Try a metric pipe thread, be gentle turning, don't force it. The threads are in aluminum which can be damaged easily.
On my older Honda I extended the oil drain, I think it was a 1/4 pipe thread, (but that was about 10-12 yrs ago, and the engine was made in the U.S.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.