1/2" Double Check Valve -  Female Thread to Female Thread Parallel threaded ends to ISO 228 Three other styles are available in this section by pressing similar products above   1/2" Female to Female Double Check Valve 3/4" Female to Female Double Check Valve 1" Female to Female Double Check Valve - 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 1/2 INCH BSP THREAD ACTUALLY MEASURES APPROXIMATELY 3/4 INCH IN DIAMETER