As you can see, it is best to know the thread type of the nut and then choose the spanner size that will then give the best fit. Once you know the thread type and size of the nut, then it is simple to look up the correct spanner size. During World War II the smaller size hexagon was adopted more widely to save metal[8] and this usage persisted thereafter. Thus it is today common to encounter a Whitworth hexagon which does not fit the nominally correct spanner and following the previous example, a more modern spanner may be marked ​7⁄16 BS to indicate that they have a jaw size of 0.710 in and designed to take either the (later) ​7⁄16 BSW or ​7⁄16 BSF hexagon.[9][10][11] In engineering circles a Spanner is often called a wrench and is generally used to grip a nut or bolt while applying torque or put more simply to help you fasten and loosen nuts and bolts.

Trying to match your spanners and bolts can be a confusing and frustrating process. You hear a lot of phrases and words being thrown about, ‘M5’ this and ‘M12’ that, and you may not understand everything being said. This guide is here to help you, and hopefully give you a better understanding and put a stop to the confusion once and for all! Types of Spanners: A standard metric M6 nut will be 10 mm across the flats, a 1/4″ UNC is just under 11mm and a 1/4″ BSW just over 11mm. Attempting to use the incorrect (oversize) spanner could cause the spanner to slip, rounding off the edges of the hexagon.What Size Spanner do I need ?” is a common question. Theimmediate answer is another question: “ What type of thread does your nut and/or bolt have?” All of these spanners can be used for different professional and DIY tasks and each has their own benefits. Certain branches of industry used Whitworth fasteners with a smaller hexagon (identical to BSF of the same bolt diameter) under the designation "AutoWhit" or Auto-Whit[citation needed] [6] and this series was formalised by the British Engineering Standards Association in 1929 as standard No. 193, with the 'original' series being No. 190 and the BSF series No. 191.[7] Combination spanners have either a doubled ended traditional spanner, or a spanner and a ratchet. The old method of how you used to measure the size of a spanner used to be; the spanner size was named after the fastener size.

These are the original, 19th Century, coarse-threaded industrial bolts designed to hold locomotives together. Because of their coarse pitch, they are more prone to vibrating loose, so are little used on motorcars. Except for threading into aluminum (e.g. crankcase studs), where a coarse thread is less prone to stripping than a fine one. It turns out that, except for 1/2" (where the British use 12 threads per inch (tpi) and Americans use 13 tpi) the thread pitches for the rest are the same as for American Unified Coarse (UNC). However, the thread form is different; Whitworth = 55 degrees; UNC = 60 degrees. In spite of this, mismatched nuts and bolts mate nicely, so you're likely to find UNC bolts or studs where BSW should have been. Just to mix things up a little; for nuts and bolts with BSW or BSF threads, you will need a spanner marked with the diameter size of the thread e.g. a 1/4″BSW or BSF nut or bolt would require a spanner marked 1/4″BSW. METRIC

For example, a ½” fastener fits a spanner with a ½’’ head. Metric fasteners often use a different sizing system to metric spanners, which could cause confusion, as the fastener size relates to the thread size rather than the head size. Metric Size Whitworth fasteners with the larger hexagons to BS 190 are now often colloquially referred to as 'pre-war' size, even though that is not strictly correct. There are many types of threads available for nuts and bolts, Metric, Unified (UNF/UNC and British Standard (BSW/BSF) are just three that we deal with, and they all have different measurements depending on the international standard they are manufactured to. For example, a metric M6 nut and bolt are very close to an imperial 1/4″ UNC/UNF thread. Measuring the thread manually you could be mistaken for thinking they are the same. Also, a 1/4″ BSW/BSF nut may sound similar but is different again in size.

Anyone out there that can explain the Whitworth sizing to me? I had read that the sizing on Whitworth spanners was to indicate the thread size and not the size of the hex-head so when I wanted to replace some fasteners with stainless I used the spanner size as a guide (1/4" spanner in this case). I also measured the hex flats (13 and a bit mm) and matched this to some tables that I found which indicated that this was 1/4" BSW. You should also be aware that the size given to any nut or bolt is relevant to the thread type and size. A metric M6 nut for example means the measurement ‘d’ shown above is approx. 6mm and this ‘M6 – 6 mm’ measurement bears no resemblance to the distance across the flats, or the spanner size required. In the 19th Century every British factory which needed to bolt something to something else devised their own fasteners to do it. Clearly, this caused all sorts of compatibility problems. So, along came Mr. Whitworth (I forget his first name right now) who invented a standardized system of coarse threads (with 55-degree thread angle and rounded roots and crests). This standardization was a Good Thing. Along with his threads came heads for the bolts that were based on the length along: the side of one flat, rather than across the flats. Hence, there is no simple fractional number for the length across the flats, which is why your imperial spanners don't fit. The fractional number on your spanners refers to the diameter of the bolt which is 1/4", 3/8" etc. not the distance across the flats (which ends up being various weird dimensions). Some years later the British decided they needed a finer pitch for some applications, so another thread series was introduced (same 55 degree angle). They also decided that the heads were too big for the bolts, so for most applications they switched to using the next size smaller heads. Because of this, and to add one more bit of confusion to life, one manufacturer will mark a particular spanner "3/8BS," while a different manufacturer will mark the same sized spanner "7/16W." They fit the same diameter bolt. The first thing any fledgling British car restorer learns is that his (or her) car has "Whitworth bolts." They think this is interesting, buy a set of "Whitworth spanners," discover these spanners fit their bolts, and believe they now know everything they need to know about British fasteners. Unfortunately, at this point they know only enough to make themselves dangerous. Instead, what they should have said to themselves is "Oh my God, what other weird and incomprehensible things have they done to the fasteners on my machine?" The answer to this question is:So - what is the correct across the flat measure for a 1/4" Whitworth nut? And if my spanner / tables are correct at 13.335mm across then why do all the suppliers think they should send me the equivalent of 3/16? For example if you were using a ¼’’ fastener size on a ¼’’ thread, the spanner would be called a ¼’’ Wide spanner. However the gap in the spanner would be 13.34mm wide, and ¼’’ is 6.35mm. Seems confusing, right? This method throws people off and only really applies if you’re using equipment and tools from pre 1970’s. Fastener Size The M stands for metric and the number after the ‘M’ refers to the width of the thread on the fastener in millimetres. The parts arrived today and both the set-screws and the nuts were smaller than the fasteners on my bike. I queried this with both suppliers and both insisted that these were indeed 1/4" Whitworth and that the across the flats measure on the 1/4" hex should be 11.3mm (a 3/16 Whitworth spanner size) SAE: Society of Automotive Engineers (American Standardoriginallyestablished for the automotive industry)