There could be a few reasons why the thread is still not sealing properly:
· The tapping process (making the hole bigger) wasn’t done properly
· The thread on the new screw/plug does not match the tapped thread
· The washer used is not a ‘crushing’ type
I would be extremely careful before letting anyone muck around with the oil sump, as it can be painful/expensive to replace the sump.
You could try a couple of things:
· Examine/measure the thread pattern on the plug used and the newly tapped thread. Try and find a perfect match
· Try using a proper ‘crush’ washer in the correct size, which would seal the gap between the plug and the sump
MrCat's post in Oil Leak From Oil Sump Screw was marked as the answer