These plastic caps simply pushes on and off rather than relying on threadss thanks to a broad and flexible inner gasket ring. SOLD INDIVIDUALLY. 2.5" refers to the diameter of the fire hose, not the connection itself. Plastic screw-on caps are available as item # 09208. Plastic break caps are another alternative; see item # 09207.
If you prefer old-school brass caps, see item # 09201.