They use a pipe thread...
there are some different pipe threads... Most pipe threads are tapered and get larger, but you have to be careful when tapping a tapered pipe thread hole, if you go too deep, then the hold becomes too large and the male pipe thread will go in too deep and bottom out and possibly not seal... If they didn't thread it in enough, it will be tight...
Then there are straight pipe threads which are the same diameter across the whole thread...
When I worked on the engine block line, they used straight pipe threads for the drain plugs and tapered pipe threads for the plugs... That way they are not as sensitive to the depth of the female thread, and the tapered male thread will seal like a cork when threaded in - a much more reliable way to use pipe threads... You have to know what threads you are working with to get the proper tap and die for them when you manufacture them...
