What Are Whitworth Tools?
Article by Mark Trotta
Whitworth is not a tool brand, it is a nut and bolt sizing. They are found on classic and vintage British cars and motorcycles.
Wrenches and sockets needed to remove and install Whitworth fasteners are differently sized than fractional or metric ones.
Cars That Have Whitworth Fasteners
Beginning somewhere in the 1940's, and into the 1970's, British vehicles were assembled from the factory with Whitworth-sized nuts and bolts. These include Austin, Jaguar, Land Rover, MG, Morris Minor, Triumph, and original Mini Coopers.
By the 1980's, they were nearly all replaced with metric, but each manufacturer did so in their own time, so there is no precise cut-off date.
British motorcycles that have Whitworth fasteners include Ariel, BSA, Matchless, Norton, Triumph, and Vincent.
There are several classic American cars, such as Nash Healey and Nash Metropolitan, that have them as well.
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Whitworth/American/Metric
On American wrenches and sockets (referred to as fractional or AF), size is noted using fractions of an inch (examples: 1/2", 9/16", etc).
NOTE: The letters AF indicates Across Flats, although some people will argue it's "American Fractional". In either case, it refers to the distance of the jaws on an open-end wrench.
On metric bolts (MM), the head size is the actual wrench size (examples: 10mm, 13mm, etc).
Whitworth sizes (W) refer to the bolt diameter (examples: 1/2W, 9/16W, etc).
Although a few sizes coincidently happened to have AF or Metric equivalents, most are somewhere in between.
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All too often, American mechanics used wrong-sized wrenches when working on vintage British cars, leaving stripped nuts and bolts for the next guy. And to make matters worse, some fasteners may have been 'converted' to Fractional or Metric.
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Here's a few examples of differences:
A 5/16W bolt has a head-size that measures 15.2 millimeters across the flats. In fractions of an inch, that's .62" or 79/128ths of an inch. So neither metric or standard wrenches or sockets will fit correctly.
The flats on a 1/4W wrench measures 13.3 millimeters, or .525 inches. Again, neither metric or standard wrenches or sockets will fit correctly.
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Here's a link to a conversion chart for Whitworth to AF and Metric sizes.
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British Standard Whitworth
Throughout the 1800's and 1900's, the Brits had several standards for threaded fasteners. The most common is British Standard Whitworth, abbreviated as BSW, and commonly referred to as just plain 'Whitworth'.
There are also tools marked as British Standard (BS).
Are Whitworth and British Standard the Same?
As for wrench-flat sizes, Whitworth (W) and British Standard (BS) are the same, just labeled differently. For example, a 1/4W wrench is the same size as a 5/16BS (British Standard) wrench. Some brands will list both on the tool.
A Whitworth tool will have a 'W' stamped next to the number.
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What Are BA Threads?
For nuts and bolts smaller than 1/8W (.34 inches), the British adopted a Swiss Standard measurement and called it British Association Thread (BA). In contrast, Americans use machine screw sizing (6-32, 8-24, 8-32, etc) for fasteners 3/16" and smaller.
Here's an example of BA sizing:
In the picture below, the two small fasteners from my 1974 Norton Commando have a head size of 8.23mm, which in fractions measures 81/250. So, neither a metric or standard socket will fit correctly.
The correct size is a 2BA Whitworth.
The long bolt in the picture has a head size of 11.4mm or 57/128's of an inch.
The correct wrench/socket size is 3/16W.
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Different Thread Pitch
Not only are Whitworth bolt head sizes different, their thread pitches are different. The angle between the sides of the thread is 65 degrees, compared to 60 degrees for AF or metric threads.
In addition, the root and crest is radiused, whereas on AF bolts the thread is flat.
You can't accurately tell what bolt threads are by looking at them, although markings on the head (if any) may give you a clue.
Checking only the TPI (threads per inch) and shank (thread) diameter is not always a reliable way to identify what you have.
To insure accuracy, both thread angle and profile can be measured with a Whitworth thread pitch gauge.
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Mixed Sizings
Some British manufacturers had a combination of fastener sizings on the same vehicle.
During my Norton Commando build, I found that all the engine nuts and bolts were Whitworth, but everywhere else on the bike was Fractional.
The 10 nuts and bolts holding on the cylinder head require 3/16W and 1/4W sockets to remove and install.
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Where Can I Buy Whitworth Tools?
You won't find Whitworth tools at Lowes or Home Depot.
The tool truck guy doesn't have them either.
As of 2024, tool companies offering Whitworth sizings include Britool, King Dick, and Rolson (British companies), as well as Kokon (a Japanese brand), and Elora (a German brand).
Buying Tools Online
While shopping for tools, I visited several online sites that advertise Whitworth spanners and sockets. After emailing three companies to check availability, only one responded. The reply was, "Currently unavailable. We don't know when or if this item will be back in stock."
I never heard back from the other two companies.
So, before you pay for tools from an online site, get some sort of confirmation from the company first.
Of course, you can always order Whitworth Wrenches and Whitworth Sockets on Amazon.
I did find a set of good used Rolson wrenches on ebay.
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Notes and Misc
Spanners (British) and wrenches (American) are the same thing, it's just different terminology.
English-built Fords (Cortina, Concul, Zephyr, etc) do not use Whitworth or BSF.
As a workaround to buying original fasteners and specialty tools, I've heard that one or two companies offer Allen head conversion kits for select British vehicles.
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