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Spanners, Sockets and Wrenches.



AF is shorthand for 'Across Flats' - a reference to the distance across the flats of a hexagon nut or bolt head.

ISO metric spanners are sized according to the across flat format, for instance a 20mm open ended metric spanner actually has jaws that are 20mm wide.

Rounding off

Rounding off is where the jaws of a spanner have been forced over the corners of a nut or bolt head. The most common causes are through using the wrong size spanner or using a loosely set adjustable wrench.

Poor quality open ended spanners and adjustable wrenches can also cause rounding off; insufficient material around the jaws and/or poor materials can allow the jaws to flex open and be forced over the corners.

Finally, limited access may contribute to rounding off, if there is no direct access to the nut or bolt and the spanner or socket can not be perfectly located, slipping round the nut or bolt head is surprisingly easy.


This is simply a reference for the 12 point drive system found in ring spanners. Bi-hex as the name tends to imply, is simply two hexagons cut into the ring. We discuss this in more detail on the next page.

Time now to take a look at the different types of spanners sockets and wrenches.

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