Take it out. Get a brass freeze/core plug (the dished ones, not the cup shaped ones) bigger than the hole, pound it flat and solder it in place. Be careful not to overheat the rest of the assembly.
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"Not all who are laundering are washed" say Bill & Heidi
'78 Excella 500,"The Silver Pullit". vacuum over hydraulic disc brakes, center bath, rear twin. '67 Travelall 1200 B 4X4 WBCCI 3737
Go to radiators.com and buy a new one (with metal end caps) so that you won't be stuck beside the road outside of Gila Bend, AZ in August with a glowing red engine.
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Crusty
"If you come to a fork in the road, take it."
Lake Travis, TX
"Rancho Deluxe"
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