I don't remember the size but it is the same thread pitch as a hydraulic hose fitting ....I put a price of aluminum and knurled it on the lath for the center where the hose would be. I used the old line for the size.
Soon I'm going to the F-Body radiator but I am going to keep the stock oil cooler assembly and run an external oil cooler. What size are the male threads that screw into the radiator? I want to locate an external cooler that has the correct female thread size to bolt right up to the stock lines.
12096 Midland Inverted Flare Fitting - Plug - 1/2" Male Inverted Flare , Brass, is ideal for use with hydraulic brake, power steering, fuel lines and transmission cooler lines, LP and natural gas.
hosewarehouse.com
Chart at the bottom has the thread sizes, measure the pipe OD for the flare thread pitch I think it's 1/2" OD pipe should be able to find a cap at a hydraulic hose shop, Parker fittings maybe NAPA Edelmann etc.