NPT threads and hose barb sizes
 

Found these helpful diagrams

first table bigger

The main thing to remember is not to mix NPT (National Pipe Tapered thread) with NPS (National Pipe Straight thread)...especially in locations where watertight integrity is required.

And under stand the difference in Pipe Schedules (40-60-80).

The following thread may assist;

http://www.jamestowndistributors.com...t.do?docId=515

> not to mix NPT (National Pipe Tapered thread) with NPS (National Pipe Straight thread)

yeah, that's important :) I know how they look, I was just never clear on nomenclature - having moved here from metric climes :)