See pix for adapter from plumbing aisle, and yes, it is 1/2 inch NPT, so there is some leakage, and you can bugger up threads if you are not careful. Two adapters used to make this up.

They also have these "rubber bladder" drain cleaners, that attach to the end of the garden hose, then swell up slightly and spray water out the end as well. Does not move a lot of volume. If using on an outlet or inlet radiator tube, would be a good idea to put a couple of hose clamps around the soft metal tube to prevent distortion.

 

I've been using these since the 70's, they work quite well.
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You do want to remove the block drains when done and then run a cycle of distilled water through before refilling with coolant.