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Hex. Head Bolts

These are threaded for just part of the shank. The length of thread for BSW and BSF bolts is 2 times the diameter. For Unified bolts, it is generally 2 times diameter plus 1/4". It can be seen that these dimensions place a minimum limit on overall length of bolt (measured from under the head to the point of the bolt). The exception is the series of BSF Short Bolts. These have a specified length of unthreaded shank for each diameter.

Hex. Head Screws

A fastener is generally termed a screw if it is threaded up to (or very nearly to) the head. Confusion arises in the terminology of hex. head screws. The term "set screw" is in wide-spread usage in the UK. Previously, this term referred to square head screws. In the USA, "set screw" refers to headless screws, known as grub screws in the UK. The thread on a hex. head screw comes to within twice the pitch of the head underside.

Machine Screws

This term is usually applied to screws that are tightened with a screw-driver - slotted or pozi-drive heads in most cases. A range of head types is available including countersunk, round-head, cheese-head, etc. Machine screws are mostly produced in smaller diameters. In the USA, these screws may be used with machine-screw nuts and washers.

Coach Bolts

These are mostly of the cup-square variety. That is, a cup shaped head, and a square section of shank under the head. These bolts are used for joining wood together or to other materials. The square under the head presses into the wood and prevents the bolt from turning during tightening. They are usually supplied with matching nut, and these nuts used to be square. The threads were usually coarse, and the AC 2 Litre has a number of BSW coach bolts holding the wood frame on to the chassis.

Coach Screws

These are similar in appearance to wood screws, but larger, and with either a square or hex. head. Diameters are measured in inches or millimetres. Square heads are found on older imperial coach screws, such as those found on the AC.

Socket Screws

Also known as Allen screws (or internal wrenching screws in the USA). These have a hexagonal recess in the head for tightening with a socket spanner (AKA socket-wrench, key, Allen key). Socket screws are most usually made from high tensile steel and are widely used on workshop machinery such as lathes. They have also been widely used in motor-sport, military applications and aerospace, although these are made to a much higher standard than commercial fasteners. There are three common head types: Cap-head (similar to cheese-head), button-head and countersunk-head. There are also headless socket-screws known as socket grub-screws in the UK, and socket set-screws in the USA.

Black Bolts and Nuts

The term "black" in this context does not refer to colour. It refers to fasteners made to a low tolerance on dimensions, of mild steel, and without any surface finishing processes. These fasteners are used for light duty construction work, and include coach bolts and a variety of special fasteners for the building trade, as well as hex. bolts. Sizes of black nuts and bolts tend to be larger than those found on motor cars, going up to at least 4 inches diameter.

Black bolts are usually supplied with matching nuts. These matching sets are referred to by a combination of head type, shape of shank under the head, and the shape of the nut. For example, cup-square-square is a coach bolt with a cup-head, square section under the head and a square nut. Hex. round hex. is also abbreviated to XOX.

Classes of Fit

Each thread series includes two or more options for fit. That is, how closely the nut and bolt threads fit each other. For BSW, BSF, BSCy and BSP (Parallel), there are 3 classes of fit: Close, Medium, and Free. For BA there are two classes: Close and Normal. For the old American threads, there are 5 classes: 1 to 5 (5 being the closest, as used for tight fitting studs screwed into hard materials). Unified threads have 3 classes of fit: 1A to 3A for screws/bolts and 1B to 3B for nuts. 3A and 3B are the closest fitting of that series. ISO Metric also has 3 classes of fit specified in the UK: Close, Medium and Free.

<< Page 1 ****** Page 3 >>

Page 1 - Screw Threads

Page 3 - Nuts

Page 4 - Nuts (Self-Locking)

Page 5 - Washers

Page 6 - Materials and Strength Grades

Page 7 - Tightening

Page 8 - Wood Screws and Nails

Page 9 - Fasteners for the AC 2 Litre Saloon



Website started 29th December 2006