Interesting Johnny! That's the 1 reason why I don't tailpipe the 510 ... lips get a burning quite quickly!
what type of ballpoint pen?
The pen I'm using is a Zebra : Z Grip Gel. I think whatever kind you like will work, they fit the 510 and 901 perfect, just have to trim the length. I'm using it on my 510 to tailpipe at 6v with no burnt lips, and I have the rubber about half a milimeter above the atty so when I hit a bump in the road the metal doesn't bash my teeth.