Bolting System

Fasteners with a non-tapered shaft
Bolt - There is not a universally accepted definition of the word, “bolt”. It generally refers to a larger threaded fastener with a non-tapered shaft.

Cap Screw (Hex Bolt) – In places the term is used interchangeably with bolt. In the past the term, “cap screw” was restricted to threaded fasteners with a shaft that is threaded all the way to the head, however this is now a non-standard usage.

Hex Cap Screw – Cap screw with a hexagonal head, designed to be driven by a wrench (spanner). An ASME B18.2.1 compliant cap screw has somewhat tighter tolerances than a hex bolt for the head height and the shaft length. The nature of the tolerance difference allows an ASME B18.2.1 hex cap screw to always fit where a hex bolt is installed but a hex bolt could be slightly too large to be used where a hex cap screw is designed in.

Hex Bolt - At times the term is used interchangeably with hex cap screw. An ASME B18.2.1 compliant hex bolt is built to different tolerances than a hex cap screw.

Socket Cap Screw – Cap screw with a hexagonal recessed drive (Allen), usually with a cylindrical head, but can also be found with a rounded button head or a countersunk flat head.

Machine screw - Generally a smaller fastener (less than 1/4 inch in diameter) threaded the entire length of its shaft that usually has a recessed drive type (slotted, Phillips, etc.).

Self Tapping Machine Screw – Similar to a machine screw except the lower part of the shaft is designed to cut threads as the screw is driven into an un-tapped hole. The advantage of this screw over a self tapping screw is that if the screw is reinstalled new threads are not cut as the screw is driven.

Set screw (grub screw) - Generally a headless screw but can be any screw used to fix a rotating part to a shaft. The set screw is driven through a threaded hole in the rotating part until it is tight against the shaft

Tap Bolt - A bolt that is threaded all the way to the head. An ASME B18.2.1 compliant tap bolt has the same tolerances as an ASME B18.2.1 compliant hex cap screw.

Stud - similar to a bolt but without the head. Studs are threaded on both ends. In some cases the entire length of the stud is threaded, while in other cases there will be an un-threaded section in the middle.

Eye Bolt – A bolt with a looped head.

Carriage Bolt (Coach Bolt) - Has a domed or countersunk head, and the shaft is topped by a short square section under the head. The square section grips into the part being fixed (typically wood), preventing the bolt from turning when the nut is tightened. A rib neck carriage bolt has several longitudinal ribs instead of the square section, to grip into a metal part being fixed.

Stove Bolt - Similar to a carriage bolt, but usually used in metal. It requires a square hole in the metal being bolted to prevent the bolt from turning.

Shoulder Screw - Screw used for revolving joints in mechanisms and linkages. The shaft of the screw is smooth and works as a bearing surface. There is a reduced diameter thread at the end of the cylindrical bearing surface.

Thumb Screw – A threaded fastener designed to be twisted into a tapped hole by hand without the use of tools.

Tension Control Bolt (TC Bolt) – Heavy duty bolt used in steel frame construction. The head is usually domed and is not designed to be driven. The end of the shaft has a spline on it which is engaged by a special power wrench which prevents the bolt from turning while the nut is tightened. When the appropriate torque is reached the spline shears off.