The MGB GT V8 water pump is a centrifugal-type pump bolted to the engine front cover, driven by a single belt from the crankshaft pulley which also drives the alternator. The pump circulates coolant from the radiator outlet through the engine block, into the cylinder heads, and back to the radiator. Because the V8 cooling system is semi-sealed and pressurised at 15 lb per square inch, the pump runs against a higher static pressure than a non-pressurised system, and the pump shaft seal is a wear item that should be inspected during any cooling-system service.
Water Pump
The water pump is supplied as a complete assembly. The pump gasket seals the pump body to the engine front cover and is renewed whenever the pump is removed, with securing screws fixing the pump to the front cover. The water pump pulley is a single V-section item, available as a replacement with three screws and spring washers. A failed water pump will typically show as either a coolant leak from the pump shaft seal, often onto the front of the engine, or as bearing noise from the pump itself, either symptom calling for renewal of the complete assembly rather than partial repair.
Thermostat
The factory V8 thermostat is rated to open at 82°C, allowing the engine to warm to operating temperature before significant coolant circulation through the radiator begins. A thermostat stuck open results in slow warm-up and reduced heater output, while one stuck closed causes rapid overheating, particularly in heavy traffic, and both conditions warrant immediate replacement. A 74°C alternative is available where lower running temperatures are wanted, typically with capacity-upgrade engines.
Thermostat Housing & Top Hose
The water-outlet elbow, commonly called the thermostat housing, is a cast item incorporating the thermostat seat and the top hose stub, secured to the inlet manifold by two screws and spring washers, with a gasket sealing the housing to its mounting face. The top hose runs from the housing to the radiator with hose clips at each end. The fan thermo-switch is mounted in the top of the inlet manifold and operates independently of the thermostat, sensing coolant temperature and triggering the front-mounted electric fans at approximately 90°C.
Fan Belt
The single fan belt drives both the water pump and alternator from the crankshaft pulley, so a worn or slipping belt impacts both cooling and charging, and the belt should be checked for cracking, glazing, and correct tension at every service.