The 1/4" pipe thread is the same as an air line fitting. Screw a male quick-connect fitting into the block and attach an air hose to it. 100-something PSI of air pressure should clear the clog - - - -just be sure to remove the radiator cap or take a hose off to protect the radiator core.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!