Bolts and screws are related threaded fasteners, but they are not always the same thing. A bolt is generally designed to pass through clearance holes and be tightened with a nut. A screw is generally designed to engage a tapped hole or form a mating thread and be tightened by its head. Some screws can also be used with nuts.
The main difference between screws and bolts is their design and intended assembly, rather than size or appearance. A hex head and a fully threaded shank do not settle the classification. For purchasing, follow the part’s dimensional standard and the joint drawing.
Bolt vs Screw at a Glance
Feature | Bolt | Screw |
|---|---|---|
Typical assembly | Passes through clearance holes and works with a nut | Engages a tapped hole, forms a thread or works with a suitable nut |
Typical tightening | Nut is turned; head is restrained as needed | Head is turned |
Access | Usually requires access to the nut side; some designs capture the head | Tapped-hole assembly can be completed from the head side |
Threaded length | Can be fully or partially threaded | Can be fully or partially threaded |
Strength | Depends on specification, material, size and joint design | Depends on specification, material, size and joint design |
Examples | Carriage bolt, specified hex bolt | Socket head cap screw, machine screw, tapping screw |
What Is the Difference Between a Bolt and a Screw?
The terminology in ASME B18.2.1 considers design characteristics and intended service. Tightening a nut is a useful practical clue, but it is not a universal classification rule. A socket head cap screw can be used with a nut without automatically changing its standard designation to “bolt.”
Consider two assemblies. A carriage bolt’s square neck prevents it from turning while the nut is tightened. A socket head cap screw in a tapped machine housing is turned by its head. Both can clamp parts, but they require different mating features and tool access.
Three Practical Differences
1. Tool Access and Assembly
A conventional bolt-and-nut joint needs access to the nut, with the head held or captured against rotation. A screw in a tapped hole needs access to the head. This makes a tapped-hole joint useful for closed housings and blind holes, provided the internal threads have adequate engagement.
2. Thread Position and Shear Loading
For a joint transferring load through the fastener in shear, a properly sized unthreaded shank in the shear plane can be preferable to having threads there. Both bolts and cap screws can have an unthreaded portion. Specify the grip and thread length rather than assuming “bolt” means partially threaded.
Tensile stress area and shear area are different design quantities. A tensile-area calculation should not be used as the shear capacity of a threaded section. The joint’s load path, fastener geometry and applicable design method determine the check.
3. Thread Engagement and Repeated Service
In a tapped-hole joint, the base material’s threads must resist stripping. Soft materials or short engagement may limit the joint before the screw reaches its rated tensile strength. Repeated servicing can also wear or damage those threads. A suitable insert or a bolt with a matched nut may be useful where the design allows it.
Is a Bolt Stronger Than a Screw?
Not necessarily. A screw made to a high-strength specification can be stronger than a lower-grade bolt of the same diameter. Compare the stated grade or property class, diameter, thread, proof load and required preload. The label “bolt” or “screw” is not a strength rating.
Specification family | Example identifiers | How to use the information |
|---|---|---|
SAE J429 steel fasteners | Grade 2, Grade 5, Grade 8 | Check grade, size range and required mechanical properties |
ISO 898-1 steel bolts, screws and studs | 8.8, 10.9, 12.9 | Use the property class with the applicable size and product specification |
ISO 3506-1 stainless fasteners | A2-70, A4-70 | Check both stainless steel grade and property class |
Thread markings and property markings describe different things. An M10 × 1.5 thread specifies nominal diameter and pitch; it does not specify strength or torque. Likewise, a material description such as 18-8 stainless is not enough to calculate a safe working load.
Fasteners Whose Names Can Be Misleading
Fastener | What to know |
|---|---|
Can be installed in a tapped hole or used with a nut; check its product standard | |
Machine screw | Uses a preformed machine thread and can mate with a tapped hole or nut |
Square neck resists rotation; normally tightened using a nut | |
Lag screw / lag bolt | Common names for a wrench-driven wood fastener; normally engages wood rather than a machine-thread nut |
Example: A Screw in a Thin Tapped Bracket
An M10 × 1.5 screw threaded through a 4 mm plate has at most 4 mm of axial engagement before allowing for chamfers or incomplete threads. That is about 2.7 thread pitches. This geometry alone does not prove the joint safe or unsafe; the load, plate material, fastener strength and actual thread geometry must be checked.
If the internal threads cannot carry the required load, increasing the screw’s property class does not solve the problem. Possible changes include a thicker tapped feature, a suitable insert or a through-fastener with a matched nut. The available access and required clamp load determine which change fits the design.
Screw vs Bolt: How to Choose
Use a bolt-and-nut arrangement when the joint provides suitable access and the nut offers a practical, replaceable mating thread.
Use a screw in a tapped hole when access is limited to one side and the internal thread has adequate engagement and strength.
For shear loading, check where the threads sit relative to the shear plane and whether the joint transfers load by friction or fastener bearing.
For wood, use a fastener and installation method specified for the timber connection. A machine screw does not become a wood screw simply because it has a sharp end.
For outdoor or wet service, choose material and coating for corrosion exposure as well as strength. Review coating and hydrogen-embrittlement controls for high-strength steel fasteners.
Select compatible nuts and washers when the assembly requires them. Thread size, nut strength and washer dimensions all affect the connection.
Browse LILY’s screws and bolts category for specific parts, or use our types of screws and bolts guide to identify the head and fastener family. For a drawing-specific part, see custom fastener services.
FAQ
Are Lag Bolts and Lag Screws the Same?
In common hardware terminology, yes: both usually refer to a lag screw with a hex or square head and a coarse wood thread. Despite the name “lag bolt,” it normally threads into wood without a machine-thread nut. Check the specified diameter, length and installation requirements; the informal name is not a performance specification.
Can I Use a Hex Bolt in a Tapped Hole?
It may be mechanically suitable if the thread system, size, pitch, fit, engaged length and strength match the design. A partially threaded bolt also needs enough threaded length. If the drawing specifies a cap screw or a particular standard, confirm an approved equivalent before substituting.
What Is the Difference Between a Bolt and a Stud?
A typical bolt has a head; a stud does not. Studs may have threads along their full length or at one or both ends. Some studs anchor in a tapped component and accept a nut on the exposed end. See threaded rods and studs.
Can I Replace a Grade 5 Bolt with a Class 8.8 Bolt?
Do not assume they interchange. SAE grades and ISO property classes belong to different specification systems, and similar strength does not establish dimensional compatibility. An inch thread and a metric thread cannot be mixed. Any conversion must address the mating threads, hole geometry, nut, preload and design approval.
Technical Reference
The NASA Fastener Design Manual provides background on thread engagement, materials, coatings and joint loading. Specify the actual part using its applicable dimensional and mechanical standards.
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