The O-ring fits into the groove of the nut to enhance the quality of the seal. Made of food grade silicone, the O-ring can withstand temperatures over 400 °F and is fairly durable. This o-ring is made from FDA Approved material. It is odorless and non-toxic. Resistant to heat, alcohol, mildew, sun and ozone. The perfect size for homebrewers 1/2″ NPT Fittings!