Spanners are divided into three main groups. Combination, Open Ended and Ring.
Box spanners are made from metal tubing and are usually double ended with a different size at each end. Box spanners are thinner than a socket and are much longer so that they can be used to loosen or tighten bolts and nuts in narrow recesses. The box spanner is used with a tommy bar which is passed through holes on the side of the spanner and acts as a handle. A box spanner of the relevant size can usually be used on a sparking plug.
The combination spanner is a combination of an open ended spanner and a ring spanner. This spanner is made with one size only per spanner as the ring and the open ended ends are the same size. he open end of the spanner is flat and the ring end s slightly cranked.
This spanner is designed to engage the nut from the ide and as it only engages on two faces of the nut, are must be taken to ensure that the correct size spanner is always used. Open ended spanners are usually double ended with a different size at either end.
As with open ended spanners the ring spanner is also double ended with a different size at either end. The ring spanner is designed to engage the nut from the top as it is totally enclosed. While the standard open ended spanner is flat, the ring spanner is cranked to enable the user to tighten or loosen bolts which are set into recesses.
Usually bihexagon (12 pointed), the ring spanner engages all six points of a hexagon nut giving a much stronger grip than the open ended.
Spanners - Sizes
On UNF, UNC and Metric the measurement of the spanner is said to be A/F or across flats. This means that the actual size of the bolt head is the measurement of the spanner. So that a 1/2in A/F spanner will fit a bolt head of UNF, UNC which will measure 1/2in. Metric is the same, a 10mm spanner will fit a 10mm across flats bolt head.
Whitworth is different. Here the size marked on the spanner is the size of the threaded part of the bolt. So that a 1/2in BSW spanner fits the thread of 1/2in BSW bolt, but the across flats size of the head is just under 1in (0.929in).
With BSF (BS) the same applies but the size of the head is 1/16in smaller than Whitworth up to 3/4in and 1/8in smaller for 7/8in upwards. You will therefore find spanners marked 1/2in BSW, 9/16in BS x 9/16in BSW, 5/8in BS or 3/4in BSW, 13/16 BS x 7/8in BSW, 1in BS.
Remember, that with Whitworth and with BS the sizes are the sizes of the threaded section of the bolt, not the A/F head sizes.
These are in most cases referred to 'across the flats' (A/F) sizes, i.e. the size across the flats of the hexagon nut or bolt head, and can be either in Metric or Imperial fractional sizes.
The exception to this is the old British standard size of bolts (BSW or BSF) where the size given is invariably the diameter of the bolt and not the head. As the BSF is a finer thread than BSW, BSF bolts need a smaller head than BSW to reduce the possibility of stripping. Thus for example a 3/8 BSF bolt has only the same head size as that of a 5/16 BSW.