Zodiac R0621000 Softube Quick Connect Fitting Black

What is the inside diameter of the barb ? in other words, what is the inside diameter of the pentair hose that will fit into the barb ?

I purchased one of these & can't get it to stay on (the end connecting to the hose) when the Polaris pump is running. Any suggestions?

These are really tight in the pump housing. Do they screw all the way in? I am worried I will break the pump.

What size thread ?