A bolt is a form of threaded fastener with an external male thread. Bolts are thus closely related to, and often confused with, screws.

Bolts vs. screws

The distinction between a bolt and a screw is commonly misunderstood. There are several practical differences, but most have some degree of overlap between bolts and screws. The defining distinction, per Machinery's Handbook, is in their intended purpose: Bolts are for the assembly of two unthreaded components, with the aid of a nut. Screws in contrast are used in components, and may cut their own internal thread channels. There is a large overlap between bolts and screws in function.

Bolts are often used to make a bolted joint. This is a combination of the nut applying an axial clamping force and also the shank of the bolt acting as a dowel, pinning the joint against sideways shear forces. For this reason, many bolts have a plain unthreaded shank (called the grip length) as this makes for a better, stronger dowel. The presence of the unthreaded shank has often been given as characteristic of bolts vs. screws, but this is incidental to its use, rather than defining. The grip length should be chosen carefully, to be around the same length as the thickness of the materials, and any washers, bolted together. Too short places the dowel shear load onto the threads, which may cause fretting wear on the hole. Too long prevents the nut from being tightened down correctly. No more than two turns of the thread should be within the hole.

Where a fastener forms its own thread in the component being fastened, it is called a screw. This is most obviously so when the thread is tapered (i.e. traditional wood screws), precluding the use of a nut, or when a sheet metal screw or other thread-forming screw is used.

Types of Fasteners

  • Hex Bolt: Hex bolts have hexagonal heads and machine threads for use with a nut or in a tapped hole. They are also commonly referred to as hex cap screws or machine bolts.
  • Stud Bolt: Stud bolts, a term used for cut-to-length all thread rod, are used for: bolting together flanges, anchor bolting, as well as general fastening.
  • Nuts: While heavy nut is a broader term used to describe a heavy pattern nut, we will focus on ASTM A194 nuts used with stud bolts. 
  • Washers: While washer is a broad fastener term used to describe a flat plate that distributes the load of a threaded fastener, we will focus on ASTM F436 washers.

Selection of bolt material

Regarding the requirement of the strength and circumstance, there are several types of material can be used for the Fastener Material Selection.

  • Steel Fasteners(Grade 2,5,8) - the level of strength
  • Stainless Steel Fasteners (Martensitic Stainless Steel, Austenitic Stainless Steel),
  • Bronze and Brass Fasteners - Water proof usage
  • Nylon Fasteners - it is used for the light material and water proof usage.
  • In general Steel is the most commonly used material of all fasteners(90%) above.

About Us

MSI Supply is a Houston, TX based full-line distributor of pipe, valves, fittings, flanges, stud bolts, gaskets, and fasteners, serving price-conscious customers who desire quick deliveries and consolidation of their material needs. With a large inventory of carbon and stainless pipe, cast and forged steel valves, tubing, and the thousands of fittings and fasteners required to put it all together, MSI can meet the needs of virtually any piping, fastener, or instrumentation project.

Also MSI is a full-line distributor of gate valves, globe valves, and check valves, with a large portfolio of: carbon steel, stainless steel, and exotic alloy. End connections for each gate valve, globe valve, and check valve include: flanged, threaded, and socket weld. With sizes ranging from 1/4” to 36", and pressure classes ranging from 150# - 2500#, MSI can supply the gate valves, globe valves, and check valves to meet the requirements of virtually any piping project.