The RAM-218-1U consists of a 1.5" ball and 0.5" NPT female thread. This part connects to any 0.5" NPT male thread, including RAM-PA82xx rigid arm series mounts.
NPT Thread and Pipe Sizing: NPT (National Pipe Thread) is a standard used in joining pipes and fittings that have a tapered thread. As a general rule, the NPT inch size is the inside diameter of the threaded pipe and is known as the NPS (Nominal Pipe Size). The outer diameter (OD) of the pipe is shown in the table below: