I also found the 1/8 NPT was not big enough; there is some metal for the threads to grab, but not much. I ran a 1/8 NPT for a year and it was obvious the exhaust was still getting through.

I drilled it out and tapped it for 1/4 NPT. It was a bit on the big side, I didn't want to drill in too far, and NPT plugs tend to snug up with some of the plug still sticking out. So after it was snugged, I just took a larger drill bit and drilled down the bit of plug that was sticking out until it was flush with the mating surface.

Wish I had taken a picture or two.. I hope that makes sense. Also this is all on a 1G head.. from what I've seen, the 2G head seems to be okay with 1/8 NPT but I've never looked at it too closely in person.