For bolt heads, see screw drive systems.

For sizes of socket wrenches to fit hex-head bolts, see inch or metric page.

For threads, see index to fastener screw thread systems.

For grades, see inch or metric page.

For thread fit classes, see inch or metric page.

When is a bolt a screw?

An engineer's definition is that a bolt is held on by a nut, while the identical object, screwed into a threaded hole in a casting, would be called a screw¹. In the local hardware store big threaded fasteners are usually called bolts and the small ones are called “machine screws.” The U.S. Customs Service issues pamphlets on the subject² for its inspectors, since the tariff on a bolt has differed from that on a screw. In the United States, the Federal Cataloging System describes threaded fasteners 0.190 inch in diameter and larger as bolts, and everything smaller as screws (machine screws, not such screws as wood screws and self-tapping screws). In the same sentence in which it notes this fact³, the SAE states, “For the purposes of this document, the term bolt will be used for all sizes.” For the purposes of this website, we will follow SAE's lead.

In Britain, a bolt is a fastener that is threaded for only part of its length. A fastener threaded for its entire length is called a “set screw.” In the United States, a set screw is a small headless threaded fastener used to fix, for example, collets or pulleys on a shaft. In Britain, such screws are called “grub screws.” Certain other types of bolts are also traditionally called screws, notably the socket screw and cap screw.

1. But see for an impassioned defense of a reasonable definition. Unfortunately, as the author himself illustrates, the definition does not describe the actual usage of the words. Further, bolts existed centuries before any standards for bolts were established.

2. U.S. Customs Service. San Francisco District. Media Resource Center.
Is it a screw-- or is it a bolt? : an illustrated introduction to fastener standards / compiled by John Fitzgerald.
San Francisco : U.S. Customs Service, San Francisco District Media Resource Center, 1992.

The above publication has been superseded by a much wordier and more detailed 21-page publication:

U.S. Dept. of Homeland Security.
Distinguishing Bolts from Screws.
Washington: U.S. Customs and Border Protection, March 2009.

Available as a pdf online at

3. Web page for SAE standard AS1132G: , accessed 3 April 2006.

Some types of bolts

Carriage bolts

carriage bolt

A carriage bolt has a wide, domed head without a recess (so they can't be driven with a screwdriver), and most have a shank which is square for about ¼ inch from the head. As the name suggests, this style of bolt was originally used to build carriages. When the bolt is driven by a hammer into a hole in wood, the square shank prevents the bolt from turning while a nut (with washer) is spun on to the other end. Today they are used to assemble things like picnic tables.

Stove bolts

Stove bolts with square heads and nuts were very common as late as the 1950s, and even had their own series of threads. They can be replaced with ordinary bolts.

Lag bolts

drawing of a lag screw

Lag screws (as they should be called) are often referred to as “lag bolts,” though they have a gimlet point and aren't used with nuts. For sizes see lag screws.

Hanger bolts

drawing of a hanger bolt

Hanger bolts are threaded on one end and have a gimlet point on the other.

Eye bolts

drawing of an eye bolt

Eye bolts are threaded on one end, with an eye on the other.

Studs (as a type of bolt)

Studs are headless bolts, threaded on both ends, sometimes with different threads.

Cap screws

Currently manufactured cap screws are SAE grade 8.

Aerospace standards

The bolts used to assemble aircraft are made to higher standards than those used for cars; the Federal Aviation Administration does not permit aircraft to be assembled with SAE-graded bolts. There are three aerospace standards for bolts:

Some of these are truly superbolts.

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