5/16" Thread Size Locking Refrigerant Cap (2 Pack);Use Standard 3/16" Hex Key to Install and Remove Cap;Uniweld Locking Refrigerant Caps Meet International Mechanical Code 1101.10 and International Residential Code M1411.6;Machined from high quality brass and nickel plated for corrosion resistance.;Help prevent unauthorized access. Reduce potential liability for inhalation induced injury or death.