Taps are marked according to the Standard System of Marking Ground Thread Taps.
The standard mark consists of four groups of symbols. They contain information about the nominal size, the number of threads per inch or thread pitch, and the appropriate thread form symbol and pitch diameter or thread class symbol.
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So, what does this mark means? And how to read taps?
Let’s find out.
Nominal Size of Tap
The nominal size marking differs for the metric and SAE systems.
Threads Per Inch
Actual for SAE sizes.
Starts from 11.
Actual for Metric sizes.
Starts from 0.5 mm.
Thread Form Symbol
|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)|
|NBUTT||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
|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, NPTR)|
|NPTF||Dryseal American National Standard Taper Pipe Thread|
|NPTR||American National Standard Taper Pipe Thread for Railing Joints (Tap marked
|NS||American National Thread-Special|
|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 Thread|
|*UN||Unified Constant Pitch Thread Series|
|*UNC||Unified Coarse Thread Series|
|*UNEF||Unified Extra Fine Thread Series|
|*UNF||Unified 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.
|UNRF||Unified Fine Thread Series with a 0.108P to 0.144P Controlled Root Radius; External thread only.|
|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|
Pitch Diameter Limit
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;
– 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.
Pitch Diameter Limits for taps to 1″ diameter
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″ dia. are ground to a .0010″ tolerance on the pitch diameter.
For example, H4 (Basic plus .0010″ to Basic plus .0020″ ).
Metric taps generally have more manufacturing tolerance than .0005 to the minus side.
U1 = Basic minus .0005 = min. tap P.D.
D1 = Basic plus .0005 = max. tap P.D.
D2 = Basic plus .0010 = max. tap P.D.
D3 = Basic plus .0015 = max. tap P.D.
D4 = Basic plus .0020 = max. tap P.D.
D5 = Basic plus .0025 = max. tap P.D.
D6 = Basic plus .0030 = max. tap P.D
Screw thread classes are distinguished from each other by the amount of tolerance and allowance.
Class 1A and Class 1B. The combination of Class 1A for external threads and Class 1B for internal threads are intended to cover the manufacture of threaded parts where the quick and easy assembly is necessary or desired, and an allowance is provided to permit ready assembly.
Class 2A and Class 2B. The combination of Class 2A for external threads and Class 2B for internal threads designed for screws, bolts, and nuts, is also suitable for a variety of other applications. A similar allowance is provided which minimizes galling and seizure encountered in assembly and use.
It also accommodates, to a limited extent, plating, finishes, or coatings.
Class 3A and 3B: The combination of Class 3A for external threads and Class 3B for internal threads are provided for those applications where the closeness of fit and accuracy of lead and angle of thread are important.
These threads are obtained consistently only by the use of high-quality production equipment supported by a very efficient system of gauging and inspection.
And finally, a complete scheme of tap’s marking system.
Editorial Board of ToolsLaboratory.