21/64" is the size.
Don't know why you want to convert though and possibly ruin a rim by drilling.
These are a much safer solution, if you have a Schraeder only pump.

http://www.jensonusa.com/!mYP5QroTv5...FbFxQgodXwwAkw

Your LBS should carry them and you only need one.
I keep one on my key chain so i can use gas station air if I get a "slower" leak, to get back home.