If its the original Carrier style then I think you need the proper kind of cap.
On my UC I have the original rad but sometime in its past life the big screw on brass thing was changed for a standard type of neck. When I needed a new cap I just bought one at Cnd Tire for a few bucks.
You should be able to get by with a rag stuffed into the hole to do a longer engine test.
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Jordan Baker
RHLI Museum,
Otter LRC
C15A-Wire3, 1944
Willys MB, 1942
10cwt Canadian trailer
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