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What is a box spanner?

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Box spanners are made from metal tubing and usually have two profiles. Box (or ‘tubular’) spanners are made from metal tubing and often have two profiles, one at each end of the tube.
Box spanners fit over the top of the nut so they make contact with all flat sides and corners of fastener head. They cover the whole of the fastener head and make contact with them on all flat sides and corners.
Box spanner profile is for one size and shape only so only fits one type of fastener nut or bolt. The profile of a box spanner is very specific and cannot be used on other sizes or shapes. Most box spanners have hexagonal profiles as they are the most common type of fastener. Box spanner sizes are available in metric, imperial AF (across flats), BA and Whitworth sizes (see What fastener sizes are available? for more information on these sizes).
Turning a box spanner by twisting a tommy bar stuck through the holes in the metal tube. The tube-shaped spanner is turned using a ‘tommy bar’ or ‘T-bar.’ Tommy bars fit through holes drilled in the spanner so they are at right angles to the tube. The tommy bar can be turned like the handles of a tap, twisting the spanner and the fastener.
Box spanners come in a set or singly with or without tommy bars. If you are buying a set of box spanners, a suitable tommy bar will come with it. If you are buying a single box spanner you may need to buy a tommy bar separately. Check which tommy bar you need in the box spanner product description because the sizes vary between spanner sizes and manufacturers.
Tommy bar can be slid in and out to avoid obstruction and still turn the box spanner. Because of their long shape, box spanners are excellent for turning fasteners in hard-to-reach recesses, particularly in vehicle machinery and plumbing. Tommy bars are also great for confined spaces because they can slide in and out of the spanner to make use of the available to space.
Electrically insulated box spanner with fixed tommy bar. Insulated box spanners have fixed tommy bars which means they are permanently attached to the tool. This is necessary to ensure the spanner is full insulated against electrical current (see What is an insulated spanner?).
Tap back nuts are often fitted and removed using box spanners during plumbing. A common use for box spanners is on plumbing back nuts. Many box spanner sets include the most common sizes for back nut application.