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Ten bucks sez JD will know.
Who me??? My MGB had a 16 lb cap, aftermarket. Not sure what the OEM cap was.
The lower radiator hose on an MGB had a "built in" connection for the heater hose. The second heater hose connected to the heater valve on the side of the block. That little "nipple" on the lower rad hose was famous for failing, leaking, etc. Seems like some company sold a metal "T" fitting that you could splice into the lower hose, replacing the rubber nipple.
Nick, later model MGB's (after about 77, IIRC?) used a closed system with a header tank like you are trying to use. Seems like they didn't have a filler on the rad itself at all. If you have a B shop manual handy, check out the plumbing from those later years, might help. Perhaps use one of those year's caps???
In my '71 MGB, I put a modern cap (dual seal, I think they're called) on the rad and routed the overflow to a catch bottle. (It was actually an old Gatorade bottle!) I put the overflow tubing through a hole in the cap of the catch bottle and ran it to within about 1/4 inch of the bottom of the bottle. The top of the catch bottle was about even with the top of the rad, but I don't think that matters. The trick is to use a "modern" cap from a closed system so that it can let the fluid flow back into the rad or, in your case, the header tank. I think...
JDK
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Quinn, Son of a...
Build LogQuinn the Slotus:Ford 302 Powered, Mallock-Inspired, Tube Frame, Hillclimb Special "Gonzo and friends: Last night must have been quite a night. Camelot moments, mechanical marvels, Rustoleum launches, flying squirrels, fru-fru tea cuppers, V8 envy, Ensure catch cans -- and it wasn't even a full moon." -- SeattleTom