A bolt is a bolt right?
Not quite.
For anyone who has watched the classic Disney animation film Cars you will know that Mater with his encylopedic knowledge of cars is able to identify a car make and model simply from a picture of the engine bay, and significantly by identifying 'them old Whiteworth bolts' used in the engine cylinder head.
Although most bolts (and corresponding nuts!) look very similiar on first look the world of vintage and classic car bolts is more complex with numerous different thread types and standards - depending on the age and coutry of manuafacture of the car.
In this guide we help to demistify them...
Thread sizes of nuts and bolts
The nuts and bolts on 'old' old classic british cars often have American (also called Unified) threads for bolts of 1/4 inch diameter upwards. The size of the hexagon bolt-head, or the nut, is determined by the diameter of the bolt thread itself, the sizes of which increase in 1/16- inch steps.
The spanners to fit them are called AF (across flats) as the size of the nut or bolthead is the distance between the opposing flat sides.
Whitworth Bolts vs. Standard Fine
Cars made before 1951 had nuts and bolts of one of two standard threads—British Standard Fine (BSF) and British Standard Whitworth (BSW).
The Whitworth bolt was one of the first standards in thread design pioneered by Josepth Whitworth in 1841 as a result of mass production of industrial goods and shipbuilding. He mandated a thread density, angle and pitch to make interchangability and reusability key. Read more about Whitworth bolts on Wikipedia for some light bedtime reading
BSF and BSW identified nut and bolt British Standard Fine (BSF) bolts have good locking quality and core strength due to the shallow pitch between the threads sizes in different ways. For example, a bolt taking a 7/16- inch BSF spanner would take a 3/8inch Whitworth spanner. But since 1951—although spanners are still marked with both sizes—Whitworth & BSF have identified bolt sizes in the same way.
Smaller threads on British cars, particularly in the electrical equipment, are usually British Association (BA) threads. The biggest, 0BA, fits a bolt of 0-236 inch diameter, and a 6BA one of 0-110 inch.
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Metric bolts and non-British classic cars
'Continental' cars (as you may see they referenced in old service manuals) have metric threads, which will eventually be used exclusively in Britain. Although some components are already metricated, the changeover was a very gradual process taking many years for mainstream adoption.
British Standard Whitworth (BSW or Whit) have fewer but deeper threads than BSF and are better for soft metals and cast iron
British Association (BA) have a limited Unified Coarse (UNC) and Unified Fine range of diameters—smaller than BSW or (UN F) are not unlike BSW and have a hex-BSF—and are used for electrical work agonal bolt for American Fine (AF) spanners
Metrics vs. Imperial Bolt Sizes - A note on interchangeability
Metric bolts have different threads AND pitches from most other bolts and, except for some BA sizes, and as such are NOT interchangeable. Close attention needs to be paid to identification and correct usage as attemping to use the wrong type in fittings or soft metals will lead to damage and component failure.
What's a bolt without a Spanner
The size marked on the spanner for BSF, BSW and BA bolts is determined by the diameter of the bolt itself. For UNC and UNF bolts, the AF spanner size is the distance between the opposing flat sides.
If your in any doubt - consult the service or workshop manual for your beloved vintage classic
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