Vent Plug, Relief/Filler, Thread Size 1/2 NPT, Height 1.4688 in, Top Cap Dia. (In.) 2 9/16 in, Overall Length (In.) 2 15/32 in, Allow Sealed Pressure Systems to Vent Off Excess Pressure as Heat in System Builds up, Air Pressure is Allowed Back into System as System Cools