According to this chart, it's 41/64, but I would check my work.

http://www.noblefix.com/helicoil/images/drill-chart.jpg

Thanks. Just got everything in. Going to try to do it tomorrow, Monday on the boat. As long as I can get it tap straight. I should be all set. I've heard various things on whether to use lock to them to heilcoil. Everyone says they will lock themselves.