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I am changing my coolant. I turned and turned the drain plug and it never started flowing. I gave it a slight (really) tug and it came out ! I can't see any threads on it anywhere ? There looks like there may have been a pin in one side.
I figured out what happened. The new design drain plug only rotates about 90 degrees counterclockwise to open. It has a pin in one side which fits into a curved slot in radiator. I must have sheared pin. I got new one from NAPA ($5.28), cheap and easy lesson.
Old chevy drain plugs screwed out. Why do they have to redesign things ! If it ain't broke, don't fix it !