MGBs produced up to September 1976 used a conventional top-entry radiator mounted in a diaphragm panel spanning the width of the engine bay. The radiator type changed during this period, and the correct replacement depends on the car’s chassis number and engine series. The earliest cars from 1962, fitted with 18G-series engines and the 3-synchro gearbox, used a rear-fill radiator where the filler cap was located at the back of the header tank on the engine side of the diaphragm panel. The cap was mounted on an elbow pipe and the system operated at 10lb pressure.
This radiator was finished in semi-matt black paint, including the header tank, and the top and bottom hoses were identical in length and profile. From the start of MkII production in 1967, a centre-fill radiator was introduced with the filler cap relocated to the front of the header tank, making the coolant level easier to check when topping up.
This type was used on all 4-synchro cars, both chrome bumper and rubber bumper, up to September 1976. The radiator cap remained a 10lb item until the 1976 models, when a revised radiator with a 13lb cap was introduced, though more significant cooling system changes were in store for the 1977 model year. Both rear-fill and centre-fill radiators are available new, reconditioned on exchange, and in high-efficiency all-aluminium versions. Aluminium radiators offer greater cooling capacity than the standard copper and brass type, making them particularly suited to modified engines, cars used in warmer climates, or vehicles driven hard on track days where the standard radiator may struggle to maintain safe operating temperatures.
Mounting Hardware & Diaphragm
The radiator is secured to the bodyshell by ties on either side, one left-hand and one right-hand, bolted to the slam panel and to the radiator itself. The middle mounting nuts on the radiator were deleted from a production change point during 1968, and replacement radiators do not have the middle nuts regardless of the car they are being fitted to. The diaphragm panel sits behind the radiator and fills the gap between the radiator and the engine bay, directing all airflow through the radiator core rather than allowing it to bypass around the sides. A rubber seal on the top edge of the diaphragm seals against the bonnet when closed.
The diaphragm panel, seal, and all associated mounting screws and washers are available individually. A missing or damaged diaphragm panel will significantly reduce cooling efficiency, as air will take the path of least resistance around the radiator rather than being forced through it. The radiator drain tap at the bottom tank was discontinued from June 1970. On cars where it was originally fitted, it can still be renewed if present.
On later cars, the cooling system is drained either from the bottom hose or from the cylinder block drain plug. When fitting a new or reconditioned radiator, the ties, mounting hardware, and diaphragm seal should all be inspected and renewed where necessary. A radiator that is not securely mounted will be subject to vibration and movement, which can cause fatigue cracking at the header tank joints and premature failure of the hose connections. The radiator and cooling system components for cars from September 1976 onwards, which use the forward-mounted radiator with electric fan and separate expansion tank, are covered in a separate section.