Socket Head Cap Screw Buyer’s Guide

7 Things to Know When Buying Socket Head Cap Screws

Socket head cap screws or socket caps, have become the standard among high-strength fasteners needed to withstand the faster speeds and higher pressure of today’s machines, instruments and assemblies.

Also known as their trademarked name, “Allen” screws, these fasteners are broadly used across various applications in many industries, including: aerospace, electrical motors, hydraulic equipment, instrumentation, tooling and fixtures, machinery, military and many more. Common uses include machine parts, die fixtures and assemblies.

1. What is a Socket Head Cap Screw? 

Socket caps are externally threaded cap screws with a cylindrical head with a flat chamfered top and a hexagonal recess for internal wrenching. Hence, socket caps need less space than alternative fasteners.

Whereas nearly 90% of all commercial and industrial fasteners have a 2A fit, which offers a combination of performance, economy, and convenience, socket products generally have a 3A thread fit. 3A threads have restrictive tolerances often used where safety is a critical design consideration.

Because of their efficient design and thread fit, socket cap screws are ideal for precision assembly work with close tolerances and applications that need a well-tooled appearance.

2. What sizes are available?

Commercial socket caps, which conform to ASME B18.3, are available off the shelf in inch sizes starting as small as 0-80 diameter and are commonly available to diameter 1-1/2-6. Metric socket caps, which conform to DIN 912, are available from M1.4 – M48 diameter and are most common in coarse thread. Screw lengths and thread lengths are related to the diameter of the screws and all lengths may not be available in all materials and styles.

3. What materials and platings are available?

“Steel” socket caps off the shelf are alloy steel black oxide through heat treatment and are generally oiled. Stainless steel grades 18-8/303, A2, 316 and A4 are also commonly available. Other exotic materials such as brass, 416, A286 or special finishes may be available for special order with a lead time.

If you need an alloy steel fastener, but require greater corrosion resistance or a different look than black, consider stainless steel or zinc plating.

4. Are there special considerations when plating socket head cap screws?

We are so glad that you asked! If you are considering zinc plated socket caps, proceed with caution! Because of their tight tolerances and 3A fit, zinc plating a socket cap will often change the thread fit to a commercial nut fit. If you need a 3A fit, you must specify that on your purchase order so the factory can meet your thread fit needs when creating and plating your screws.

5. Other than size, material and plating, what other options are available?

The other main considerations when selecting socket caps are the head style and the drive type.

There are four main available head styles, and each serves a slightly different purpose.

• Standard Head – The standard in socket head cap screws (ASTM A574, F835, DIN912)
• Low Head – Use in parts too thin for standard height heads and where clearance is limited (DIN7984)
• Flat Head– Countersunk heads at 82 degrees provide maximum flushness and side wall contact (ASTM F835, F879, DIN7991)
• Button Head – For use in materials too thin to countersink (ASTM F835, F879, DIN7380)

Most socket caps come with a standard hexagonal socket drive. Other options include security pin-in-hex, pin-in-torx, or six-lobe torx versions in certain sizes and materials.

6. Are there other factors to consider?

We’ve covered the most common variables including head style, diameter and thread, length, material and plating or finish. Here are a few of the other considerations you might be faced with based on the project and the bill of materials:

• Do you need domestic parts or will imported screws work?
• Do you require parts that conform to a specific mil-spec number, such as MS16995, MS16996, NAS1351 and NAS1352 with their exacting tolerances and traceability, or will standard commercial parts work?
• Does your bill of materials calls out a specific manufacturer, such as Holo-krome or Blue Devil?

7. What level of certification is required?

Generally, a Certificate of Conformance or a COC is sufficient for your customer. Full certification with material certs, DFARS certification and test reports are often available for USA-made and mil-spec parts for a fee.

Socket Head Cap Screws Available at MF Supply

MF Supply is a leading supplier of Socket Head Cap Screws in alloy and stainless steel, including the brands Holo-krome and Blue Devil. We offer functional equivalents to the most popular national brands such as Fastenal, McMaster-Carr, MSC Industrial and Grainger.

Here at MF Supply, we offer:

• The full range of commercial and mil-spec parts, including parts that are DFARS and ROHS compliant.
• Brand names, generic equivalents, domestic and import.
• Custom made parts per print in non-standard and exotic materials and finishes.
• Inch and Metric sizes.
• Stainless steel in 18-8, 316, A286, A2 and A4 (ASTM F837, F879).
• Mil-spec MS16995, MS16996, NAS1351 and NAS1352.

For more detailed information about socket head cap screws, visit our website. And if you don’t see it listed, as always, ask us. “Finding the right screw for you” is our tag line after all!

For more than 40 years, MF Supply has helped American manufacturers streamline operations, saving them time and money with our sourcing expertise and unique supply chain strategies. We are a WBE/WOSB certified stocking distributor of fasteners and electronic components.

We work with the best-established factories in the United States and DFARS-certified countries, and stock a huge inventory including all major brands and equivalents. MF Supply provides solutions including: same-day drop ship direct from the factory; custom sizes, materials and plating options; and DFARS parts with full paperwork and Certificates of Conformance.

We creatively solve common problems including hard-to-find parts and long lead times. Our sourcing and reworking expertise includes fast turnaround for modification of existing parts, hard to find standards and specials, special threads, non-standard diameters and lengths, exotic materials, special platings and short runs. We provide functional equivalents to expensive brand name fasteners and help our customers save money while reducing lead times.

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