Guide to Square Head Machine Bolts

What Is a Square Head Bolt?

Square head bolts, also called square head machine bolts or square head screws, are externally threaded fasteners. It consists of a square head and a screw with external threads, typically paired with square nuts, hex nuts, or directly screwed into threaded holes. It is often used in narrow or obstructed spaces.

Before hex bolts became the industrial standard, square-head bolts were the mainstream choice. Today, they are primarily used in specialized applications requiring high torque, specific self-locking functions, or a retro aesthetic.

Square bolts are classified by high-strength grade as: Grade 4.8 bolts, Grade 8.8 bolts, Grade 10.9 bolts, Grade 12.9 bolts; ASTM Grade 5/Grade 8; SAE J429 Grade 5, etc. Screws are classified by material into iron bolts and stainless steel bolts; stainless steel bolts include SUS304 stainless steel bolts, SUS201 stainless steel bolts, and SUS316 stainless steel bolts.

Applications

In early mechanical manufacturing and steel structure engineering, square-head bolts were one of the mainstream standard fasteners and remain widely used today in:

• Heavy Machinery and Equipment
• Railway Track Systems
• Petrochemical Equipment
• Steel Structure Engineering
• Power and Energy Facilities

Specifications for Square Bolts

Dimensions	Diameter: Metric M6 to M72, Imperial 5/16″ to 2″. Length: Metric 10 to 500mm, Imperial 5/8″ to 19″. Or custom specifications.
Thread	Full thread, half thread.
Material	Carbon steel, stainless steel, aluminum, brass, copper-nickel alloy, titanium alloy.
Surface treatment	PTFE coating, hot-dip galvanizing, aluminum-zinc coating, black phosphating, galvanizing, nickel plating
Strength Grade	Grade: 4.8, 8.8, 10.9, 12.9; ASTM A307 Grade A Steel Machine Bolts; A2-70, A4-70, A4-80.
Standard	EU, IFI, CSN, SAE, DIN, ISO, ASME, Italian, British BS, Polish PN.
Delivery time	7 to 45 days

Square Head Bolts Dimensions Chart

(inch)	1/4	5/16	3/8	7/16	1/2	5/8	3/4	7/8	1	1-1/8	1-1/4	1-3/8	1-1/2
P	20|28|32	18|24|32	16|24|32	14|20|28	13|20|28	11|18|24	10|16|20	9|14|20	8|12|20	7|12|18	7|12|18	6|12|18	6|12|18
ds max	0.26	0.324	0.388	0.452	0.515	0.642	0.768	0.895	1.022	1.149	1.277	1.404	1.531
ds min	0.237	0.298	0.36	0.421	0.482	0.605	0.729	0.852	0.976	1.098	1.223	1.345	1.47
s max	0.375	0.5	0.562	0.625	0.75	0.938	1.125	1.312	1.5	1.688	1.875	2.062	2.25
s min	0.362	0.484	0.544	0.603	0.725	0.906	1.088	1.269	1.45	1.631	1.812	1.994	2.175
e max	0.53	0.707	0.795	0.884	1.061	1.326	1.591	1.856	2.121	2.386	2.652	2.917	3.182
e min	0.498	0.665	0.747	0.828	0.995	1.244	1.494	1.742	1.991	2.239	2.489	2.738	2.986
k max	0.188	0.22	0.268	0.316	0.348	0.444	0.524	0.62	0.684	0.78	0.876	0.94	1.036
k min	0.156	0.186	0.232	0.278	0.308	0.4	0.476	0.568	0.628	0.72	0.812	0.872	0.964
r max	0.03	0.03	0.03	0.03	0.03	0.06	0.06	0.06	0.09	0.09	0.09	0.09	0.09
r min	0.01	0.01	0.01	0.01	0.01	0.02	0.02	0.02	0.03	0.03	0.03	0.03	0.03

Note: ASME/ANSI B18.2.1-Table1

Standard for Square Head Bolts

• ASME B18.2.1 (ANSI)
• ASTM A307 Grade A
• DIN 478
• DIN 479
• DIN 480
• DIN 21346
• ISO 8677 / 8678
• AASHTO / SAE Specs

Types of Square Head Bolts

Advantages of Square Bolts

• Lower Manufacturing Costs

In the traditional forging process, the production process of square head bolts is relatively simple and the material utilization rate is high

• Good Anti-Rotation Properties

The square head can be embedded into channel steel or structural grooves to reduce the risk of rotation.

• Large Load-Bearing Area

The relatively large contact area at the head helps distribute the force.

• High Strength

Sturdy construction, suitable for high-strength connections.

• Easy to Install

In early structural designs, square heads were easier to machine and install than hex heads.

Disadvantages of Square Bolts

• Poor tool compatibility: Socket wrenches and box-end wrenches are primarily designed for hex heads (e.g., 12-point sockets). Square-head bolts typically require open-end wrenches or specialized square-hole tools, resulting in limited versatility.
• Weight and Material Consumption: Under identical strength requirements, the volume of square heads is typically larger than that of hex heads, increasing both the fastener’s own weight and raw material costs.
• Uneven stress distribution: Compared to polygonal heads, square heads exhibit less uniform torque distribution than hex heads when subjected to extremely high torque. In extreme cases, this can more readily cause head deformation or damage.
• Prone to snagging: The sharp edges of square heads are more pointed and protruding than hex heads, making them more likely to snag clothing or scratch operators in exposed installation locations.

How Are Square-Head Bolts Manufactured?

1️⃣ Cold Heading

Forming the head structure using a high-speed multi-station cold heading machine.

2️⃣ Thread Rolling

External threads are formed using a thread rolling process.

3️⃣ Heat Treatment

Quenching and tempering of high-strength grade products.

4️⃣ Surface Treatment

Electrogalvanizing, hot-dip galvanizing, blackening treatment, Dacromet coating, etc.

What Types of Bolt Heads Are There?

Common types include Square Head Bolt, Hex Head Bolt, Countersunk Bolt, Carriage Bolt, T-Head Bolt, and Round Bolt.

Summary

Square-head bolts are now primarily used in specialized applications requiring extremely high gripping force (e.g., for easy rotation with heavy tools) or to prevent bolt rotation within holes (e.g., when paired with square-hole washers or T-slots). In precision mechanical assemblies prioritizing efficiency and space utilization, they have largely been replaced by hex bolts or hex socket bolts.

Custom Quality Square Head Machine Bolts

As a professional fastener manufacturer, BD Fastener offers:

• Metric and Imperial Square-Head Bolts
• Custom Sizes and Non-Standard Products
• Hot-Dip Galvanized and High-Strength Grade Products
• Support for Small Batches and Bulk Export Orders
• OEM / ODM Services

For detailed specifications, quotations, or sample support, please contact the BD Fastener team for technical consultation.