housing covers were used across production, corresponding with the three main cooling system configurations.
Cars with 18G, 18GA, and 18GB engines used one type suited to the rear-fill radiator, cars from the 18GD to September 1976 used a revised cover with the outlet pointing to the left-hand side of the radiator suited to the centre-fill type, and from September 1976 a further revised cover was introduced for the electric fan cooling system, incorporating a different filler cap arrangement. The thermostat itself is a wax-pellet type that opens at a calibrated temperature, available in three temperature ratings, 74°C for hot climates, 82°C as the standard fitment, and 88°C for cold climates. The 74°C thermostat was briefly adopted as standard from late 1964 to increase oil pressure and reduce oil temperature, but the 82°C was re-introduced as standard from March 1969 and remained to the end of production, all three ratings using the same body and fully interchangeable. A thermostat gasket should always be renewed when the housing is disturbed.
Filler Caps & Gaskets
The filler cap on the thermostat housing cover changed with the introduction of the electric fan system in September 1976. Earlier cars use a radiator-mounted cap, while post-1976 cars have a cap on the thermostat housing itself, with the main system pressure cap on the separate expansion tank, and both brass and plastic filler caps are available for the post-1976 arrangement together with the associated O-ring or cork seal. Water pump gaskets are supplied as standard with all replacement water pumps. The range covers the full selection of water pumps, thermostats, thermostat housing covers, gaskets, filler caps, and all associated seals and fixings to cover every MGB engine type across the full production run.