Watertight well caps consist of two round disks which sandwich a flexible rubber donut in between. The lower disk is threaded so that when the bolts on top are tightened, the disks come together and expand the rubber out against the inside walls of the casing. This locks the seal like a plug inside any standard 6" ID well casing.
The EarthStraw 6 inch well seal is made from structurally reinforced ABS and comes with the EarthStraw gripper in the center. It also has a 3/4" NPT access hole which measures about 7/8" in diameter. This provides an additional access point and can also hold EarthStraw for pumping.