How to identify various markings on a North American Tool tap:

PITCH DIAMETER LIMIT SYMBOLS

All standard ground thread taps are marked with the letter “G” to designate Ground Thread. The letter “G” will be followed by the letter “H” to designate above basic; or the letter “L” to designate below basic. The number following “H” or “L” signifies the number of .0005″ steps above or below the basic pitch diameter.

For instance, the tap pictured above is a 3/8″ dia. tap with 20 threads per inch (pitch) and has a NS (American National Special Thread) thread form. The tap is made from High Speed Steel, and the GH-3 pitch diameter limit symbol indicates a Ground Thread tap with pitch diameter limits .0010 to .0015 over basic.

Pitch Diameter Limits for taps to 1″ diameter inclusive:

L1 = Basic to Basic minus .0005
H1 = Basic to Basic plus .0005
H2 = Basic plus .0005 to Basic plus .0010
H3 = Basic plus .0010 to Basic plus .0015
H4 = Basic plus .0015 to Basic plus .0020
H5 = Basic plus .0020 to Basic plus .0025
H6 = Basic plus .0025 to Basic plus .0030

Taps larger than 1″ diamter are ground to a .0010″ tolerance on a pitch diameter and are, for example:

H4 (Basic plus .0010″ to Basic plus .0020″).

THREAD FORM SYMBOL

ACME-C Acme Thread-Centralizing
ACME-G ACME-G Acme Thread-General Purpose
AMO American Standard Microscope Objective Thread
ANPT Aeronautical National Form Taper Pipe Thread (Ground thread tap marked NPT)
BA British Association Standard Thread
BSF British Standard Fine Thread Series
BSP British Standard Pipe
BSPP British Standard Pipe (Parallel) Thread
BSPT British Standard Taper Pipe Thread
BSW British Standard Whitworth Coarse Thread Series
M Metric ScrewThread Series
N American National 8, 12 and 16 Thread Series (8N, 12N, 16N)
N BUTT American Buttress Screw Thread
NC American National Coarse Thread Series
NEF American National Extra Fine Thread Series
NF American National Fine Thread Series
NGO National Gas Outlet Thread
NGT National Gas Taper Thread (see “SGT”)
NH American National Hose Coupling and Fire Hose Coupling Threads
NPS For tap marking only (See NPSC, NPSM)
NPSC American National Standard Straight Pipe Thread in Pipe Couplings (Tap marked NPS)
NPSC Dryseal American National Standard Fuel Internal Straight Pipe Thread
NPSF Dryseal American National Standard Fuel Internal Straight Pipe Thread
NPSH American National Standard Straight Pipe Thread for Hose Couplings
NPSI Dryseal American National Standard Intermediate Internal Straight Pipe Thread
NPSL American National Standard Straight Pipe Thread for Loose Fitting Mechanical Joints with Locknuts
NPSM American National Standard Straight Pipe Threads for Free-Fitting Mechanical Joints for Fixtures (Tap marked NPS)
NPT American National Standard Taper Pipe Thread (See ANPT, NPT)
NPTF Dryseal American National Standard Taper Pipe Thread
NPTR American National Standard Taper Pipe Thread for Railing Joints (Tap marked NPT)
NS American National Thread-Special
PG Panzer Gewinde
PTF Dryseal SAE Short Taper Pipe Thread
SGT Special Gas Taper Thread
SPL-PTF Dryseal Special Taper Pipe Thread
STI Special Thread for Helical Coil Wire Screw Thread Inserts
STUB ACME Stub Acme Stub Acme Thread
*UN Unified Constant Pitch Thread Series
*UNC Unified Coarse Thread Series
*UNEF Unified Extra Fine Thread Series
UNJ** Unified Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNJC Unified Coarse Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNJF Unified Fine Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNM Unified Miniature Thread Series
UNR Unified Constant Pitch Thread Series with a 0.108P to 0.144P Controlled Root Radius; Ext. thread only.
UNRC Unified Coarse Thread Series with a 0.108P to 0.144P Controlled Root Radius; Ext. thread only.
UNRF Unified Fine Thread Series with a 0.108P to 0.144P Controlled Root Radius; External thread only.
*UNS Unified Thread-Special
V A 60°”V” thread with Truncated Crest and Root. The theoretical “V” Form is usually flatted to the user’s specifications.
WHIT British Standard Whitworth Special Thread

*Taps are not marked with “U” but with the symbol for the corresponding American Standard thread form with which it is compatible.
** See UNJ Thread Form Overview for additional information on UNJ taps.

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