Sorry that I didn't read your first post where you said 1.5" bulkheads BTW.
The hole size for 1.5" bulkhead is I believe 2.5" (65mm), at least according to this page it's 2.5" (I'll check when I get home from work, I have some 1.5" bulkheads I can measure to be sure).
http://www.wetdryfilter.com/drilling_holes.htm
Anyhow I just came into a situation where I need to drill a hole for 1.5" bulkhead too, so I just ordered me a 2.5" glass bit. If you're still in need of getting the stuff drilled when this arrives, you can borrow it... I'll post back when I get it.