In Taper Pipe threads, the L1 Length of “hand-tight” engagement. It is the distance that the male and female parts should be threaded by normal hand force. It is one of the most critical dimensions to control when manufacturing an NPT thread. The typical inspection method uses a RIng Gage for external threads and a Plug Gage for internal threads.
Us the NPT Thread Calculator to obtain the required L1 value for each NPT thread
L1 Plug Gage for inspecting Internal NPT thread in fittings
The taper pipe thread plug measures the L1 dimension of the internal thread in NPT fittings. It has a step at the L1 distance from its small end. When turning “Hand Tight” into the thread (do not force the gage), the step is required to be aligned with the large end of the fitting within a tolerance of plus or minus one turn.
L1 Ring Gage for inspecting External NPT thread in Pipes
The NPT L1 ring gauge has a thickness equal to L1. The ring’s large end is turned “hand tight” onto the external pipe thread (do not force the gage). The ring gauge’s small end is required to align with the pipe’s small end within a tolerance of plus or minus one turn.