WW/AF/UNC/UNF confusion.

[JimK]

I have grasped that there is a set of thread and spanner sizes called Whitworth. I've just bought a set of eight combination spanners for £9, so I'm happy.

However, I can't work out if/how AF, UNC and UNF are related, and what size spanners I need. Can somebody help?

Ta.

[lowedb]

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.

[JimK]

Brilliant, thanks. I'll go and get that set of 12 AF spanners for £7 then.

Is a WW spanner sized according to the thead? It's clearly not the head size.

[bigginger]

W seems to be labelled by some secret engineers system involving magic and/or witchcraft. An engineer will tell you that it's "perfectly logical" and brilliant, but like the rules of tennis, it's impossible to comprehend.

[alex_holden]

It is rather complicated and somebody will probably be along in a minute to correct the bits I get wrong.

Joseph Whitworth developed the first standardised bolts, and the standard was called British Standard Whitworth or BSW. The standard specified both the thread sizes and the sizes of the bolt heads, so a 1/4" Whitworth spanner is a spanner designed to be used with bolts that have a 1/4" Whitworth thread. The head sizes are rather odd measurements and were chosen based on the strength of steel used for making bolts at the time.

BSW is a bit too coarse (too few turns per inch) for many applications, so British Standard Fine (BSF) was developed. It has a finer thread but uses the same spanners as BSW, so a 1/4" BSW spanner will also work on a 1/4" BSF bolt.

BSW and BSF don't go down to really small sizes, so there was a third standard called British Association (BA) developed for small screws, and they have their own spanner sizes. BA is numbered backwards - the biggest one is size 0, and above about 8 or 9 is very small indeed.

There's yet another standard called British Standard Pipe (BSP) which is used for pipe fittings - the thread form is optimised for screwing together hollow things as opposed to solid bolts. I'm not totally sure what the situation is with the spanner sizes for BSP fittings, as I've seen at least three different sizes used on 1/4" BSP fittings, but I suspect they're supposed to use Whitworth spanners. BSPT is a tapered version of BSP that's used in applications where the thread itself needs to provide a good seal.

I think America had their own equivalents of BSW and BSF (ANC and ANF), but after the Second World War, various people agreed it was a bit silly the way mechanics had had to carry two sets of tools around to work on allied equipment, and they agreed on the Unified Thread Standards (UTS) which included a coarse thread (UNC) and a fine thread (UNF). The bolts which used these threads were to have more sensible head sizes that were common fractions of an inch. AF isn't a thread type, it's "Across Flats" and is just a way of measuring the size of a bolt head, so UNF and UNC bolts have head sizes that are specified by measuring them Across the Flats instead of relating them to the thread size as with Whitworth and BA.

The "new" pair of standards that the whole world is moving over to is ISO metric coarse and metric fine, which use bolt heads that are sensible multiples of 1mm AF.

[JimK]

Great information, thanks.

I've copied it here for future reference, along with some information from older threads about where you are likely to find each type of thread on your car.

[HarryMango]

It has a finer thread but uses the same spanners as BSW, so a 1/4" BSW spanner will also work on a 1/4" BSF bolt.

Not quite - they use the same spanners but are 1 size different, so 1/4W is same spanner as 5/16BSF, 5/16W is same as 3/8BSF etc.

Rog

[bigginger]

AAAAAAAAAAAAAAAAGH!

[Kevin]

I am for one grateful for definitive answers, and grateful again when all the exception show up

[simmitc]

At least the 4 lb club spanner fits them all