Guide to Hole saw sizes
 

I keep forgetting to post this:

Bit sizes are
1 1/8 inch 28mm perfect for a 1/2 inch bulkhead for 1/2 inch pipe.
1 1/2 inch 38mm for 3/4 inch bulkhead for 3/4 inch pipe,
1 3/4 inch 45mm for 1 inch bulkhead 1 inch pipe,
2 3/8 60mm for 1 1/2 inch bulkhead 1 1/2 inch pipe,
3 inch 75mm for 2 inch bulkhead 2 inch pipe.
Plus a 5/8 inch (16mm) for water top off units.

When I was redoing my return pump found 3 different sized 1.5" bulkheads.