3/8" Bottle Air Vents with Check Valves - Top Vent Supplied by either Watts or Giacomini - IMPORTANT NOTE, MOST PLUMBING PRODUCTS ARE STATED AS BSP IMPERIAL THREAD MEASUREMENT WHICH MEANS THAT ACTUAL DIAMETER OF THE THREAD SIZE IS APPROXIMATELY 1/4 INCH LARGER THAN THAT STATED IN BSP TERMS. FOR EXAMPLE THIS 3/8 INCH BSP THREAD ACTUALLY MEASURES APPROXIMATELY 5/8 INCH IN DIAMETER