Imperial are another range of sizes completely;tend to be found on older UK vehicles. Most people take imperial to be inch sizes which are AF. If you're thinking of Whitworth then you'll struggle to get a whit set for cheap money these days. Most AF sizes are useful for when the metric nut is rounded etc.
These bolts have a fine thread, with a different thread count from an American National Fine (also known as SAE). The BSF threads have an included angle as the Whitworth threads, with an included angle of 55° where as the American National Fine (SAE) threads have an included angle of 60°.
Society of Automotive Engineers (SAE) measurement3/8 in, 7/16 in, 1/2 in, 9/16 in, 5/8 in, 11/16 in, 3/4 in, 13/16 in, 7/8 in, 15/16 in, 1 in, 1-1/16 in, 1-1/8 in, 1-3/16 in, 1-1/4 in.
British Standard Whitworth (BSW) is an imperial-unit-based screw thread standard, devised and specified by Joseph Whitworth in 1841 and later adopted as a British Standard.
The most common Imperial standard in classics built after 1960 is the 'increments of one inch across flats' or 'AF' system. A good quality AF socket set will fit the vast majority of fittings on a British classic car.
Width across flats is the distance between two parallel surfaces on the head of a screw or bolt, or a nut, mostly for torque transmission by positive locking. The term width across flats (AF) is used for the following forms: 2-socket = round material with two surfaces.
These are both machine thread (and hex) standards based on fractional inch diameters. Whitworth is rarely used any more, while SAE is still in use, though metric hardware has become more common in most industries. Thread counts are different, hex sizes (nuts and bolt heads) are different sizes.
Imperial and Metric Spanner Sizes
| BOLT DIAMETER (INCHES) | AF SPANNER SIZE (INCHES) | SPANNER SIZE (MM) |
|---|
| 3/8 INCH | 9.52 MM |
| 1/4 INCH | 7/16 INCH | 11.11 MM |
| 5/16 INCH | 1/2 INCH | 12.7 MM |
| 3/8 INCH | 9/16 INCH | 14.29 MM |
Whitworth heads are NOT the same size as the equivalent UNC/UNF - they are larger. For this reason, it is imperative that a set of Whitworth wrenches are purchased before working on any antique British machinery. Otherwise, expect rounded-off heads and busted knuckles, trademarks of the careless craftsman.
In simple terms AF iis the size of spanner used on bolts / nuts with UNC and UNF threads. AF meaans acroos flats and is the imperial size of the head. Both UNC and UNF (Unified National Coarse / Fine) use AF heads.
BSF and BSW are Britisch measurement systems, while UNF and UNC are American measurement systems. BSF and UNF are the fine thread systems, while BSW and UNC are the coarse thread systems.
Modern spanner sizes refer to the distance across the flats of the hexagonal nut or bolt they would fit. A 17mm spanner therefore fits a nut 17mm across its flats and this is normally a nut to fit a 10mm diameter bolt (M10).
Wrench Size And Conversion Table
| Inches | Millimeters | Spanner |
|---|
| 1.024 | 26mm | 26mm |
| 1.063 | 27mm | 1 1/16 AF; 27mm |
| 1.100 | | 5/16 Wworth; 11/16 BSF |
| 1.125 | | 1 1/8 AF |
Metric Spanner Sizes
| Nominal Size | Spanner Size |
|---|
| M10 | 17mm |
| M12 | 19mm |
| M14 | 22mm |
| M16 | 24MM |
AF can stand for American Fractional; another term used for imperial. However, when applied to metric sockets, AF means 'Across Flats'. This means a 19mm AF socket will fit a nut or bolt head that measures 19mm across its flats.
SAE to Metric Conversion Chart
| SAE | Metric | Inch |
|---|
| 5mm | 0.197 |
| 5.5mm | 0.216 |
| 7/32" | | 0.219 |
| 6mm | 0.236 |
If the bolt size is up to M14,
- Spanner size =(Bolt size*1.5)+1.
- Eg. If Bolt size is M8, Then spanner size is (8*1.5)+1 =13.
- If bolt size is above M14,
- Spanner size = (Bolr size *1.5)
- Eg. If bolt size is M16, Then spanner size is (16*1.5) = 24.
BA size chart
| Screw Numbers | Diameter of Screw Over Thread | Size |
|---|
| 3 BA | 0.1610 inch | Number drill 29 3.4 mm 0.1339 inch |
| 2 BA | 0.1850 inch | Number drill 22 4 mm 0.1575 inch |
| 1 BA | 0.2090 inch | Number drill 16 4.5 mm 0.1772 inch |
| 0 BA | 0.2360 inch | Number drill 9 5.1 mm 0.2008 inch |
Conversion table inches to mm
| Dimensions — Inches to Metric | |
|---|
| 0.188” | 3/16” | 4.78 mm |
| 0.250” | 1/4” | 6.35 mm |
| 0.313” | 5/16” | 7.95 mm |
| 0.375” | 3/8” | 9.53 mm |
Metric / Standard Wrench Conversion Chart
| Bolt Diameter | Metric | Standard |
|---|
| 1/2" | 19mm | 3/4" |
| 9/16" | 21mm | 13/16" |
| 22mm | 7/8" |
| 5/8" | 24mm | 15/16" |
A Whitworth wrench is sized according to the diameter of the shank of the bolt, not the head. A 1/4 W (Whitworth) wrench is actually a bit larger than a 1/2″ American wrench—0.525″ to 0.500″. As if that wasn't enough, in 1924 it was decided that the heads of the Whitworth bolts were too large, so they were down-sized.
Both are considered Imperial,as well as a couple of others. Id be happily corrected on this. Yes, you are correct, A/F means 'across the flats' as opposed to the old Whitworth & BSW standards which were based on a proportion of the thread diameter (BSW being the smaller proportion of the 2).
