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Identifying Taps

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

Identifying Taps

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-CAcme Thread-Centralizing
ACME-GACME-G Acme Thread-General Purpose
AMOAmerican Standard Microscope Objective Thread
ANPTAeronautical National Form Taper Pipe Thread (Ground thread tap marked NPT)
BABritish Association Standard Thread
BSFBritish Standard Fine Thread Series
BSPBritish Standard Pipe
BSPPBritish Standard Pipe (Parallel) Thread
BSPTBritish Standard Taper Pipe Thread
BSWBritish Standard Whitworth Coarse Thread Series
MMetric ScrewThread Series
NAmerican National 8, 12 and 16 Thread Series (8N, 12N, 16N)
N BUTTAmerican Buttress Screw Thread
NCAmerican National Coarse Thread Series
NEFAmerican National Extra Fine Thread Series
NFAmerican National Fine Thread Series
NGONational Gas Outlet Thread
NGTNational Gas Taper Thread (see "SGT")
NHAmerican National Hose Coupling and Fire Hose Coupling Threads
NPSFor tap marking only (See NPSC, NPSM)
NPSCAmerican National Standard Straight Pipe Thread in Pipe Couplings (Tap marked NPS)
NPSCDryseal American National Standard Fuel Internal Straight Pipe Thread
NPSFDryseal American National Standard Fuel Internal Straight Pipe Thread
NPSHAmerican National Standard Straight Pipe Thread for Hose Couplings
NPSIDryseal American National Standard Intermediate Internal Straight Pipe Thread
NPSLAmerican National Standard Straight Pipe Thread for Loose Fitting Mechanical Joints with Locknuts
NPSMAmerican National Standard Straight Pipe Threads for Free-Fitting Mechanical Joints for Fixtures (Tap marked NPS)
NPTAmerican National Standard Taper Pipe Thread (See ANPT, NPT)
NPTFDryseal American National Standard Taper Pipe Thread
NPTRAmerican National Standard Taper Pipe Thread for Railing Joints (Tap marked NPT)
NSAmerican National Thread-Special
PGPanzer Gewinde
PTFDryseal SAE Short Taper Pipe Thread
SGTSpecial Gas Taper Thread
SPL-PTFDryseal Special Taper Pipe Thread
STISpecial Thread for Helical Coil Wire Screw Thread Inserts
STUB ACMEStub Acme Stub Acme Thread
*UNUnified Constant Pitch Thread Series
*UNCUnified Coarse Thread Series
*UNEFUnified Extra Fine Thread Series
UNJ**Unified Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNJCUnified Coarse Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNJFUnified Fine Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNMUnified Miniature Thread Series
UNRUnified Constant Pitch Thread Series with a 0.108P to 0.144P Controlled Root Radius; Ext. thread only.
UNRCUnified Coarse Thread Series with a 0.108P to 0.144P Controlled Root Radius; Ext. thread only.
UNRFUnified Fine Thread Series with a 0.108P to 0.144P Controlled Root Radius; External thread only.
*UNSUnified Thread-Special
VA 60°"V" thread with Truncated Crest and Root. The theoretical "V" Form is usually flatted to the user’s specifications.
WHITBritish 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.