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On this page we have listed dimensions for sizes of various thread forms which may help you identify those more awkward screws etc.

An SWG (standard wire gauge) comparison table is also listed below.

Sending us a sample for identification

Many customers send us samples to identify - we provide this service free of charge, and can almost always source matching nuts and screws. Samples should be sent to the address at the bottom of our home page
Please make sure that samples are very well wrapped - if they are sent in a plain paper envelope with
no additional packing they almost certainly will not reach us intact as the post office sorting machinery
will push the sample through the side of the envelope.
 
Please also make sure you stamp your package using the appropriate postage rate, taking note that any envelopes posted in the UK over 5mm in thickness must be stamped at 'large letter' rate, and over 25m thick at 'packet' rate.
If you need the sample returned, please include a stamps to the appropriate value. If you order items following our advice regarding your sample, the stamps will be returned to you.

You must also include an e mail address so we can contact you.

Further information on UK postage rates are available here

Some Explanation of Terms Used

These are intended to give some basic guidance to customers that may be unfamiliar with the technical details of the many screw thread forms available.

We could fill this page with masses of technical thread data, but some basic definition of the terms used in the table below will hopefully give enough guidance without putting you to sleep !

If you do require more in depth data, please do not hesitate to e mail us with your questions.

Please also remember that we are happy identify samples sent to us by post at no cost.

We now stock a range of taps and dies to match our metric, UNC, UNF and BA fixings range (other threads  also available on request) - please click here for details.

'Nom Dia'

This is the nominal diameter over the thread, i.e. the approx diameter over the thread peaks. In practice if measured very accurately, the diameter across the tips of the thread peaks will be very slightly less than the nominal diameter

'Pitch'

The distance between the peaks of adjacent threads. 25.4 divided by the pitch in millimetres figure will give the TPI figure.

'TPI'

A term often stated for 'imperial' threads  = Threads per Inch.    25.4 divided by the TPI figure will give the pitch dimension in millimetres.  American threads (UNC and UNF) are often quoted as the gauge size followed by the TPI figure, for instance No 4 UNC is often quoted as '4-40' (screw size No 4 with 40 TPI)

Tap Drill

The diameter of the drill required to give the correct pilot hole before tapping to the specified thread size and form. The sizes listed below are the standard size for maximum thread engagement. Larger sizes can be used for increased tap life  but reduced thread engagement. Our drill bits page lists alternative sizes for 65% engagement for non critical applications.

The information below is given in good faith, however we accept no responsibility for any consequences of its use.

Metric Coarse Pitch Metric Fine Pitch
Size Nom Pitch Tap Drill Size Nom Pitch Tap Drill
ref Dia (mm) (mm) Dia (mm) ref Dia (mm) (mm) Dia (mm)
M1 1.0 0.25 0.75 M2 2.0 0.45* 1.55
M1.6 1.6 0.35 1.25 M2.3 2.3 0.4 1.9
M2 2.0 0.4 1.6 M2.6 2.6 0.45 2.15
M2.5 2.5 0.45 2.05 M3 3.0 0.35 2.65
M3 3.0 0.5 2.5 M3 3.0 0.6* 2.4
M3.5 3.5 0.6 2.9 M4 4.0 0.5 3.5
M4 4.0 0.7 3.3 M4 4.0 0.75* 3.25
M5 5.0 0.8 4.2 M5 5.0 0.5 4.5
M6 6.0 1.0 5.0 M5 5.0 0.75 4.25
M8 8.0 1.25 6.8 M6 6.0 0.75 5.25
M10 10.0 1.5 8.5 M8 8.0 0.75 7.25
M12 12.0 1.75 10.2 M10 10.0 1.25 8.75
M16 16.0 2.0 14.0 M12 12.0 1.5 10.5
          Please note that this is not an exhaustive list and other pitches are available
 for some diameters.
Pitches marked with a * are actually coarser than the metric coarse equivalent
size - this is not an error!
Unified Coarse (UNC) Unified Fine (UNF)
Size Nom Pitch Tap Drill Size Nom Pitch Tap Drill
ref Dia (mm)* (mm)* Dia (mm) ref Dia (mm)* (mm)* Dia (mm)
1-64 1.854 0.397 1.6 0-80 1.524 0.318 1.4
2-56 2.184 0.454 1.8 1-72 1.854 0.353 1.7
3-48 2.515 0.529 2.1 2-64 2.184 0.397 2.0
4-40 2.845 0.635 2.3 3-56 2.515 0.454 2.3
5-40 3.175 0.635 2.65 4-48 2.845 0.529 2.6
6-32 3.505 0.794 2.8 5-44 3.175 0.577 2.9
8-32 4.166 0.794 3.5 6-40 3.505 0.635 3.0
10-24 4.826 1.058 3.9 8-36 4.166 0.706 3.5
12-24 5.486 1.058 4.5 10-32 4.826 0.794 4.1
1/4-20 6.35 1.27 5.1   12-28 5.486 0.907 4.7
        1/4-28 6.35 0.907 5.5
* Dia and pitch converted from inches to 3dp, 1" = 25.4mm * Dia and pitch converted from inches to 3dp, 1" = 25.4mm
British Standard Whitworth (BSW) British Standard Fine (BSF)
Size Nom Pitch Tap Drill Size Nom Pitch Tap Drill
ref Dia (mm)* (mm)* Dia (mm) ref Dia (mm)* (mm)* Dia (mm)
1/16" 1.588 0.423 1.2 3/16" 4.763 0.794 4.0
3/32" 2.381 0.529 1.9 7/32" 5.556 0.907 4.6
1/8" 3.175 0.635 2.55 1/4" 6.350 0.977 5.3
5/32" 3.969 0.794 3.2 9/32" 7.144 0.977 6.0
3/16" 4.763 1.058 3.7 5/16" 7.938 1.155 6.8
7/32" 5.556 1.058 4.5 3/8" 9.525 1.27 8.3
1/4" 6.350 1.270 5.1 7/16" 11.113 1.411 9.7
9/32" 7.144 1.27 5.8 1/2" 12.70 1.588 11.1
5/16" 7.938 1.411 6.5 9/16" 14.288 1.588 12.7
3/8" 9.525 1.588 7.9          
7/16" 11.113 1.814 9.3        
1/2" 12.70 2.117 10.5          
9/16" 14.288 2.117 12.1          
British Association (BA) Metric Spark Plug Threads
Size Nom Pitch Tap Drill Size Nom Pitch Tap Drill
ref Dia (mm) (mm) Dia (mm) ref Dia (mm)* (mm)* Dia (mm)
12 1.3 0.28 1.05 M10SP 10.0 1.0 9.0
11 1.5 0.31 1.2 M12SP 12.0 1.25 10.75
10 1.7 0.35 1.4 M14SP 14.0 1.25 12.75
9 1.9 0.39 1.55 M18SP 18.0 1.5 16.5
8 2.2 0.43 1.8        
7 2.5 0.48 2.05 Metric Conduit Threads
6 2.8 0.53 2.3 Size Nom Pitch Tap Drill
5 3.2 0.59 2.64 M16C 16.0 1.5 14.5
4 3.6 0.66 3.0 M20 20.0 1.5 18.5
3 4.1 0.73 3.4   M25C 25.0 1.5 23.5
2 4.7 0.81 4.0 M32C 32.0 1.5 30.5
1 5.3 0.9 4.4          
0 6.0 1.0 5.1          

 

Dimensions for Metric Hex Nuts

Thread Size Thread Pitch A/F Dim A/C Dim Thickness
F G H
Max Min Nom Max Min
M1.6 0.35 3.2 3.02 3.41 1.3 1.05
M2 0.4 4 3.82 4.32 1.6 1.35
M2.5 0.45 5 4.82 5.45 2.0 1.75
M3 0.5 5.5 5.32 6.01 2.4 2.15
M3.5* 0.6 6 5.82 6.60 2.8 2.55
M4 0.7 7 6.78 7.66 3.2 2.9
M5 0.8 8 7.78 8.79 4.7 4.4
M6 1.0 10 9.78 11.05 5.2 4.9
M8 1.25 13 12.73 14.38 6.8 6.44
M10 1.5 16 15.73 17.77 8.4 8.04
M12 1.75 18 17.73 20.03 10.8 10.37
M14 2.0 21 20.67 23.35 12.8 12.1
M16 2.0 24 23.67 26.75 14.8 14.1

*Dimensions for M3.5 are as measured from stock samples

A/F (Across flats) dimensions are also valid for Nylocs and AerotightTM nuts

 

Wire Gauge Conversion Table

S.W.G Millimetres Inches
6 4.88 0.192
8 4.06 0.16
10 3.25 0.128
12 2.64 0.104
14 2.03 0.08
16 1.63 0.064
18 1.22 0.048
20 0.91 0.036
22 0.71 0.028
24 0.56 0.022
26 0.46 0.018
28 0.38 0.015
30 0.32 0.012

 

 

This page was last modified on : November 22, 2014

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