Tool

Thread Callout Decoder

Paste the thread callout from your drawing. See what each part means: series, size, pitch, class, external or internal, hand, and how a pipe thread seals.

Reads Unified inch threads (UNC, UNF, UNEF, UN, UNR), ISO metric threads, NPT and NPTF, and British pipe threads R, Rc, Rp and G, with the old JIS names PT, PS and PF.

Type it as it appears on the drawing. Spaces, x or ×, and capitals do not matter, except in a metric class: 6g and 6H are different threads.

This page explains what a callout says. It does not check a part. A thread is accepted with a gauge of the right system and class; where a gauge and this page disagree, the gauge and the standard govern.

Questions

What does 2A mean on a thread callout?

2A is a thread class from ASME B1.1 for Unified inch threads. The number is the class: 1 is the loosest, 2 is the general-purpose class and the most common, 3 is the closest. The letter says which thread: A is external, B is internal. A 2A external thread is made slightly under basic size, an allowance that leaves room for plating; 3A and the internal B classes have no allowance. In 1/4-20 UNC-2A, 1/4 is the size in inches, 20 the threads per inch, UNC the coarse series and 2A the class.

What is the difference between 6g and 6H?

6g is a tolerance class for an external metric thread, 6H for an internal one, both from ISO 965-1. The number is the tolerance grade; 6 is the medium grade. The letter is the tolerance position, lower case for external threads and capital for internal. H starts the internal thread's tolerance zone at basic size; g puts the external thread's zone a small step below basic size, so a 6g bolt and a 6H nut always leave a small clearance. For M1.6 and larger, 6H/6g is the medium-quality pairing ISO 965-1 applies when a drawing states no class, and the usual choice for general-purpose nuts and bolts.

What is the difference between NPT and NPTF?

NPTF is the Dryseal version of NPT: it is designed to seal without a sealant. Both are 60° taper pipe threads with the same taper and the same threads per inch. NPT, under ASME B1.20.1, is made up wrench-tight with a sealant. NPTF, under ASME B1.20.3, controls the crests and roots so that they crush together on assembly and close the leak path. An NPTF thread is gauged with more than the L1 gauge used for NPT, and Class 2 NPTF also has its crest and root truncation inspected.

What do A and B mean after a G pipe thread?

They are the two tolerance classes for an external G thread under ISO 228-1. Class B allows twice the pitch-diameter tolerance of class A. An internal G thread has a single class and carries no letter: G 1/2 A is an external thread to class A, and G 1/2 is the internal thread. G threads do not seal on the threads; a pressure-tight joint seals on a gasket or O-ring between two faces.

What is the difference between R, Rc and Rp?

All three are ISO 7-1 pipe threads that seal on the threads, with a sealant. R is a taper external thread, Rc a taper internal thread and Rp a parallel internal thread. An R thread is made to go into either an Rc or an Rp. In older JIS callouts, still seen on drawings from Japan and Taiwan, R and Rc were written PT and Rp was written PS.

How is a left-hand thread shown in a callout?

With LH at the end of the callout, for example 1/2-13 UNC-2A-LH, M10x1-6g-LH or G 1/2 A LH. A callout without LH is a right-hand thread.

Decoding rules follow ASME B1.1 (Unified), ISO 965-1 (metric tolerance classes), ASME B1.20.1 (NPT), ASME B1.20.3 (Dryseal NPTF), ISO 7-1 (R, Rc, Rp), GB/T 7306 (R1, R2) and ISO 228-1 (G), checked against published references.

This is general information, not engineering advice. Thread standards are revised; check the edition named on your drawing, and confirm gauging requirements with your own quality department.

If you are sourcing a turned part with these threads, send the drawing.

Not sure which pipe thread you have? Try the Pipe Thread Finder.

To see the limits of size for a thread you have identified, use the Thread Size Lookup.