How to Determine Wrench Sizes for Bolts and Nuts: A Comprehensive Guide

How to Determine Wrench Sizes for Bolts and Nuts: A Comprehensive Guide

The table of contents

  • Determining the wrench number by the diameter of the fastener thread
  • Selecting Inch Wrenches
  • Wrench sizes for hexagonal socket screws

 

Anyone, even non-mechanically inclined, has to unscrew and tighten bolts and nuts in everyday life. To do this, most often we use a wrench, of which there are about fifteen varieties today. The classic and most common tool is a double-sided open-ended wrench, which can be found in the toolbox of every man. The dimensions of its working profiles are marked on its handle, for example: 7x8 or 17x19, etc. These numbers indicate the distance from one jaw to the other in millimeters.

size of wrench for bolt

Wrench size 24

In the technical manuals of fasteners, the size "turn-key" is indicated by the letter "S". But we do not see it on the fasteners themselves. Experienced mechanics can visually determine the size of the fastener's head to what size "wrench" should be taken for a bolt M10, M12 or M16? And inexperienced craftsmen can easily make a wrong choice. And when the connection is in a hard-to-reach place, you will have to pick up the tool by trial and error. If a small wrench simply will not fit, a large wrench with a loose fit can "eat away" the edges of the part and then further unscrewing can be problematic.

Sizes of wrench

Hex key marking

Determining the wrench number by the thread diameter of the fastener



The key size of a hexagonal bolt or nut is the distance between two parallel lying faces. You can quickly and accurately find it out, without resorting to a ruler or caliper, by the diameter of the threaded part, which is listed in the technical information for the fastener. The fact is that each standard thread diameter corresponds to a certain size of the working profile of the fastener - basic (normal), reduced and increased. Bolts with reduced and increased size of the head are much rarer.

According to the table, you can easily determine what size wrench is suitable for a particular bolt from M1 to M110.

Table 1. Wrench sizes for metric bolts and nuts.

Thread diameter, М                                                 Turn key size
 main S, мм  reduced S, мм  enlarged S, мм
    М1     3.2     -     -
    М1.2     3.2     -      -
    М1.4     3.2     -      -
    М1.6     3.2     -      -
    М2     4     -      -
    М2.5     5     -      -
    М3     5.5     -      -
    М4     7     -     -
    М5     8     -     -
    М6     10     -     -
    М7     11     -     -
    М8     13     12     -
    М10     17 (16)     14     -
    М12     19 (18)     17     21 (22)
    М14     22 (21)     19     24
    М16     24     22     27
    М18     27     24     30
    М20     30     27     32 (34)
    М22     32 (34)     30     36
    М24     36     32     41
    М27     41     36     46
    М30     46     41     50
    М33     50     -     55
    М36     55     50     60
    М39     60     55     65
    М42     65     60     70
    М48     75     -     75
    М52     80     -      80
    М56     85     -      -
    М60     90     -      -
    М64     95     -      -
    М68     100     -      -
    М72     105     -      -
    М76     110     -     -
    М80     115     -     -
    М85     120     -     -
    М90     130     -     -
    М95     135     -     -
    М100     145     -     -
    М105     150     -     -
    М110     155     -     -

 

Some modern ISO standards set other dimensions of hexagonal heads, in the table they are written in brackets.

The sizes of the hexagon socket are defined in the standard GOST 6424-73. When properly selected, the gap between the jaws of the tool and the faces of the fastener does not exceed 0.1 - 0.3 mm. 

Selecting Inch Wrenches

The size of the inch wrench is expressed in inches and refers not to the width of the wrench's shed, but to the diameter of the threads of the inch fasteners it is designed to work with. The hexagonal working profile of inch bolts and nuts differs in size from the metric standard by a fraction of a millimeter. Therefore, metric wrenches will either not fit on the faces of inch fasteners, or they won't fit at all. You need inch wrenches and sockets to work with it.

Table 2. UNC/UNF threads and key nut size.

Thread diameter (wrench size), inch Wrench nut size, inch Wrench nut size, mm
 1/4  7/16  11.11
 5/16  1/2  12.7
 3/8  9/16  14.29
 7/16  5/8  15.88
 1/2  3/4  19.05
 9/16  13/16  20.63
 5/8  15/16  23.81
 3/4  1 1/8  28.58
 7/8  1 5/16  33.34
 1  1 1/2  38.10
 1 1/8  1 11/16  42.86
 1 1/4  1 7/8  47.63
 1 3/8  2 1/16  52.39
 1 1/2  2 1/4  53.15
 1 3/4  2 5/8  66.68
 2  3  76.20
 2 1/4  3 3/8  85.73
 2 1/2  3 3/4  95.25
 2 3/4  4 1/8  104.76
 3  4 1/2  114.30

 

Wrench sizes for bolts with internal hexagon



In the mechanical engineering and instrumentation along with hexagonal fasteners are widely used bolts with cylindrical head, for screwing which the hexagonal angle wrench is used. They are made according to DIN 912 or GOST 11738-84, according to which each thread diameter must correspond to a certain size "under the key" (the distance between the opposite faces of the internal hexagon).

Crescent wrench with ball for Allen screws

Crescent wrench with ball for Allen screws

Compliance Table:

Thread, M  М4  М5  М6  М8  М10  М12  М14  М16  М18  М20  М22  М24  М27  М30  М33  М36
Turnkey size, mm  3  4  5  6  8  10  12  14  14  17  17  19  19  22  24  27

 Before you start disassembling a car, servicing a bicycle or repairing garden equipment, it's a good idea to make sure you have all the tools you need. It is a good idea to have a table of key sizes for bolts and nuts handy. When repairing old machinery often faces the problem of unscrewing damaged parts. In order to work with fasteners with worn-out sides you can purchase special twist heads AN0400120 at the "Krepkom" store and for removing screws with ripped-off heads we offer a set of thread extractors AG010048.

Bondhus crescent wrench

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