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.