Threaded thread, drain plug oil

[sg67]

Hello

A very bad adventure this afternoon: I did the emptying of my bike but almost all the tapping came with the cap :(( . It was very difficult to turn it, not at all normal. The previous owner had much too tight.
I do not see myself dismantling the engine entirely either. I thought of drilling delicately and tapped with grease, plus then reverse the oil in to be sure that there is no more chip inside.
What do you think ? Already meet this problem ?
Thank you
Nice evening
Serge