Shoulder Bolts - Shoulder Screws

Shoulder screws, or shoulder bolts, are machine screws that have an unthreaded shoulder under the head that is larger in diameter than the threads. It is this enlarged shoulder that determines the nominal size of the screw. The head is the largest diameter, while the thread has a major diameter that is slightly smaller than the diameter of the shoulder. When installed, the smooth shoulder extends beyond the surface the thread is driven into, allowing it to act like a shaft or dowel for the rotation or sliding of attached parts including pulleys, gears, mechanical assemblies and the like. Because they are also used to attach stripper plates, shoulder screws are sometimes referred to as stripper bolts.

Both metric and imperial shoulder screws (shoulder bolts) are available in standard sizes. Inch shoulder screw sizes are specified by the shoulder diameter and shoulder length, followed by the overall length. Metric shoulder bolts are denoted by shoulder diameter, then thread diameter, then shoulder length.

How to Measure Socket Head Shoulder Screws / Shoulder Bolts

 How to measure socket shoulder screws

Inch Shoulder Screws:

1. List shoulder diameter, then

2. List thread diameter/size, then

3. List shoulder length

Example: 1/4"-10-24 x 7/8" = a shoulder screw with a 1/4" diameter shoulder, #10-24 thread size and 7/8" shoulder length

Thread size is one standard size smaller than the shoulder diameter and doesn't need to 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.

Metric Shoulder Screws:

1. List shoulder diameter, then

2. List thread size, then

3. List shoulder length

Example: M6-M5 x 20 mm = a shoulder screw with a 6 mm shoulder diameter, a 5 mm thread diameter and 20 mm shoulder length

Commercial Grade Shoulder Screws vs. Precision Shoulder Screws

Commercial grade shoulder bolts primarily possess a hex socket drive that is driven into place via a hex key or allen wrench. A socket head is beneficial when installing the screw in tight spaces where there is not enough clearance for regular driving or wrenching. Hex socket shoulder screws are manufactured from either alloy steel with a black oxide finish or uncoated stainless steel.

Precision shoulder screws are available with either a hex-socket drive or a slotted drive. Additionally, some precision screw varieties feature special tamperproof drives including Torx and six-lobe styles. Manufactured from stainless steel, precision shoulder screws are useful as a precision spacer and bearing surface for a wide variety of applications. Undercuts on each side of the shoulder ensure perfect seating for precise low friction assembly. Finally, the tolerance on the shoulder diameter of the precision screw is far more exact than that of a commercial grade screw, which makes them vital in assemblies that require a high level of accuracy in their component parts.

Threadlocking Shoulder Screws

AFT offers stock sizes of threadlocking shoulder screws that feature either a nylon-pellet or a nylon-patch, which are nylon locking elements applied to the threads of the screw that increase the holding power of the screw and ensure a tight fit in high-vibration applications. All of our should screws and bolts come in alloy steel or stainless steel.

As always, if you don't see the size, finish or threadlocking feature you are looking for, let us know. We specialize in custom parts

Shoulder Screw Resources