The different types of screws and bolts are easiest to identify by head shape, drive, thread and purpose. Hex head screws suit wrench access; socket head cap screws use an internal drive; flat head screws sit in a countersink. Tapping screws create a mating thread, while set and shoulder screws perform more specialized jobs.
This guide compares 12 common product families used in machine building and maintenance. These categories overlap: a tapping screw can also have a rounded head, and a shoulder screw can have a hex socket. Threaded rods and studs are included as related threaded fasteners, rather than classified as headed bolts.
For the design distinction between screws and bolts, see our bolt vs screw guide. To source a part after identifying it, browse LILY’s screws and bolts by size, material and drive.
12 Types of Screws and Bolts at a Glance
Type | Head or defining feature | Typical use |
|---|---|---|
Hex head screw | External hex; wrench or socket | Machine frames and general assembly |
Socket head cap screw | Cylindrical head; internal hex or Torx | Fixtures and joints with limited side access |
Flat head screw | Countersunk head; several drive styles | Joints requiring a flush surface |
Rounded head screw | Pan, button, truss and related profiles | Covers, sheet assemblies and equipment panels |
Thumb screw | Head shaped for hand tightening | Covers and adjustments requiring frequent access |
Shoulder screw | Unthreaded shoulder; smaller threaded end | Pivots, guides and linkages |
12-point screw | External 12-point head | Specified machinery joints with restricted tool access |
Tapping screw | Thread-forming or thread-cutting design | Suitable pilot holes in metal or plastic |
Set screw | Usually headless; point engages a component | Positioning collars, pulleys and gears on shafts |
Captive panel screw | Retainer keeps screw attached to the panel | Access panels where loose screws are undesirable |
Carriage bolt | Domed head and square neck; nut tightened | Wood joints or metal with suitable square holes |
Threaded rod / stud | Headless; full-length or end threads | Tie rods, adjustable connections and stud assemblies |
Screw Head Types and Their Uses
Head shape affects the bearing surface, installed height and tool access. It does not establish the fastener’s strength grade. Check the drawing and mechanical specification separately.
Hex Head Screws
A hex head screw has a six-sided external head for a wrench or socket. It is useful where there is room around the head to engage the tool. Check whether the part is fully or partially threaded and whether its grade suits the required preload. See hex head screws.
Socket Head Cap Screws
A socket head cap screw typically has a cylindrical head with an internal hex drive; Torx versions are also available. The tool approaches along the screw axis, making this style useful in fixtures and closely spaced assemblies. Low-profile versions can have different load limits from standard-height cap screws. See socket head cap screws.
Flat Head Screws
A flat head screw has a countersunk bearing surface that can sit flush with the part. The countersink angle and diameter must match the screw. A flush head is useful where a protruding fastener would interfere with movement or a mating surface. See flat head screws.
Rounded Head Screws
Pan, button and truss heads are common raised-head options. A truss head offers a broad bearing face; a button head offers a low rounded profile. Fillister and cheese heads are other profiles found in this product family. Select the actual head dimensions rather than relying on the family name. See rounded head screws.
Thumb Screws
Thumb screws are designed for hand tightening, often using a knurled or wing-style head. They suit covers and adjustments that need frequent access. Hand tightening does not give a controlled preload, so use a specified tightening method where clamp load matters. See thumb screws.
Shoulder Screws
A shoulder screw has an unthreaded shoulder and a smaller threaded end. The shoulder can guide a moving part or provide a pivot surface, while the threaded end retains the assembly. Specify shoulder diameter and length separately from thread size; the moving component must not be trapped by an incorrectly sized shoulder. See shoulder screws.
12-Point Screws
An external 12-point head is driven with a compatible 12-point socket or box wrench. It provides more tool indexing positions than an external hex head. Head dimensions and the specified grade determine suitability; the number of points alone does not prove greater strength or torque capacity. See 12-point screws.
Screw Head Shapes at a Glance
These side profiles help distinguish common head shapes. They are illustrative rather than dimensioned product drawings.
Schematic illustration; use the product drawing for dimensions and tool fit.
Screw Drive Types: Phillips, Torx, Hex and More
The drive is the feature engaged by the tool. An external hex head takes a wrench or socket; an internal hex drive takes a hex key or bit. A hex head screw and a hex socket screw therefore need different tools.
Phillips and Pozidriv are different cross-recess systems. Use the correct bit type and size for each.
Slotted drives take a flat-blade bit that fits the slot width and depth.
Torx uses a six-lobe recess. A correctly sized bit provides secure engagement and helps reduce cam-out.
Internal hex and square drives require matching keys or bits; worn or undersized tools can damage the recess.
One-way and other tamper-resistant drives restrict removal or require a dedicated tool. They are not interchangeable with standard recesses.
Schematic illustration; use the product drawing for dimensions and tool fit.
What Head Do Nut Drivers Fit?
Nut drivers fit external hexagonal heads and hex nuts. They do not fit Phillips, slotted, Torx or internal hex recesses. Match the driver to the width across flats of the head, which is different from the screw’s nominal thread diameter.
Tapping Screws: Forming or Cutting a Mating Thread
A self-tapping screw forms or cuts its mating thread during installation. Many tapping screws still require a correctly sized pilot hole. Thread-forming designs displace material; thread-cutting designs remove material. Choose the type for the substrate, thickness and specified hole size.
Self-drilling screws also create the hole using a drill-like point. Self-tapping and self-drilling are therefore related terms, but they do not mean the same thing. Check the product’s drilling capacity before using it in sheet metal. Browse tapping screws.
Set Screws: Securing Components on Shafts
The common headless set screw threads into a collar, pulley or gear and bears against a shaft. Cup points can indent the surface; flat and nonmarring points suit applications where surface damage must be limited. Dog points can locate in a matching hole or slot.
Point style, shaft hardness, applied torque and vibration all affect retention. A set screw alone may not provide enough holding capacity for a demanding drive. Check whether the design needs a key, clamp or another retention method. See set screws.
Captive Panel Screws: Retained During Servicing
A captive panel screw stays attached to the panel after disengaging from the mating thread. Its retainer reduces the chance of losing a screw during service. Check panel thickness, mounting method and available screw travel. Quarter-turn fasteners serve a similar access-panel function but use a different locking mechanism. See captive panel screws.
Carriage Bolts, Threaded Rods and Studs
Carriage Bolts
A carriage bolt has a domed head and a square neck that resists rotation in wood or a suitable square hole. The nut supplies the tightening action. In metal, an ordinary round clearance hole does not provide the same anti-rotation support. See carriage bolts.
Threaded Rods and Studs
Threaded rod commonly has threads along its length and can be used with nuts for adjustable connections. Studs can be fully threaded or threaded at one or both ends. A stud may anchor into a tapped component while a nut clamps the mating part. These headless fasteners are related to bolts and screws but form a distinct product family.
Check the specified material, property class and thread engagement. A rod’s shape or an informal grade description is not enough to establish an allowable working load. See threaded rods and studs.
How Screw Sizes Are Written
An inch thread callout gives diameter, threads per inch and length. For example, #8-32 × 1 in. describes a #8 thread with 32 threads per inch and a nominal 1-inch length. A metric callout such as M6 × 1.0 × 20 gives a 6 mm nominal thread diameter, 1.0 mm pitch and 20 mm nominal length. Inch and metric threads are not interchangeable.
Flat countersunk screws are generally measured from the top of the head to the end. Hex, socket and pan head screws are generally measured from beneath the head. Shoulder screws use additional shoulder dimensions, and special head styles can have different conventions. Use the product drawing for the final callout.
Schematic illustration; use the product drawing for dimensions and tool fit.
Materials and Finishes for Screws and Bolts
Select material for both the load and the environment. A material name alone does not specify tensile strength, proof load or installation torque.
Material or finish | Typical reason to select it | What to check |
|---|---|---|
Carbon or alloy steel | Machinery joints requiring a specified strength grade | Grade, heat treatment and corrosion protection; black oxide offers limited protection on its own |
18-8 stainless steel | General corrosion resistance | Actual mechanical rating; it is not a direct substitute for every alloy-steel grade |
316 stainless steel | Improved resistance to chloride-related pitting compared with common 304 / 18-8 grades | Exposure severity, crevices and rated strength; 316 is not immune to seawater corrosion |
A286 | Specified fasteners requiring elevated-temperature strength | Heat treatment, product specification and service temperature; do not treat it as a synonym for 316 |
Brass or bronze | Electrical, appearance or application-specific corrosion requirements | Exact alloy, strength and compatibility with surrounding metals |
Nylon or other plastic | Electrical insulation and low weight | Creep, temperature, chemical exposure and allowable clamp load |
Aluminum or titanium | Weight-sensitive assemblies | Alloy, mechanical rating and galvanic compatibility |
For marine screws and bolts, evaluate saltwater exposure and the whole joint rather than selecting “stainless” by name. Stainless fasteners in aluminum can create a galvanic couple when moisture provides an electrolyte. An insulating washer alone does not isolate threads or the shank; isolation, sealing and compatible materials must address the complete contact path.
How to Choose the Right Type
Define the job: clamping a joint, guiding motion, retaining a panel or locating a component on a shaft.
Check head clearance, drive access and whether a flush surface is required.
Specify diameter, pitch, length, thread engagement and any shoulder or countersink dimensions.
Select the required strength and corrosion protection from the joint design. Avoid choosing a grade solely from a general application label.
Confirm the tightening method and whether service access or vibration requires a retention feature.
For non-standard dimensions or special screws and bolts, LILY’s custom fastener service can review a drawing and application requirements.
FAQ
Can I Identify a Replacement Screw from Its Head Alone?
No. Two screws can have the same head but different diameters, pitches, lengths, materials or strength ratings. Record the thread and dimensions, then compare the product drawing. For a critical joint, use the specified part or an approved equivalent.
Can Hanger Bolts and Dowel Screws Replace Each Other?
Usually not. A typical hanger bolt has a wood-screw thread at one end and a machine thread at the other for a nut or threaded fitting. A typical dowel screw has wood-screw threads at both ends. Choose the end-thread combination required by the connection.
Can I Replace a Phillips Screw with a Torx Screw?
Possibly, if the replacement matches the required thread, length, head seating dimensions, material, strength and other specified features. Changing the drive means changing the tool as well. Check the replacement specification rather than assuming the recess is the only difference.
Does “Stainless Steel” Specify a Fastener’s Strength?
No. Stainless grade describes the material family, while the product’s mechanical rating depends on its specification and condition. Compare the stated strength or property class before substituting a stainless fastener for a steel one.
Technical Reference
The NASA Fastener Design Manual discusses material selection, corrosion, thread engagement and joint loading. Use the applicable product standard and drawing for the fastener being specified.
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