refer Zoro's reply, he said it all. Tapered is indeed the way to go, the surfaces aren't flat at all. Also, use red locktite to 'glue' them in, you don't want them to sweat or leak. Clean screen & flush afterwards eh?
This mod gives enormous piece of mind oil-wise, seeing (whilst riding!) is believing.
On the TM40, I also have a little 'play' between rod & plunger, to leave a bit of delay before the squirting starts.