"Any idea if a 3/4" self-tapping drain plug would work? I think 3/4" is smaller than 20mm but maybe with the threads etc. it's somewhat oversized."

I asked a mechanic friend of mine, he said the only way to go is a self tapping over sized plug. Also be VERY careful it goes in straight. Heili coils do not work on old drain plugs. Good luck. <img src="/forums/images/graemlins/patriot.gif" alt="" />