Size conversion
A printed list of the exact metric sizes of imperial spanners, sockets & drill bits is handy. This is because some imperial tools make a good fit for metric nuts, bolts, etc, and vice versa (though of course many don't). A good example is 3/4", which is 19.05mm. A size conversion list can be stuck to the lids of socket, spanner & drill bit boxes.
Such a list
- saves needing to find a spanner/socket/drill bit already in use
- provides 2 of the same size tool for some sizes, which is occasionally very useful
- enables old imperial tools to replace missing metric ones
- avoids need to buy replacements sometimes
- enables some metric tools to be used on old imperial equipment.
- enables purchase & use of now very cheap old imperial tools
- effectively expands your metric drill bit collection to include fractional mm sizes
This is definitely not a recommendation to use the sizes of socket & spanner that don't fit well. Doing so can damage nuts & spanners, and injure hands when the tool slips off.
Size tables
AF
Most imperial sockets & spanners are measured AF, across flats. These are marked with the size, and no following letters. Conversion is a straightforward inch to mm conversion.
____ / \ AF means from / \ top to bottom \ / on this diagram \____/
64ths Inch mm Fits 1/64 0.40 2/64 = 1/32 0.79 3/64 1.19 4/64 = 1/16 1.59 1.5 5/64 1.98 2 6/64 = 3/32 2.38 7/64 2.78 8/64 = 1/8 3.12 3 9/64 3.57 10/64 = 5/32 3.97 4 11/64 4.37 12/64 = 3/16 4.76 13/64 5.16 14/64 = 7/32 5.56 5.5 15/64 5.95 16/64 = 1/4 6.35 17/64 6.75 18/64 = 9/32 7.14 7 19/64 7.54 20/64 = 5/16 7.94 8 21/64 8.33 22/64 = 11/32 8.73 23/64 9.13 24/64 = 3/8 9.52 9.5 25/64 9.92 26/64 = 13/32 10.32 27/64 10.72 28/64 = 7/16 11.11 11 29/64 11.51 30/64 = 15/32 11.91 31/64 12.30 32/64 = 1/2 12.7 33/64 13.1 34/64 = 17/32 13.49 35/64 13.89 36/64 = 9/16 14.29 37/64 14.68 38/64 = 19/32 15.08 15 39/64 15.48 40/64 = 5/8 15.87 41/64 16.27 42/64 = 21/32 16.67 43/64 17.07 44/64 = 11/16 17.46 17.5 45/64 17.86 46/64 = 23/32 18.26 47/64 18.65 48/64 = 3/4 19.05 19 49/64 19.45 50/64 = 25/32 19.84 51/64 20.24 52/64 = 13/16 20.64 53/64 21.03 54/64 = 29/32 21.43 55/64 21.83 56/64 = 7/8 22.22 22 57/64 22.62 58/64 = 29/32 23.02 59/64 23.42 60/64 = 15/16 23.81 61/64 24.21 62/64 = 31/32 24.61 63/64 25.0 64/64 = 1" 25.4mm 25
Sizes are only listed under the 'fits' column where the imperial size is in general use (or was). 29/32" may be a good metric fit, but no-one has such size tools.
Whitworth
The Whitworth system dates from 1841, and was the first non-proprietary bolt size system to be widely adopted. It describes bolt head sizes and bolt threads. There are 4 whitworth bolt designations, known as:
- W (Whitworth)
- BSW (BS Whitworth)
- BSC (British Standard Cycle)
- BSF (British Standard Fine thread)
BS and BSW sizes are often seen on old tools. In some countries they're still in common use, and Whitworth is still used here for some niche applications.
Whitworth sizes are not Across Flats measurements.
Whit BSW/BSF Inch AF mm
1/16 W - 0.256 6.90
3/32 W - 0.297 7.54
1/8 W 3/16 0.340 8.64
3/16 W 1/4 0.445 11.30
1/4 W 5/16 0.525 13.34 (Camera tripod mounting)
5/16 W 3/8 0.600 15.24
3/8 W 7/16 0.710 18.03
7/16 W 1/2 0.820 20.83
1/2 W 9/16 0.920 23.37
9/16 W 5/8 1.010 25.65
5/8 W 11/16 1.100 27.94
11/16 W 3/4 1.200 30.48
3/4 W 7/8 1.300 33.02
13/16 W 15/16 1.390 35.31
7/8 W 1. 1.480 37.59
1. W 1.1/8 1.670 42.42
1.1/8 W 1.1/4 1.860 47.24
1.1/4 W 1.3/8 2.050 52.07
1.3/8 W 1.1/2 2.220 56.39
1.1/2 W 1.5/8 2.410 61.21
1.5/8 W 1.3/4 2.580 65.35
1.3/4 W 2. 2.760 70.10
1.7/8 W - - 76.70
2.1/4 3.150 80.01
2. W
2.1/2 3.550 90.17
2.3/4 3.890 98.81
3. 4.180 106.17
3.1/4 4.530 115.06
3.1/2 4.850 123.19
3.3/4 5.180 131.57
4. 5.550 140.97
4.1/2 6.380 162.05
BA
BA Inch AF mm 8 BA 0.152 3.86 7 BA 0.172 4.37 6 BA 0.193 4.90 5 BA 0.220 5.59 4 BA 0.248 6.30 3 BA 0.282 7.16 2 BA 0.324 8.23 1 BA 0.365 9.27 0 BA 0.413 10.49 = 7/32 BS
Number & Letter
What size is ok?
Sockets & spanners
It may be helpful to first explain that spanners and sockets aren't actually the size marked on them. Rather they're a size which will fit over nuts of the marked size. A slight degree of slack between tool and nut is required, otherwise spanners would often not fit on, and would often be an impractically stiff fit. So tools are already calculatedly oversize.
The question then is how much oversize is ok. And its not simple to answer, as it depends on several factors. There should be just a little free play between tool and nut. If the gap is too big, the tool can damage the nut and slip off under pressure. The acceptable gap also depends on the nut size, for example a difference of 1mm is ok on a 1" nut, but entirely non-functional on a 3mm nut. Steel nuts are more tolerant of oversize tools than brass. And the acceptable oversize also depends on the torque to be applied; at very low torque levels large amounts of oversize are ok. And finally it sometimes depends on whether the tool in use is hex, bi-hex or parallel jawed.
Perhaps someone will come up with an engineering table for safe gap limits some day. Until then I can only say that if there is any noticable amount of rotation of the tool relative to the nut, its no good, so the acceptable tolerances are fairly small.
The one exception to that is where the torque applied is very low. If a socket is to be rotated directly by hand, with no bar, then generally anything that even loosely fits will do.
Drill bits
With some jobs, drill bit size must be precise. But for most DIY this is far from true, and simply selecting the nearest size is usually absolutely fine. Thus more or less all imperial drill bit sizes are good for re-use in a metric world.
Size is critical when drilling a hole in metal that will be tapped, and close substitutes should never be used for this.
Labelling
If you want to use old tools as a regular part of a set of metric tools, remarking them in mm makes life easier.
Painting sizes on tends to wear off in use, it doesn't work well.
A good option is a die grinder fitted with any small abrasive tool, which can be used to write on metal like a pen.
Another option is to paint the whole outside of the spanner/socket/etc, and put a blob of that colour paint on the hole where it fits in the toolbox. (Each size uses a different colour paint.)
Issues
Some vehicles use a mixture of bolt systems by design. Land Rovers and some other old designs are known for this. Until 1955 Morris and MG used metric threaded bolts with their heads made to fit Whitworth sizes.