Hydrostatic Valves are basically a safety valve for your pool to release excess water from underneath your pool. When water pressure builds up under the pool due to water run-off during a storm for example, the valve opens releasing the water harmlessly into your pool. If this valve fails the walls can deform and split, floors can hump upward and crack open, or the entire pool could pop out of the ground.
The 50mm Hydrostatic valve thread measures 60mm approx. 70 diameter across the top.