Square head design allows the user to install or remove these by hand or with a wrench. Used to protect NPT threads against contaminants and moisture. Nylon is recommended when strength and abrasion resistance are desired. The nylon material absorbs paint, keeping paint flakes to a minimum. Nylon is able to withstand higher temperatures than HDPE. Nylon is recommended for intermittent temperatures of 320F (160C) to -40F (-40C).