Atlantic Fasteners Technical Tips: Socket Head Cap Screws

• Head height is equal to shank diameter.
• Shouldn’t be mated with a regular hex nut, which isn’t as strong.

• Head height is approximately half the shank diameter.

• Caution: Inch and metric have different countersink angles.
• Mismatching fastener and hole countersink angles can result in premature fastener failure

• A larger head diameter makes it more appropriate for holding thin materials like sheet metal guards.
• Because of its internal hex drive style, it’s ideal for tamper-proofing applications.
• A good substitute for other drive styles that are prone to stripping like Phillips and slotted.

• Typically used as a pivot point or axle because shoulders are ground to a tight tolerance.

• Socket head cap screws 1″ diameter and under have class 3A fit (tight tolerance).
• Over 1″ have a class 2A fit (typical hex cap tolerance).
• All other head styles and diameters have a class 3A fit.

Inch has one strength level, alloy steel.

Metric has two strength levels, property classes 12.9 and 8.8.

8.8 is about 30% weaker.
8.8 is also 20 – 50% less expensive.
Caution: Don’t substitute 8.8 for 12.9.

For metric M5 and larger, the property class and manufacturer’s I.D. is marked on the head.

To specify inch shoulder screws:

Call out shoulder diameter, then the shoulder length
Example: 5/8″ x 2″ = a shoulder screw with a 5/8″ diameter shoulder, 2″ long.

Thread size is one standard size smaller than the shoulder diameter and needn’t be specified, except for:

A screw with a 1/2″ diameter shoulder has a 3/8 – 16 thread.
A 5/8″ diameter shoulder has a 1/2 – 13 thread.

Why? 7/16″ and 9/16″ are not stocked shoulder diameter thread sizes.

To specify metric shoulder screws:

Call out shoulder diameter, then the thread diameter, then the shoulder length
Example: 12/M10 x 30 = a shoulder screw with a 12mm shoulder diameter, a 10mm thread diameter, and a 30mm shoulder length.