From September 1976, the MGB cooling system was substantially redesigned. The radiator was moved forward in the engine bay and a thermostatically controlled electric fan replaced the mechanical engine-driven fan. A separate expansion tank was introduced, and the system was pressurised to 15lb, creating a sealed, no-loss cooling arrangement that represented a significant improvement over the earlier open system. The radiator used on these cars is specific to the post-September 1976 configuration and is not interchangeable with the earlier rear-fill or centre-fill types.
The filler cap is no longer fitted directly to the radiator but instead sits on the expansion tank, which acts as the system’s pressure vessel and coolant reservoir. It is available new, reconditioned on exchange, and in a high-efficiency all-aluminium version. The aluminium radiator offers greater cooling capacity than the standard unit, an advantage for modified engines, cars used in demanding conditions, or those fitted with air conditioning.
Expansion Tank & Cap
The expansion tank is available in the original metal type and a plastic alternative. It is mounted by a strap in the engine bay and connected to the radiator by a small-bore hose. An overflow hose runs from the expansion tank to vent excess coolant safely if the system overpressures. Both the tank-to-radiator hose and the overflow hose are available by the metre.
The radiator cap on these cars is a 15lb rated item and must not be substituted with a lower-pressure cap from an earlier car. The higher system pressure raises the coolant’s boiling point, which is fundamental to the sealed system’s ability to maintain stable operating temperatures. Using an incorrect cap will compromise the system and can lead to premature coolant loss and overheating. The radiator is secured by left-hand and right-hand ties bolted to the slam panel, with long and short mounting screws at the base.
These ties differ from those used on pre-1976 cars and are specific to the forward-mounted radiator position. All mounting hardware, screws, washers, and nuts, is available individually. The electric fan is controlled by a thermostatic switch fitted directly into the radiator header tank. Two switch types were used, an earlier type with pin terminals and a later type with spade terminals, introduced later in production.
The correct seal and retaining clip should be matched to the switch fitted. The fan motor, blade, guard, motor bracket, and lower clamp are all available separately and are covered in detail in the Cooling Fan, Electrical section.
When fitting a new or replacement radiator on a post-1976 car, the expansion tank condition should be checked at the same time. A corroded metal tank or a cracked plastic one will allow coolant loss that may not be immediately obvious, as the sealed system can lose fluid gradually without a visible external leak. The expansion tank strap, hose clips, and all associated mounting hardware should be renewed where necessary for a complete and reliable installation. The MGOC Spares range covers radiators, expansion tanks, radiator caps, mounting ties, and all associated fixings for the post-September 1976 MGB cooling system.