3/4" Male Thread Hydra-Rester is manufactured by Sioux Chief. A hammer arrestor prevents what is called a "water hammer". The hammering noise happens most commonly when water reaches a closing valve. When the valve is closed a hammering sound will occur and the noise can be supressed by an arrestor. If an arrestor is not used, the appliances and pipes can be damaged or even destroyed by the vibration that comes from the "water hammer".