Not a bike topic but maybe someone here would have experience. The sedimenter on my old diesel pickup has a nylon drain plug on the bottom. Stuck. Had to cut head off, used a mill to cut out the center. I have really stuck nylon remaining in the threads. Dental pick got some out.

It’s a M10 1.25 thread pitch. I have a tap, but not enough exposed threads to start it in.

Heat? Solvents barely touch nylon. Vinegar to work on the corrosion, or some weak phosphoric?