Nuts, Bolts and Washers

Nuts and Bolts are available in a wide variety of sizes, types of screw thread, shapes of head and materials to provide a very commonly used means of fastening components together such that they can be removed if needed.

A screw thread is one of the Basic Machines comprising a small thread cross-section wrapped around a cylinder as a Helix - which in itself can be simulated by wrapping a triangle around a cylinder. It follows that coarser threads have bigger thread cross sections and are therefore stronger, especially in softer materials, but fine threads follow the Helix traced by a more Acute angled triangle and grip tighter because they are a thinner Wedge or Inclined Plane - (e.g. the cylinder head Studs of a high revving engine may have fine threads at the top to resist vibration but coarse threads where they screw into the soft aluminium crankcase).

Machine Screws are very similar to Bolts except that, in general, Bolts have a plain section of shank below the head whereas Machine Screws are threaded right up to the head.

Sizes are now most commonly standardised in terms of their diameter in millimetres (e.g. a 10mm bolt has an external diameter of 10mm) although British Association (B.A.) threads may still occasionally be found in the smaller sizes of screws. Older, now largely obsolete, standards include British Standard Whitworth (i.e. B.S.W.; B.S.F.; B.S.C. and B.S.B.), British Standard Pipe (or B.S.P. in which the size number was originally based on the inner diameter of a steel tube for which the external thread was intended), and the American Unified Thread Standard (i.e. U.N.C. and U.N.F.) - which are still in use in USA and Canada although there is movement towards the increasingly universal ISO Metric standard.

Bolt and Screw Head Shapes

Hexagonal	These six-sided headed bolts and screws are now the most common in general use. Most Spanners and Sockets are designed to work with Hexagonal Nuts and Bolts
Square	These were more common before modern manufacturing techniques made hexagonal bolts and screws cost effective and hence preferred. They are still available and tend to be used for the more rugged applications.
Round	Round head machine screws provide a smooth finish less likely to snag loose clothing for example. Originally, all screws of all types had Slot Heads for use with a standard flat bladed Screwdriver but Phillips and Pozidrive type heads are now more common together with Socket Head types and various Tamper Resistant designs.
Cheese Head	Machine Screws with a cylindrical head of height approximately half the head diameter. They are a traditional screw used in precision work and, like Cap Screws, they can be sunk below the surface of the material in a Counterbored hole.
Countersunk	Abbreviated to Csk.Hd., these bolts and screws have a cone shaped head which allow them to sit within a Countersunk hole such that they can finish flush with the surface of the material.
Wing Bolts	B
Coach Bolts (aka Carriage Bolts)	B
Roofing Bolts	B

Cap Screws

Socket Head Cap Screws

Commonly referred to a Allen Screws, these precision cap screws are similar to Cheese Head Screws and used for similar purposes but they are often made from High Tensile Steel (H.T.S.) and use a Allen Key to tighten them.

Grub Screws

Grub Screws (aka Set Screws)

These headless screws are commonly used for holding components in place on rotating shafts for example. Originally with a Slot Head for use with a flat blade Screwdriver, the Socket Head or Allen Screw versions are now preferred

Types of Nut

Hexagonal	Their six-sided shape allows spanners and wrenches to be used with them in relatively confined spaces.
Square	B	[[File:###|125px|right]]
Locking Nut	B	[[File:###|125px|right]]
Wing Nut	B

Washers

Plain	B	[[File:###|125px|right]]|
Spring	B	[[File:###|125px|right]]
Repair	B	[[File:###|125px|right]]
Spring	B	[[File:###|125px|right]]

Studs

Studs	These can be made from Threaded Rod or purpose made - sometimes with a different thread at each end (a coarse thread at one end for entry into a material coupled with a fine thread at the other end for greater vibration resistance).
Screw Stud	Variously known as Hanger Screws or Dowel Screws, these studs have a screw thread at one end and a wood screw at the other to faciliate the task of joining wood to components of various materials.