Here are Two Ways to Determine the Size Kit You Need:
Measure across the bottom of the Poppet (item 12 in picture above); the distance across the bottom of the Poppet will determine which size kit you need: 1/2" & 3/4" Kits: Poppet measures 1- 1/8" to 1- 3/16" across the bottom (diameter). 1" & 1-1/4" Kits: Poppet measures 1- 7/8" across the bottom (diameter). 1-1/2" & 2" Kits: Poppet measures 3- 7/16" across the bottom (diameter).
Measure across the bottom of the Bonnet Assembly; the distance across the bottom of the Bonnet Assembly (threaded part) will determine which size kit you need: 1/2" & 3/4" Kits: Bonnet Assembly measures 1- 5/16" across the bottom (diameter). 1" & 1-1/4" Kits: Bonnet Assembly measures 2- 3/16" across the bottom (diameter). 1-1/2" & 2" Kits: Bonnet Assembly is made of brass and measures 5- 1/8" across the bottom (diameter).
90% of the Pressure Vacuum Breakers (PVB) installed for a typical residential home are 1" PVB's
How to Replace the Bonnet and Poppet (you should always change both):