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MG MGA - radiator cap

I have a 1957 MGA 1500 and it appears that the radiator cap has a very loose fit. I ordered a new 7 lb cap from Moss and that also has a loose fit.
It seems to me that when you put the cap on it should require a little bit of pushing down and then turning so you know the rubber washer is sealing the opening to the radiator.
Any suggestions for a radiator cap with a longer reach?
W Chandler

Radiator necks and caps come in two depths. Early neck for MGA is 1 inch deep, and cap is 1-1/8 inch deep from top seal to bottom seal. The cap will compress 1/8 inch when assembled to apply the preload on the pressure foot.

Later model MGB uses 3/4 inch deep neck and 7/8" deep cap, also compressing 1/8 inch when assembled. Measure depth of the filler neck and depth (reach length) of the cap to be sure they match.
Barney Gaylord

Just a comment about the 7 lb cap. Think about how nearly doubling the pressure in your cooling system stresses your hoses, radiator and core plugs. After I went to a 7 lb cap and blew out a core plug I went back to the 4 lb cap.
Cheers,
GTF
G T Foster

If you blew out a core plug at 7 psi it needed to be replaced anyway. If it was a rebuilt engine with new core plugs, then the engine builder is incompetent, and you had best check installation of the rest of the plugs as well. Core plugs in good condition will not blow out at 25 psi. In fact if you freeze water in the block it is more likely to break the block than to dislodge a core plug.
Barney Gaylord

Barney is right we pressure test all cooling systems to 20 psi. NO failure should ever happen at 20. To lose a core plug at 7 indicates imminent failure or, as Barney suggests, incompetence.
R J Brown

And 7lbs is the recommended pressure for an MGA rad cap.
Peter.
P. Tilbury

This thread was discussed between 29/09/2011 and 03/10/2011

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