Grade A Square Head Machine Bolt

The Xiaoguo® Grade A Square Head Machine Bolt in stock feature a square design with a square head. They can achieve mechanical anti-torsion locking, which can enhance safety levels in specific working environments.

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Product Description

Grade A Square Head Machine Bolt is a type of fastener with a square head. It consists of a square head and a threaded rod section. It is usually used in conjunction with square nuts or hexagonal nuts, or can be directly screwed into threaded holes. Square head bolts were the mainstream fastener from the 19th century to the early 20th century. They were widely used in infrastructure construction such as bridges and railways due to their ease of forging and excellent anti-torsion performance. Nowadays, thanks to its unique design advantages, it still plays a role in heavy machinery, railway tracks, equipment maintenance, and restoration of vintage buildings.

Structural Characteristics

Summary of the structural characteristics of Grade A Square Head Machine Bolt: The correlation between geometric features and functional design.

Structural components Key features Core functions

Head Square shape, wide opposite sides, large height. Provides high torque transmission, anti-slip function, friendly for blind assembly, and convenient for imprinting marks.

Neck Flat neck (anti-rotation), flange (bearing), short pin (guidance) Mechanical anti-rotation, protecting the workpiece surface, blind hole guidance

Shaft section Partially threaded (smooth shaft section) Resists shear load, provides precise positioning, and avoids stress concentration in the threads

Threaded part Mainly coarse teeth, tolerance 6g. High-strength connection, reliably cooperating with the nut.

End type Flat end / Chamfered end / Cylindrical end Suitable for different installation conditions

Grade A Square Head Machine Bolt

Mon	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	16\|24\|32	14\|20\|28	13\|20\|28	11\|18\|24	10\|16\|20	9\|1420	8\|1220	7\|12\|18	7\|12\|18	6\|12\|18	6\|12\|18
ds max	0.388	0.452	0.515	0.642	0.768	0.895	1.022	1.149	1.277	1.404	1.531
s max	0.562	0.625	0.75	0.938	1.125	1.312	1.5	1.688	1875	2.062	2.25
s min	0.544	0.603	0.725	0.906	1.088	1.269	1.45	1.631	1.812	1.994	2.175
e max	0.795	0.884	1.061	1.326	1.591	1.856	2.121	2.386	2.652	2.917	3.182
e min	0.747	0.828	0.995	1.244	1.494	1.742	1.991	2.239	2.489	2.738	2.986
k max	0.268	0.316	0.348	0.444	0.524	0.62	0.684	0.78	0.876	0.94	1.036
k min	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	0.06	0.06	0.06	0.09	0.09	0.09	0.09	0.09
r min	0.01	0.01	0.01	0.02	0.02	0.02	0.03	0.03	0.03	0.03	0.03

Preparations before use

Before you plan to use the Grade A Square Head Machine Bolt, we would recommend that you check these items.

（1）Confirm the matching of bolt specifications

Thread specification: Ensure that the thread diameter and pitch of the square head bolt are exactly the same as those of the matching nut or threaded hole.

Length selection: The length of the bolt should be such that after the nut is tightened, 2 to 3 complete threads are exposed at the end of the bolt.

Head side dimensions: Ensure that the width of the square head matches the opening size of the used wrench.

Strength grade: Select the appropriate grade as needed.

（2）Inspect the hole positions of the workpiece

Round hole vs square hole: Ordinary square-head bolts can pass through the round hole. However, if anti-turning is required, a pre-drilled installation surface for the square hole must be made on the workpiece, allowing the square head neck to be inserted.

Diameter tolerance: The diameter of the round hole should be 0.5 to 1.0 mm larger than the diameter of the bolt shank.

Corner rounding of the hole: If the Grade A Square Head Machine Bolt head needs to fully adhere to the surface of the workpiece, the hole should be rounded or countersunk.

（3）Cleaning and Lubrication

Remove the oil stains, iron filings, old coatings and other debris from the threads.

For stainless steel materials or high-temperature environments, it is recommended to apply anti-adhesion agent or molybdenum disulfide lubricating grease on the threads.

For ordinary carbon steel bolts, a small amount of oil can be applied to reduce the friction coefficient.