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Thread Nomenclature:


English Sizes:

Threads are described by the major diameter of the thread followed by the number of threads per inch (TPI)


                 1/4-20:  .250” basic major diameter with 20 TPI


                  Each Machine Screw Number = .013” larger than .060”.


#0 tap = .060 + .000 = .060” major diameter

                                  #1 tap = .060 + .013 = .073” major diameter

#2 tap = .073 + .013 = .086” major diameter                                                                                                                   

Metric Sizes:

Threads are described by the major diameter of the thread followed by the pitch or spacing of the thread.


M6 x 1.0: 6mm basic major diameter with a pitch of 1.0mm.        


Major Diameter: 

The basic reference dimension and is not the actual dimension of the tap or the screw going in the hole.            


Thread pitch =     1                                                                                                                      



Pitch Diameter:                                                                               

                  The imaginary line that is half way up the height of the thread.  

                                                See Figure 3 in ANSI B1.13M.     


         Basic Pitch Diameter = minimum PD for an internal thread (Go Gage)                                     


English = Major Dia. - .64952/TPI              


#2-56   Basic Pitch Dia.  =  .086 - .64952/56 = .0744”

1/4-20 Basic Pitch Dia.  =  .250 - .64952/20 = .2175”


Metric = Major Dia. - .64952 x Pitch         

M6 x 1.0 Basic Pitch Dia. = 6 - .64952 x (1.0) = 5.350mm 

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Gaging Threads & Classes of fit                                         



There is only one thread specification.  This is for cutting threads and forming threads.


Pin Gages: These inspect the Minor diameter.


         Thread Plug gages: These inspect the Pitch Diameter and Major Diameter.


”Go Gage” = Minimum Pitch Diameter (Most of the time this is the Basic Pitch Diameter)


”No Go Gage” = Minimum Pitch Diameter + Tolerance (See Figure 6 in ANSI B1.13M)


“No Go" thread gage limits are established by classes of fit.  Tolerances for classes of fit are established by formulas based on the diameter of the thread and the pitch.  Fine pitch threads have tighter tolerances than coarse pitch threads. 


English Sizes: 2B and 3B are most common. 

2B is a loose class of fit (standard)                                                                           

3B is a tight class of fit


Metric Sizes:  6H and 4H are most common. 


6H is a loose class of fit (standard) – comparable to a 2B                                

4H is a tight class of fit – comparable to a 3B       

6g is for external threads

6G is an oversized internal thread


Unified Screw Thread series tables provide all this information.

Tap Tolerances:                                                                            


English taps are measured in "H" numbers.  1 H = .0005 in.                 


Metric taps are measured in "D" numbers.    1 D = .0005 in.                                                                  


The tap tolerance is expressed as a multiple of "H” or "D” numbers and is measured as an increase above the Basic Pitch Diameter (GO GAGE) of the thread size:  


H1 or D1 = basic pitch diameter + .0005

H2 or D2 = .0005 to .001   over basic                                                            

H3 or D3 = .001   to .0015 over basic  

H4 or D4 = .0015 to .002   over basic      

H5 or D5 = .002   to .0025 over basic

H6 or D6 = .0025 to .003  over basic         ****(H6 is not the same as 6H)****       


¼-20 2B: Class of fit

¼-20 H6: Tap Size

M6 x 1 6H: Class of fit

M6 x 1 D5: Tap Size

M6 x 1 H5: Tap Size  (Some customers specify metric taps with H limits) 

“H” Size Selection:


  • Cutting taps cut oversize and Forming taps form threads undersize due to springback.                                                                     


Therefore, forming tap H and D#'s are always 2-3 numbers higher than the same size cutting tap. 


Cutting taps will screw into the tapped hole by hand, and forming taps will not screw into the tapped hole by hand.    


This is also why as long as the forming tap is smaller than the NO GO Gage, there is almost no chance for the tap to make oversize threads.

  • Use our Balax recommended H numbers and hole size chart.\


The competition makes their taps too small.  Consequences:  poor tap life.

  • For soft materials or parts with thin walls that will have larger amount of springback, use higher recommended “H” numbers.  


  • For hard materials, use lower “H” numbers to reduce the amount of cold forming work.


  • If a part gages tight, increase the “H” number.  If the part gages loose, reduce the H”” number.                                                                                                                                                  


  • Plating:                                                                                        


Plating of parts after tapping requires a larger pretap H or D number in order to adjust for the buildup on the threads after tapping.  The formula to adjust the tap size is as follows:                                                                                                      


Add the additional H or D# to the standard tap H or D#.


Additional H#'s =    4 x plating thickness                                                                



Most plating problems are caused by excessive buildup in the threads after plating which ruins the thread gaging of the parts.  Small diameter threads and fine thread pitches are more difficult to plate because the threads tend to fill with material.  If customers are unsure about plating thickness they should try a range of H numbers to evaluate actual results.

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