AF spanners measure the width across flats of a bolt head, while Whitworth spanners are sized by the diameter of the threaded stud, so the same nominal size (e.g., 1/2 in) refers to different physical dimensions.
What does AF stand for on spanners?
AF stands for “Across Flats,” the distance between two parallel surfaces on a bolt or nut head.
Basically, this number tells you which spanner or socket will grip the bolt head properly. In the imperial world you’ll usually see AF sizes as fractions of an inch, whereas metric versions are quoted in millimetres. Knowing the AF size means you can pick the right tool without worrying about the thread type at all. Honestly, it’s one of the simplest ways to avoid stripping a bolt.
Source: Wikipedia
Are Whitworth and AF spanners the same?
No; Whitworth spanners are sized by the thread diameter, whereas AF spanners are sized by the width across flats of the bolt head.
The Whitworth system measures the thread diameter, so a 1/2‑in Whitworth spanner actually fits a different head than a 1/2‑in AF spanner. Because of that, the two sets aren’t interchangeable even though the numbers match. Bottom line: you’ll want to check which standard your fastener uses before you grab a tool. Honestly, mixing them up is a quick way to round off a bolt.
Source: Britannica
What are Whitworth spanners?
Whitworth spanners are tools sized to match the nominal diameter of a Whitworth thread, not the head width across flats.
Take a 1/4‑in Whitworth spanner: its opening matches that of a 5/16‑in BSF spanner, since the Whitworth label is all about thread diameter. As a result, a Whitworth set will cover the same range of BSF nuts and bolts, even though the numbers stamped on the tools are different. This was the go‑to system in UK workshops before metrication took over. Honestly, it’s a bit confusing at first, but once you see the pattern it makes sense.
Source: Wikipedia
Is Whitworth the same as imperial?
Whitworth is an imperial‑based thread standard, but not all imperial fasteners follow the Whitworth profile.
Imperial just means anything measured in inches, but Whitworth is a specific 55‑degree thread form that Joseph Whitworth introduced back in 1841. Besides Whitworth, you’ve got BSW, BSF, and the later UNC/UNF families. So yeah, Whitworth sits inside the imperial family as a subset. Honestly, calling all imperial threads ‘Whitworth’ is a common slip‑up.
Source: Britannica
What is 16mm in AF?
16 mm corresponds roughly to a 5/8 in AF spanner (0.625 in across flats).
If you do the math, 16 mm divided by 25.4 gives you about 0.6299 in, which most charts round to a 5/8‑in AF spanner. Different manufacturers might have tiny tolerances, but a 5/8‑in AF tool will usually seat nicely on a 16 mm hex head. Still, if you’re working to tight tolerances, you’ll want to double‑check the fit. Honestly, it’s worth a quick test fit before you apply any torque.
Source: Wikipedia
What does AF mean in sockets?
In sockets, AF denotes “Across Flats,” the measurement of the socket’s internal width that matches the bolt or nut head.
See a socket marked 19 mm AF? That means its opening is exactly 19 mm, so it’ll grip a hex head that measures 19 mm flat‑to‑flat. The AF label sets socket sizing apart from thread‑based systems such as Whitworth or SAE. Pick the right AF size and you’ll avoid rounding or slipping when you apply torque. Honestly, grabbing the wrong size is a fast way to strip a bolt head.
Source: Wikipedia
Are Whitworth threads still used?
Yes, Whitworth threads remain in niche applications such as vintage machinery, restoration projects, and certain pipe fittings.
Even though the standard was declared obsolete back in 1948 in favor of Unified and metric threads, you’ll still find Whitworth fasteners on plenty of older British engines, bicycles, and industrial gear. That’s why enthusiasts and heritage engineers keep Whitworth taps, dies, and spanners around. Swapping in a modern replacement usually isn’t straightforward without an adapter. Honestly, it’s satisfying to keep those old pieces running with the original hardware.
Source: Wikipedia
What does SAE stand for?
SAE stands for the Society of Automotive Engineers, a standards‑setting organization for the automotive and aerospace industries.
SAE started out in 1905 as the Society of Automobile Engineers, then widened its focus to cover aircraft and eventually became SAE International. These days the organization writes standards for everything from fuel viscosity to fastener dimensions, including the familiar SAE bolt grades. All that work helps keep parts compatible and safe in factories worldwide. Honestly, it’s hard to imagine modern manufacturing without their guidelines.
Source: SAE International
What is an AF thread?
There is no distinct “AF thread”; AF merely describes the width across flats of a fastener’s head.
AF isn’t a thread type at all; it just tells you how wide the fastener’s head is across the flats. Thread standards like UNC, UNF, or Whitworth deal with the helical ridge, whereas AF is purely about head size. So you could have an UNC‑threaded bolt whose head is sized in millimetres or inches AF. Mix them up and you’ll likely pick the wrong tool. Honestly, it’s an easy mistake to make when you’re new to spanners.
Source: Wikipedia
When did Whitworth stop being used?
Whitworth and its fine counterpart BSF were officially declared obsolete in 1948 after the UK, US, and Canada agreed to adopt Unified thread standards.
In 1948 the UK, US, and Canada agreed to phase out Whitworth and its fine‑pitch cousin BSF in favor of Unified threads, hoping to streamline postwar logistics and boost parts interchangeability. Even after the declaration, plenty of older machines kept their Whitworth fasteners, so the threads lingered in service for decades. These days you’ll mostly see them in restoration work or specialty engineering niches. Honestly, it’s kind of neat that a standard from the 1800s still shows up in hobby shops.
Source: Wikipedia
How do you identify Whitworth threads?
Identify Whitworth threads by measuring the nominal diameter and counting threads per inch (TPI), then comparing to Whitworth tables.
To spot a Whitworth thread, measure its nominal diameter and count the threads per inch, then check those numbers against a Whitworth chart. Unlike metric threads, Whitworth pitch is given in TPI, so a thread gauge or calibrated ruler works well for the count, while micrometers or calipers handle the diameter. When both match the chart, you’ve got a Whitworth thread — this also helps you tell it apart from the similar BSW or BSF varieties. Honestly, once you’ve done it a couple of times, the process feels second nature.
Source: Wikipedia
What is the difference between AF and Imperial?
AF is a measurement method (width across flats), while Imperial denotes any inch‑based system, which may use AF, thread‑based, or other sizing.
Think of imperial as the big family of inch‑based measurements, while AF is just one way members of that family size their heads — measuring the distance across the flats. So every AF size is imperial, but you’ll also find imperial parts sized by thread diameter, like a 1/4‑in BSW bolt. Knowing whether a spec is talking about head size or thread size saves you from grabbing the wrong spanner or bolt. Honestly, mixing those up is a frequent source of frustration in the workshop.
Source: Wikipedia
How do I know if my threads are metric or imperial?
Measure the pitch: metric threads use millimetres between crests, imperial threads use threads per inch.
The quickest way to tell is to look at the pitch. With a metric ruler or caliper, measure from one thread crest to the next; if the distance lines up with a whole or half‑millimetre, you’re dealing with metric. If the spacing matches a whole number of threads per inch — say, 20 TPI — then it’s imperial. Pairing the pitch with a diameter measurement gives you a pretty reliable ID. Honestly, it’s a handy trick that saves a lot of guesswork.
Source: Wikipedia
Is BSP a Whitworth thread?
BSP (British Standard Pipe) threads use a Whitworth‑type 55° thread form, but they are a pipe‑thread standard, not a fastener thread like BSW.
BSP threads do indeed use the same 55‑degree angle as Whitworth, but that’s where the similarity ends. BSP specs differ in diameter, taper, and purpose — they’re made for sealing pipe joints, not for fastening bolts like BSW. You’ll encounter the tapered (BSPT) and parallel (BSPP) versions mostly in plumbing and hydraulic setups. So while they share the Whitworth angle, BSP stands in its own category. Honestly, it’s easy to assume they’re interchangeable because of the angle, but the applications are quite different.
Source: Wikipedia
What size is a 9/16 spanner?
A 9/16 in AF spanner opens to 14.29 mm across flats, fitting a bolt with a 9/16 in head size.
In the AF line, the spanner size is literally the head width, so a 9/16‑in tool will fit a 9/16‑in hex or square head. Convert that to metric and you get 9/16 × 25.4 = 14.288 mm, which most charts round to 14.3 mm. You’ll see this size a lot on American‑made gear and on older British kit that uses AF sizing. Honestly, it’s one of those handy midsize spanners that shows up in just about every toolbox.
Source: Wikipedia
