MGA Radiator Caps

This article attempts to answer a question about whether radiator caps with a reach of 1” will give an effective seal in a 1” deep neck. If a cap is not sealing correctly it may allow coolant to vent off too early and cause coolant loss. If a cap is holding too high a pressure it may cause stress on hoses, clips and soldered joints. Radiators which “vent off” in summer temperatures whilst performing normally at other times of the year may have a radiator cap which is not operating as it should. At the time of writing many specialists and other suppliers sell what they describe as a “long reach” cap for the MGA with a 1” deep radiator neck. Visually these appear to be identical caps sold under different brand names and numbers eg. First Line number FRC60. QH number FC41. Moss & MGOC Parts GRC103. NTG number B350. It may or may not be the case that all these caps have been made by the same manufacturer and only the part numbers are different.

The tests results recorded here were obtained using a simple home-made test rig, like this:

In this article “depth” refers to the distance from the top of the radiator’s filler neck to the sealing flange on the bottom of the neck. The word “reach” refers to the cap and is the measurement from the lower face of the top seal (whether metal or rubber) to the lower face of the bottom seal. In other words, the sealing faces. Other measurements such as overall length from the top of the cap to the very bottom metal ring can be ignored as they are irrelevant; it is the distance between the seals which is being considered here.

Objective: The objective of these tests was to establish how well a cap with a 1” reach seals in a 1” deep radiator neck.

Method: Each cap was tested 5 times. The “test tube” was pressurised using a bicycle pump to replicate the build-up of pressure in a radiator when the coolant heats up and expands. The point at which air bubbles were seen in the glass jar replicates the point at which the pressure in the radiator exceeds the rated cap pressure and vents off coolant into the overflow pipe. The pressure readings at which bubbles appeared in the glass jar were recorded. The results shown below are the lowest figure recorded for each cap with the exception of the unbranded 4lb cap (with the metal upper seal) where the result shown is the highest recorded.

Apart from the unbranded 4lb cap all of the “long reach” caps tested appeared to demonstrate that the lower seal was being adequately compressed onto the bottom lip of the filler neck. This would prevent any leakage when the coolant was cold and will probably be tight enough to seal the radiator once the coolant gets hot and the radiator is pressurised. However, based on the sample of 8 caps tested it can’t be taken as a given that any 1” cap will hold pressure. Nor can it be taken for granted that the cap will release at the claimed pressure rating. The unbranded 4lb cap failed to hold any useful pressure and only one cap vented off at the pressure stated on the cap. The only cap which held the stated pressure was a NOS Waxstat WPC6 (4lb).

Discussion:
A 50/50 mix of glycol/water boils at 223dF at sea level so if the cap is failing to seal under pressure then the radiator will vent off coolant at this temperature. The coolant’s boiling point increases by 3.25dF for each psi of applied pressure meaning that a 4psi cap, which is working correctly, will vent off coolant at 236dF. A 7psi cap takes the boiling point up to about 245dF. See this www.mgaguru.com article (*2).

So if your radiator “vents off” on hot days then it could be that you have a cap which doesn’t hold the coolant under sufficient pressure.

It is worth noting that the early MGB radiator also had a 1” deep neck and the cap listed for that is often sold as a direct replacement for the MGA 4lb cap. (see MG Service Memorandum MG/289 referred to on www.mgaguru.com (*1) However, the reach on this cap is also 1” (25mm) meaning that if this cap does not seal properly in a 1” (25mm) deep neck then the poundage is irrelevant. See this www.mgaguru.com article (*2).

If you can find a NOS AC-Delco RC-4 you will see a size chart on the side of the box. This implies that for a 1” deep neck a cap with length of more than 1” is required. However, the cap which came in this box has a reach of 1” and has the same issue as the other 1” reach caps on test. Although, apart from the No.2 unbranded 7lb cap, it was the only one which held a pressure close to the value stated.

Fitting a radiator cap of the correct length and pressure will not make your engine run cooler, but it may well stop the radiator venting off when the
engine gets hot. It may also prevent the vicious cycle of; running temperature increases, coolant temperature/volume increases, radiator vents off increased volume of coolant meaning engine is running with less coolant in the system, meaning the volume of coolant left is unable to absorb sufficient heat meaning the engine runs hotter still, the coolant further increases in temperature/volume so more coolant is vented off thus reducing the volume of coolant in the system still further and the cycle starts again. If you want to reduce the running temperature of your engine you need to look for another solution.

Fitting a radiator cap which functions correctly will improve the efficiency of the cooling system and allow it to work as originally designed. It will not compensate for clogged up waterways or a blocked radiator.

Sources:
(*1) (https://mgaguru.com/mgtech/care/csm/mg289.pdf
(*2) https://mgaguru.com/mgtech/cooling/cool_200d.ht

Written by Joe Walsh, November 2023.