These plastic caps are used to protect fire hydrant threads and are used where theft of more expensive brass caps is a problem. These caps are threaded and replacement gaskets are available if you need them. SOLD INDIVIDUALLY. 2.5" refers to the diameter of the fire hose, not the connection itself. The inner diameter of this item is 3" and inner depth is 3/4". Plastic break caps are another alternative; see item # 09207. A push-on unthreaded cap is available as item # 09209.
If you prefer old-school brass caps, see item # 09201.