There have been posts about this issue before..
After 12 years of leakages I've found original looking(as close as possible)taps that don't leak;
The main difference is that 'push on' and 'push off' have been lasered instead of engraved. €19 a piece and suitable for (at least) Norton and BSA.
www.jvmotors.nl
Whichever type of taps you use (I prefer the brass racing type) always use ones with the attached filter as per previous photos. There are plenty out there with no filters at all.