To form the seal, the assembly continues with a wrench an additional distance called “Wrench Allowance”. This distance is called “Hand Tight Engagement Length”, and it is usually measured by an “ L1 Gauge”. When assembling with “hand force”, the male and female parts of a tapered thread will turn several rotations until the clearance is closed, and a “stop” is reached.

Internal (Fitting) thread drawing with all its relevant dimensions.

External (Pipe) thread drawing with all its relevant dimensions.

An assembly drawing of a pipe (External thread) and fitting (Internal thread) showing only the parameters that are either common to both or are relevant to the assembly.To make the explanations and drawings more straightforward, we split them into 3 sections: It changes depending on the distance from the beginning of the thread.īecause of that, the set of parameters and measuring techniques is very different (and often confusing) compared to Unified Inch and metric threads. However, the pitch diameter is not a constant value. The profile properties, such as height, pitch, and truncations, are constant. Depending on the application, it can be used with or without a thread sealant. The taper on NPT threads allows them to form a seal when torqued as the flanks of the external and internal threads compress against each other (no clearance remains between the crests and roots of the threads because of the taper).

The main difference is that the thread profile is tapered at an angle of 1☄7′ (0.75 in/ft or 62.5 mm/m) relative to the axis. The basic profile is a 60° truncated triangle similar to the Unified Inch threads (UNC/UNF). NPT stands for National Pipe Tapered threads.

Form tap minor diameter chart