Screw

Screws come in a variety of shapes and sizes for different purposes. U.S. quarter coin (diameter 24 mm) shown for scale.

A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the internal thread formed into it. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted. The most common uses of screws are to hold objects together and to position objects.

A screw will always have a head, which is a specially formed section on one end of the screw that allows it to be turned, or driven. Common tools for driving screws include screwdrivers and wrenches. The head is usually larger than the body of the screw, which keeps the screw from being driven deeper than the length of the screw and to provide a bearing surface. There are exceptions; for instance, carriage bolts have a domed head that is not designed to be driven; set screws have a head smaller than the outer diameter of the screw; J-bolts have a J-shaped head which is not designed to be driven, but rather is usually sunk into concrete allowing it to be used as an anchor bolt. The cylindrical portion of the screw from the underside of the head to the tip is known as the shank; it may be fully threaded or partially threaded. The distance between each thread is called the "pitch".

The majority of screws are tightened by clockwise rotation, which is termed a right-hand thread; a common mnemonic device for remembering this when working with screws or bolts is "righty-tighty, lefty-loosey." Screws with left-hand threads are used in exceptional cases. For example, when the screw will be subject to counterclockwise torque (which would work to undo a right-hand thread), a left-hand-threaded screw would be an appropriate choice. The left side pedal of a bicycle has a left-hand thread.

More generally, screw may mean any helical device, such as a clamp, a micrometer, a ship's propeller or an Archimedes' screw water pump.

Differentiation between bolt and screw

A carriage bolt with square nut

A structural bolt with a hex nut and washer.

There is no universally accepted distinction between a screw and a bolt. Machinery's Handbook describes the distinction as follows:

A bolt is an externally threaded fastener designed for insertion through holes in assembled parts, and is normally intended to be tightened or released by torquing a nut. A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head. An externally threaded fastener which is prevented from being turned during assembly and which can be tightened or released only by torquing a nut is a bolt. (Example: round head bolts, track bolts, plow bolts.) An externally threaded fastener that has thread form which prohibits assembly with a nut having a straight thread of multiple pitch length is a screw. (Example: wood screws, tapping screws.)

This distinction is consistent with ASME B18.2.1 and some dictionary definitions for screw and bolt.

The issue of what is a screw and what is a bolt is not completely resolved with Machinery's Handbook distinction, however, because of confounding terms, the ambiguous nature of some parts of the distinction, and usage variations. Some of these issues are discussed below:

Machine screws

ASME standards specify a variety of "Machine Screws" in diameters ranging up to 0.75 in (19.05 mm). These fasteners are often used with nuts as well as often driven into tapped holes. They might be considered a screw or a bolt based on the Machinery's Handbook distinction. In practice, they tend to be mostly available in smaller sizes and the smaller sizes are referred to as screws or less ambiguously as machine screws, although some kinds of machine screws can be referred to as stove bolts.

Hex cap screws

ASME standard B18.2.1-1996 specifies Hex Cap Screws that range in size from 0.25–3 in (6.35–76.20 mm) in diameter. These fasteners are very similar to hex bolts. They differ mostly in that they are manufactured to tighter tolerances than the corresponding bolts. Machinery's Handbook refers parenthetically to these fasteners as "Finished Hex Bolts". Reasonably, these fasteners might be referred to as bolts but based on the US government document, Distinguishing Bolts from Screws, the US government might classify them as screws because of the tighter tolerance. In 1991 responding to an influx of counterfeit fasteners Congress passed PL 101-592 "Fastener Quality Act" This resulted in the rewriting of specifications by the ASME B18 committee. B18.2.1 was re-written and as a result they eliminated the "Finished Hex Bolts" and renamed them the "Hex Cap Screw"—a term that had existed in common usage long before, but was now also being codified as an official name for the ASME B18 standard.

Lug bolts and head bolts

These terms refer to fasteners that are designed to be threaded into a tapped hole that is in part of the assembly and so based on the Machinery's Handbook distinction they would be screws. Here common terms are at variance with Machinery's Handbook distinction.

Lag bolt

Lag screws, also called lag bolts.

Another view.

Lag bolts, also called lag screws, are basically "large wood screws". A typical lag bolt can range in diameters from 1/4" to 3/4", and lengths from 1" to 24" or longer, with coarse threads of a wood-screw or sheet-metal-screw threadform (but larger). The head is typically an external hex. The materials are usually carbon steel substrate with a coating of zinc galvanization (for corrosion resistance). The zinc coating may be bright (electroplated), yellow (electroplated), or dull gray hot-dip galvanized. Lag bolts are used to lag together lumber framing, to lag machinery feet to wood floors, and other heavy carpentry applications. These fasteners are clearly "screws" when defined by the Machinery's Handbook distinction. The term "lag bolt" has been replaced by "lag screw" in the Machinery's Handbook. However, in the minds of most tradesmen, they are "bolts", simply because they are large, with external-hex heads. In the United Kingdom, lag bolts/screws are known as coach screws though this can also refer to Carriage Bolts (Round head).

Government standards

The US government made an effort to formalize the difference between a bolt and a screw because different tariffs apply to each. The document seems to have no significant effect on common usage and does not eliminate the ambiguous nature of the distinction between screws and bolts for some threaded fasteners. The document also reflects (although it probably did not originate) significant confusion of terminology usage that differs between the legal/statutory/regulatory community and the fastener industry. The legal/statutory/regulatory wording uses the terms "coarse" and "fine" to refer to the tightness of the tolerance range, referring basically to "high-quality" or "low-quality", but this is a poor choice of terms, because those terms in the fastener industry have a different meaning (referring to the steepness of the helix's lead).

Historical issue

Old USS and SAE standards defined cap screws as fasteners with shanks that were threaded to the head and bolts as fasteners with shanks that were partially unthreaded. This is now an obsolete distinction (but standard in UK).

Controlled vocabulary versus natural language

The distinctions above are enforced in the controlled vocabulary of standards organizations. Nevertheless, there are sometimes differences between the controlled vocabulary and the natural language use of the words by machinists, auto mechanics and others. These differences reflect linguistic evolution shaped by the changing of technology over centuries. The words bolt and screw have both existed since before today's modern mix of fastener types existed, and the natural usage of those words has evolved retronymously in response to the technological change. (That is, the use of words as names for objects changes as the objects themselves change.) Non-threaded fasteners predominated until the advent of practical, inexpensive screw-cutting in the early 19th century. The basic meaning of the word screw has long involved the idea of a helical screw thread, but the Archimedes screw and the screw gimlet (like a corkscrew) preceded the fastener.

The word bolt is also a very old word, and it was used for centuries to refer to metal rods that passed through the substrate to be fastened on the other side, often via nonthreaded means (clinching, forge welding, pinning, wedging, etc.). The connection of this sense to the sense of a door bolt or the crossbow bolt is apparent. In the 19th century, bolts fastened via screw threads were often called screw bolts in contradistinction to clench bolts.

In common usage, the distinction (not rigorous) is often that screws are smaller than bolts, and that screws are generally tapered while bolts are not. For example, cylinder head bolts are called "bolts" (at least in North American usage) despite the fact that by some definitions they ought to be called "screws". Their size and their similarity to a bolt that would take a nut seem linguistically to overrule any other factors in this natural word choice proclivity.

Other distinctions: Bolts have been defined as headed fasteners having external threads that meet an exacting, uniform bolt thread specification (such as ISO metric screw thread M, MJ, Unified Thread Standard UN, UNR, and UNJ) such that they can accept a nontapered nut. Screws are then defined as headed, externally threaded fasteners that do not meet the above definition of bolts. These definitions of screw and bolt eliminate the ambiguity of the Machinery's handbook distinction. And it is for that reason, perhaps, that some people favour them. However, they are neither compliant with common usage of the two words nor are they compliant with formal specifications.

Types of screws and bolts

Threaded fasteners either have a tapered shank or a non-tapered shank. Fasteners with tapered shanks are designed to either be driven into a substrate directly or into a pilot hole in a substrate. Mating threads are formed in the substrate as these fasteners are driven in. Fasteners with a non-tapered shank are designed to mate with a nut or to be driven into a tapped hole.

Fasteners with a tapered shank (self-threading screws)

	American name	British name	Description
	chipboard screw particle board screw		Similar to a drywall screw except that it has a thinner shaft and provides better resistance to pull-out in particle board, while offset against a lower shear strength. The threads on particle board screws are asymmetrical.
	concrete screw Tapcons masonry screw confast screw blue screw self-tapping screw Titen		A stainless or carbon steel screw for fastening wood, metal, or other materials into concrete or masonry. Concrete screws are commonly blue in colour, with or without corrosion coating. They may either have a Phillips flat head or a slotted hex washer head. Heads sizes range from 0.1875 to 0.375 in (4.763 to 9.525 mm) and lengths from 1.25 to 5 in (32 to 127 mm). Typically an installer uses a hammer drill to make a pilot hole for each concrete screw.
	deck screw		Similar to drywall screw except that it has improved corrosion resistance and is generally supplied in a larger gauge. Most deck screws have a type-17 (auger type) thread cutting tip for installation into decking materials.
	double ended screw dowel screw hanger bolt		Similar to a wood screw but with two pointed ends and no head, used for making hidden joints between two pieces of wood. A hanger bolt has wood screw threads on one end and machine threads on the other. A hanger bolt is used when it is necessary to fasten a metal part to a wood surface.
	drywall screw		Specialized screw with a bugle head that is designed to attach drywall to wood or metal studs, however it is a versatile construction fastener with many uses. The diameter of drywall screw threads is larger than the shaft diameter.
	eye screw screw eye		Screw with a looped head. Larger ones are sometimes called lag eye screws. Designed to be used as attachment point, particularly for something that is hung from it.
	lag bolt lag screw	coach screw	Similar to a wood screw except that it is generally much larger running to lengths up to 15 in (381 mm) with diameters from 0.25–0.5 in (6.35–12.70 mm) in commonly available (hardware store) sizes (not counting larger mining and civil engineering lags and lag bolts) and it generally has a hexagonal drive head. Lag bolts are designed for securely fastening heavy timbers ( post and beams, timber railway trestles and bridges) to one another, or to fasten wood to masonry or concrete. Lag bolts are usually used with an expanding insert called a lag in masonry or concrete walls, the lag manufactured with a hard metal jacket that bites into the sides of the drilled hole, and the inner metal in the lag being a softer alloy of lead, or zinc alloyed with soft iron. The coarse thread of a lag bolt and lag mesh and deform slightly making a secure near water tight anti-corroding mechanically strong fastening.
	mirror screw		This is a flat-head wood screw with a tapped hole in the head, which receives a screw-in chrome-plated cover. It is usually used to mount a mirror.
	sheet metal screw		Has sharp threads that cut into a material such as sheet metal, plastic or wood. They are sometimes notched at the tip to aid in chip removal during thread cutting. The shank is usually threaded up to the head. Sheet metal screws make excellent fasteners for attaching metal hardware to wood because the fully threaded shank provides good retention in wood.
	Twinfast screw		A Twinfast screw is a type of screw with two threads (i.e. a lead of 2), so that it can be driven twice as fast. Dry wall screws designated as fine are the most common screws to use the twinfast style of threads.
	wood screw		A metal screw with a sharp point designed to attach two pieces of wood together. Wood screws are commonly available with flat, pan or oval-heads. A wood screw generally has a partially unthreaded shank below the head. The unthreaded portion of the shank is designed to slide through the top board (closest to the screw head) so that it can be pulled tight to the board it is being attached to.
	Security head screw		These screws are use for security purpose. The head of this type of screw is impossible to reverse. It requires special tools or mechanisms like spanners, tri-wings, torxes, square drivers, etc. In some screws, the head can be removed by breaking it after installing the screw.

Fasteners with a non-tapered shank

	American name	British name	Description
	anchor bolt		A special type of bolt that is set in wet concrete, with the screw threads protruding above the concrete surface.
	breakaway bolt		A breakaway bolt is a bolt with a hollow threaded shank, which is designed to break away upon impact. Typically used to fasten fire hydrants, so they will break away when hit by a car. Also used in aircraft to reduce weight.
Narrow definition Wide definition	cap screw		The term cap screw refers to many different things at different times and places. Currently, it most narrowly refers to a style of head (see the gallery below). More broadly, and more commonly, it refers to the group of screws: shoulder screws, hex heads, counter-sunk heads, button heads, and fillister heads. In the US, cap screws are defined by ASME B18.6.2 and ASME B18.3. In the past, the term cap screw, in general, referred to screws that were supposed to be used in applications where a nut was not used, however the characteristics that differentiated it from a bolt vary over time. In 1910, Anthony defined it as screw with a hex head that was thicker than a bolt head, but the distance across the flats was less than a bolt's. In 1913, Woolley and Meredith defined them like Anthony, but gave the following dimensions: hex head cap screws up to and including 7⁄16 inches (11.1125 mm) have a head that is 3⁄16 inches (4.7625 mm) larger than the shank diameter; screws greater than 1⁄2 inches (12.7 mm) in diameter have a head that is 1⁄4 inches (6.35 mm) larger than the shank. Square head cap screws up to and including 3⁄4 inches (19.05 mm) have a head 1⁄8 inches (3.175 mm) larger than the shank; screws larger than 3⁄4 inches (19.05 mm) have a head 1⁄4 inches (6.35 mm) larger than the shank. In 1919, Dyke defined them as screws that are threaded all the way to the head. A socket cap screw, also known as a socket head capscrew, socket screw or Allen bolt, is a type of cap screw with a cylindrical head and hexagonal drive hole. The term socket head capscrew typically refers to a type of threaded fastener whose head diameter is nominally 1.5 times that of the screw shank ( major) diameter, with a head height equal to the shank diameter (1960 series design). Forged heat-treated alloy examples are high strength fasteners intended for the most demanding mechanical applications, with special alloy formulations available that are capable of maintaining strength at temperatures in excess of 1000 degrees F (587 degrees C). In addition to the 1960 series design, other head designs include low head, button head and flat head, the latter designed to be seated into countersunk holes. A hex key (sometimes referred to as an Allen wrench or Allen key) or hex driver is required to tighten or loosen a socket screw. Socket head capscrews are commonly used in assemblies that do not provide sufficient clearance for a conventional wrench or socket.
	carriage bolt		A carriage bolt, also known as a coach bolt, has a domed or countersunk head, and the shank 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.
	elevator bolt		An elevator bolt is a bolt similar to a carriage bolt, except the head is thin and flat. There are many variations. Some do not have a square base, but rather triangular sections of the flat head are folded down to form "fangs" that cut into wood and hold it secure.
	eye bolt		An eye bolt is a bolt with a looped head.
	hex cap screw hex bolt		A hex cap screw is a 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 shank 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.
	Fine adjustment screw		The term fine adjustment screw typically refers to screws with threads from 40-100 TPI (Threads Per Inch) (0.5mm to 0.2mm pitch) and ultra fine adjustment screw has been used to refer to 100-254 TPI (0.2mm to 0.1mm pitch). These screws are most frequently used in applications where the screw is used to control fine motion of an object.
	machine screw		A machine screw is generally a smaller fastener (less than 1⁄4 inches (6.35 mm) in diameter) threaded the entire length of its shank that usually has a recessed drive type (slotted, Phillips, etc.). Machine screws are also made with socket heads (see above), in which case they may be referred to as socket head machine screws.
	plow bolt		A plow bolt is bolt similar to a carriage bolt, except the head is flat or concave, and the underside of the head is a cone designed to fit in a countersunk recess. There are many variations, with some not using a square base, but rather a key, a locking slot, or other means. The recess in the mating part must be designed to accept the particular plow bolt.
	self-drilling screw Teks screw		Similar to a sheet metal screw, but it has a drill-shaped point to cut through the substrate to eliminate the need for drilling a pilot hole. Designed for use in soft steel or other metals. The points are numbered from 1 through 5, the larger the number, the thicker metal it can go through without a pilot hole. A 5 point can drill a 0.5 in (12.7 mm) of steel, for example.
	self-tapping machine screw		A self-tapping machine screw is similar to a machine screw except the lower part of the shank is designed to cut threads as the screw is driven into an untapped hole. The advantage of this screw type over a self-drilling screw is that, if the screw is reinstalled, new threads are not cut as the screw is driven.
	set bolt	tap bolt	A bolt that is threaded all the way to the head. An ASME B18.2.1 compliant set/tap bolt has the same tolerances as an ASME B18.2.1 compliant hex cap screw.
	set screw	grub screw	A set screw is 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. The most often used type is the socket set screw, which is tightened or loosened with a hex key.
	shoulder bolt shoulder screw	stripper bolt	A shoulder screw differs from machine screws in that the shank is ground to a precise diameter, known as the shoulder, and the threaded portion is smaller in diameter than the shoulder. Shoulder bolt specifications call out the shoulder diameter, shoulder length, and threaded diameter; the threaded length is fixed, based on the threaded diameter, and usually quite short. As a rule, socket head shoulder screws are produced from alloy heat-treated steel for maximum strength and wear resistance. Common applications for shoulder screws include rotating mechanism joints, linkage pivots, and guides for the stripper plate of a metal forming die set. In the latter application, the term stripper bolt is often substituted.
	stove bolt		A stove bolt is a type of machine screw that has a round or flat head and is threaded to the head. They are usually made of low grade steel, have a slot or Phillips drive, and are used to join sheet metal parts using a hex or square nut.
	tension control bolt		A tension control bolt (TC bolt) is a heavy duty bolt used in steel frame construction. The head is usually domed and is not designed to be driven. The end of the shank 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.
	thread rolling screws		These have a lobed (usually triangular) cross-section. They form threads in a pre-existing hole in the mating workpiece by pushing the material outward during installation. In some cases the properly prepared hole in sheetmetal uses an extruded hole. The extrusion forms a lead-in and extra thread length for improved retention. Thread rolling screws are often used where loose chips formed by a thread cutting operation cannot be tolerated.

Other threaded fasteners

Superbolt, or multi-jackbolt tensioner

A superbolt, or multi-jackbolt tensioner is an alternative type of fastener that retrofits or replaces existing nuts, bolts, or studs. Tension in the bolt is developed by torquing individual jackbolts, which are threaded through the body of the nut and push against a hardened washer. Because of this, the amount of torque required to achieve a given preload is reduced. Installation and removal of any size tensioner is achieved with hand tools, which can be advantageous when dealing with large diameter bolting applications.

Hanger screw or hanger bolt

A hanger screw is a headless fastener that has machine screw threads on one end and self-tapping threads on the other designed to be driven into wood or another soft substrate. Often used for mounting legs to tables. Also known as a dowel screw.

Materials

Screws and bolts are usually made of steel. Where great resistance to weather or corrosion is required, like in very small screws or medical implants, materials such as stainless steel, brass, titanium, bronze, silicon bronze or monel may be used.

Galvanic corrosion of dissimilar metals can be prevented (using aluminium screws for double-glazing tracks for example) by a careful choice of material. Some types of plastic, such as nylon or polytetrafluoroethylene (PTFE), can be threaded and used for fastenings requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation.

Often a surface coating is used to protect the fastening from corrosion (e.g. Bright Zinc Plating for steel screws), to impart a decorative finish (e.g. jappaning) or otherwise alter the properties of the base material.

Selection criteria of the screw materials include: size, required strength, resistance to corrosion, joint material, cost and temperature.

Bolted joints

Rusty hexagonal bolt heads

The American Institute of Steel Construction ( AISC) 13th Edition Steel Design Manual section 16.1 chapter J-3 specifies the requirements for bolted structural connections. Structural bolts replaced rivets due to decreasing cost and increasing strength of structural bolts in the 20th century. Connections are formed with two types of joints: slip-critical connections and bearing connections. In slip-critical connections, movement of the connected parts is a serviceability condition and bolts are tightened to a minimum required pretension. Slip is prevented through friction of the "faying" surface, that is the plane of shear for the bolt and where two members make contact. Because friction is proportional to the normal force, connections must be sized with bolts numerous and large enough to provide the required load capacity. However, this greatly decreases the shear capacity of each bolt in the connection. The second type and more common connection is a bearing connection. In this type of connection the bolts carry the load through shear and are only tightened to a "snug-fit." These connections require fewer bolts than slip-critical connections and therefore are a less expensive alternative. Slip-critical connections are more common on flange plates for beam and column splices and moment critical connections. Bearing type connections are used in light weight structures and in member connections where slip is not important and prevention of structural failure is the design constraint. Common bearing type connections include: shear tabs, beam supports, gusset plates in trusses.

Mechanical classifications

The numbers stamped on the head of the bolt are referred to the grade of the bolt used in certain application with the strength of a bolt. High-strength steel bolts usually have a hexagonal head with an ISO strength rating (called property class) stamped on the head. And the absence of marking/number indicates a lower grade bolt with low strength. The property classes most often used are 5.8, 8.8, and 10.9. The number before the point is the ultimate tensile strength in MPa divided by 100. The number after the point is 10 times the ratio of tensile yield strength to ultimate tensile strength. For example, a property class 5.8 bolt has a nominal (minimum) ultimate tensile strength of 500 MPa, and a tensile yield strength of 0.8 times ultimate tensile strength or 0.8(500) = 400 MPa.

Ultimate tensile strength is the stress at which the bolt fails. Tensile yield strength is the stress at which the bolt will receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2% offset strain. When elongating a fastener prior to reaching the yield point, the fastener is said to be operating in the elastic region; whereas elongation beyond the yield point is referred to as operating in the plastic region, since the fastener has suffered permanent plastic deformation.

Mild steel bolts have property class 4.6. High-strength steel bolts have property class 8.8 or above.

The same type of screw or bolt can be made in many different grades of material. For critical high-tensile-strength applications, low-grade bolts may fail, resulting in damage or injury. On SAE-standard bolts, a distinctive pattern of marking is impressed on the heads to allow inspection and validation of the strength of the bolt. However, low-cost counterfeit fasteners may be found with actual strength far less than indicated by the markings. Such inferior fasteners are a danger to life and property when used in aircraft, automobiles, heavy trucks, and similar critical applications.

Inch

SAE J429 defines the bolt grades for inch-system sized bolts and screws. It defines them by grade, which ranges from 0 to 8, with 8 being the strongest. Higher grades do not exist within the specification. SAE grades 5 and 8 are the most common.

Head markings and properties for inch-system hex-head cap screws
Head marking	Grade, material, and condition	Nominal size range (in)	Proof strength ( ksi/MPa)	Yield strength, min. (ksi/MPa)	Tensile strength, min. (ksi/MPa)	Core hardness ( Rockwell)
	SAE Grade 0	Strength and hardness is not specified
	SAE grade 1 ASTM A307 Low carbon steel	1⁄4–1-1⁄2	33/230		60/410	B70–100
	ASTM A307 - Grade B Low or medium carbon steel	1⁄4–4			60/410 minimum 100/690 maximum	B69–95
	SAE grade 2 Low or medium carbon steel	1⁄4–3⁄4	55/380	57/390	74/510	B80–100
		Greater than 3⁄4	33/230	36/250	60/410	B70–100
	SAE grade 4 Medium carbon steel; cold worked	1⁄4–1-1⁄2		100/690	115/790
	SAE grade 3 Medium carbon steel; cold worked	1⁄4–1	85/590		100/690	B70–100
	SAE grade 5 Medium carbon steel; quench and tempered	1⁄4–1 (inc.)	85/590	92/630	120/830	C25–34
		1–1-1⁄2	74/510	81/560	105/720	C19–30
	ASTM A449 - Type 1 Medium carbon steel; quench and tempered	1–1-1⁄2 (inc.)	74/510		105/720	C19–30
		1-1⁄2–3	55/380		90/620	Brinell 183–235
	SAE grade 5.1 Low or medium carbon steel; quench and tempered	No. 6–1⁄2	85/590		120/830	C25–40
	SAE grade 5.2 Low carbon martensitic steel; quench and tempered	1⁄4–1	85/590		120/830	C26–36
	ASTM A449 - Type 2 Low carbon martensitic steel; quench and tempered					C25–34
or	ASTM A325 - Type 1 Medium carbon steel; quench and tempered	1⁄2–1 (inc.)	85/590	92/630	120/830	C24–35
		1–1-1⁄2	74/510	82/570	105/720	C19–31
	ASTM A325 - Type 3 Atmospheric corrosion resistant steel; quench and tempered	1⁄2–1	85/590	92/630	120/830	C24–35
		1–1-1⁄2	74/510	82/570	105/720	C19–31
	ASTM A354 - Grade BC Medium carbon alloy steel; quench and tempered	1⁄4–2-1⁄2 (inc.)	105/720	109/750	125/860	C26–36
		2-1⁄2–4	95/660	99/680	115/790	C22–33
	SAE grade 7 Medium carbon alloy steel; quench and tempered	1⁄4–1-1⁄2	105/720	115/790	133/920
	SAE grade 8 Medium carbon alloy steel; quench and tempered	1⁄4–1-1⁄2	120/830	130/900	150/1,000	C32–38
	ASTM A354 - Grade BD	1⁄4–2-1⁄2 (inc.)	120/830	130/900	150/1,000	C33–39
		2-1⁄2–4	105/720	115/790	140/970	C31–39
	SAE grade 8.2 Medium carbon boron martensitic steel; fully kilned, fine grain, quench and tempered	1⁄4–1	120/830		150/1,000	C33–39
	ASTM A490 - Type 1 Medium carbon alloy steel; quench and tempered	1⁄2–1-1⁄2	120/830	130/900	150/1,000 minimum 170/1,200 maximum	C33–38
	ASTM A490 - Type 3 Atmospheric corrosion resistant steel; quench and tempered
	18-8 Stainless Stainless steel with 17–19% chromium and 8–13% nickel	1⁄4–5⁄8 (inc.)		40/280 minimum 80/550 – 90/620 typical	100/690 – 125/860 typical
		5⁄8–1 (inc.)		40/280 minimum 45/310 – 70/480 typical	100/690 typical
		Over 1			80/550 – 90/620 typical

Metric

The international standard for metric screws is defined by ISO 898, specifically ISO 898-1. SAE J1199 and ASTM F568M are two North American metric standards that closely mimic the ISO standard. In case of inch sizes the grade is dictated by the number of radial shapes plus a value of two. Inch-system bolts use integer values to indicate grades but metric bolts use numbers with one decimal. The two North American standards use the same property class markings as defined by ISO 898. The ASTM standard only includes the following property classes from the ISO standard: 4.6, 4.8, 5.8, 8.8, 9.8, 10.9, and 12.9; it also includes two extra property classes: 8.8.3 and 10.9.3. ASTM property classes are to be stamped on the top of screws and it is preferred that the marking is raised.

Head markings and properties for metric hex-head cap screws
Head marking	Grade, material, and condition	Nominal size range (mm)	Proof strength (MPa/ksi)	Yield strength, min. (MPa/ksi)	Tensile strength, min. (MPa/ksi)	Core hardness ( Rockwell)
	Class 3.6	1.6–36	180/26	190/28	330/48	B52–95
	Class 4.6 Low or medium carbon steel	5–100	225/32.6	240/35	400/58	B67–95
	Class 4.8 Low or medium carbon steel; fully or partially annealed	1.6–16	310/45	340/49	420/61	B71–95
	Class 5.8 Low or medium carbon steel; cold worked	5–24	380/55	420/61	520/75	B82–95
	Class 8.8 Medium carbon steel; quench and tempered	Under 16 (inc.)	580/84	640/93	800/120
		17–72	600/87	660/96	830/120	C23–34
	Class 8.8 low carbon Low carbon boron steel; quench and tempered
	Class 8.8.3 Atmospheric corrosion resistant steel; quench and tempered
	ASTM A325M - Type 1 Medium carbon steel; quench and tempered	12–36
	ASTM A325M - Type 3 Atmospheric corrosion resistant steel; quench and tempered
	Class 9.8 Medium carbon steel; quench and tempered	1.6–16	650/94	720/104	900/130	C27–36
	Class 9.8 low carbon Low carbon boron steel; quench and tempered
	Class 10.9 Alloy steel; quench and tempered	5–100	830/120	940/136	1,040/151	C33–39
	Class 10.9 low carbon Low carbon boron steel; quench and tempered
	Class 10.9.3 Atmospheric corrosion resistant steel; quench and tempered
	ASTM A490M - Type 1 Alloy steel; quench and tempered	12–36
	ASTM A490M - Type 3 Atmospheric corrosion resistant steel; quench and tempered
	Class 12.9 Alloy steel; quench and tempered	1.6–100	970/141	1,100/160	1,220/177	C38–44
	A2 Stainless steel with 17–19% chromium and 8–13% nickel	Up to 20		210/30 minimum 450/65 typical	500/73 minimum 700/100 typical
	ISO 3506-1 A2-50 304 stainless steel-class 50 (annealed)			210/30	500/73
	ISO 3506-1 A2-70 304 stainless steel-class 70 (cold worked)			450/65	700/100
	ISO 3506-1 A2-80 304 stainless steel-class 80			600/87	800/120

Screw head shapes

(a) pan, (b) dome (button), (c) round, (d) truss (mushroom), (e) flat (countersunk), (f) oval (raised head)

Combination flanged-hex/Phillips-head screw used in computers

Pan head

A low disc with chamfered outer edge

Button or dome head

Cylindrical with a rounded top

Round head

A dome-shaped head used for decoration.

Mushroom or Truss head

Lower-profile dome designed to prevent tampering

Countersunk or flat head

Conical, with flat outer face and tapering inner face allowing it to sink into the material. The angle of the screw is measured as the full angle of the cons.

Oval or raised head

A decorative screw head with a countersunk bottom and rounded top. Also known as "raised countersunk" (UK)

Bugle head

Similar to countersunk, but there is a smooth progression from the shank to the angle of the head, similar to the bell of a bugle

Cheese head

Disc with cylindrical outer edge, height approximately half the head diameter

Fillister head

Cylindrical, but with a slightly convex top surface. Height to diameter ratio is larger than cheese head.

Flanged head

A flanged head can be any of the above head styles (except the countersunk styles) with the addition of an integrated flange at the base of the head. This eliminates the need for a flat washer.

Some varieties of screw are manufactured with a break-away head, which snaps off when adequate torque is applied. This prevents tampering and also provides an easily inspectable joint to guarantee proper assembly. An example of this is the shear bolts used on vehicle steering columns, to secure the ignition switch.

Types of screw drives

Modern screws employ a wide variety of drive designs, each requiring a different kind of tool to drive in or extract them. The most common screw drives are the slotted and Phillips in the US; hex, Robertson, and Torx are also common in some applications, and Pozidriv has almost completely replaced Phillips in Europe. Some types of drive are intended for automatic assembly in mass-production of such items as automobiles. More exotic screw drive types may be used in situations where tampering is undesirable, such as in electronic appliances that should not be serviced by the home repair person.

Tools

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An electric driver screws a self-tapping phillips head screw into wood