Shoulder Bolt Selection Guide
Shoulder bolts are precision fasteners with a smooth cylindrical shoulder between the head and the threaded end. The shoulder—not the threads—is the functional element of the fastener. It provides the bearing surface for pivoting parts, a precision guide for sliding components, a standoff for clearance gaps, or a press-fit alignment pin. Specifying a shoulder bolt correctly requires understanding four distinct dimensions—shoulder diameter, shoulder length, thread diameter, and thread length—and how each affects fit in the assembly. This guide covers the complete specification and selection process.
In This Guide:
- Shoulder Bolt Anatomy
- How to Specify a Shoulder Bolt
- Shoulder-to-Thread Relationship
- Clearance Hole Sizing
- Bearing Fit Tolerances
- Common Applications
- Material Options
- Frequently Asked Questions
Shoulder Bolt Anatomy
A shoulder bolt has five distinct dimensions that must be specified:
- Shoulder Diameter: The precision-ground OD of the smooth cylindrical section. This is the primary dimension—it determines bearing fit and clearance hole size. Common inch shoulder diameters: 3/16", 1/4", 5/16", 3/8", 1/2", 5/8", 3/4", 1".
- Shoulder Length: The length of the smooth cylindrical section, measured from under the head to the beginning of the thread relief. This is the working length in most applications—it controls stack height in pivot assemblies and standoff distance.
- Thread Diameter: Always smaller than the shoulder diameter. For inch shoulder bolts per ASME B18.3, the thread diameter is standardized per shoulder size (see table below).
- Thread Length: The length of the threaded section. Long enough to engage the receiving tapped hole with at least 1.5 thread diameters of engagement.
- Head Diameter / Height: The socket head cap, driven by an internal hex key. Head diameter is larger than the shoulder to provide a bearing shoulder against the mating part.
How to Specify a Shoulder Bolt
Shoulder bolts are specified by shoulder diameter × shoulder length, with thread size understood from the shoulder diameter per standard. When ordering, state: shoulder diameter, shoulder length, material, and finish. Example: “3/8" shoulder × 1" long, alloy steel, black oxide.”
The thread size is not a free variable—it is fixed by the shoulder diameter per ASME B18.3. This ensures the thread is always strong enough relative to the shoulder and the head bears properly against the assembly. The table below lists the standard shoulder-to-thread size relationship for inch shoulder bolts:
| Shoulder Diameter | Thread Size | Hex Key Size |
|---|---|---|
| 3/16” | #10-24 | 3/32” |
| 1/4” | 1/4-20 | 1/8” |
| 5/16” | 1/4-20 | 5/32” |
| 3/8” | 5/16-18 | 3/16” |
| 1/2” | 3/8-16 | 1/4” |
| 5/8” | 1/2-13 | 5/16” |
| 3/4” | 5/8-11 | 3/8” |
| 1” | 3/4-10 | 1/2” |
Shoulder-to-Thread Size Relationship
A critical dimension to understand: the thread diameter is always smaller than the shoulder diameter. This means the thread end passes through the clearance hole and engages the tapped hole in the receiving plate, while the shoulder itself bears against the bore in the pivot or sliding part. The head clamps the assembly from above.
The receiving tapped hole must be sized for the thread, not the shoulder. If you are using a 3/8" shoulder bolt, the tapped hole receives a 5/16-18 thread—not a 3/8" thread. This is a common error when retrofitting standard socket screws with shoulder bolts; the tapped hole often needs to be re-tapped to the shoulder bolt’s thread size.
Clearance Hole Sizing for the Shoulder
The shoulder passes through a clearance hole in the pivot or sliding part. The fit between the shoulder and this hole determines whether the assembly pivots freely, slides smoothly, or is locked in position. Three standard fits apply:
| Shoulder Dia. | Close Fit (H7) | Free Fit (H8) | Loose Fit |
|---|---|---|---|
| 3/16” | 0.1880” | 0.1890” | 0.1910” |
| 1/4” | 0.2510” | 0.2520” | 0.2540” |
| 5/16” | 0.3135” | 0.3145” | 0.3165” |
| 3/8” | 0.3760” | 0.3770” | 0.3790” |
| 1/2” | 0.5010” | 0.5020” | 0.5040” |
| 5/8” | 0.6260” | 0.6275” | 0.6295” |
| 3/4” | 0.7510” | 0.7530” | 0.7550” |
Bearing Fit Tolerances
Inch shoulder bolts per ASME B18.3 are ground to a tolerance of +0.000 / –0.0005" on shoulder diameter. This close tolerance enables consistent bearing fits when the receiving bore is machined to match:
- Close fit (locating): Bore at +0.001 / +0.002" over shoulder diameter. Used when the pivot or slider must be precisely located with minimal play—instrument panels, precision cams, linkages.
- Free fit (rotating): Bore at +0.002 / +0.004" over shoulder diameter. Used for freely rotating parts like pulleys, links, and rollers that turn on the shoulder without a separate bearing.
- Loose fit (general): Bore at +0.004–+0.006" over shoulder diameter. Used for clearance applications, non-precision guides, and stops where exact position is not critical.
For high-cycle pivot applications, use a needle roller bearing, bronze bushing, or polymer sleeve in the bore rather than direct metal-to-metal contact. The shoulder bolt’s precision OD tolerance makes it a natural bearing journal; adding a bushing extends service life dramatically in rotating applications.
Common Shoulder Bolt Applications
- Pivot pins: Links, cams, rocker arms, door hinges, and lever assemblies where parts rotate on the shoulder OD. The shoulder is the bearing surface; the threaded end clamps into the frame or base plate.
- Stripper bolt / spring guide: In punch press dies and injection molds, shoulder bolts guide stripper plates and die components up and down. The shoulder controls travel distance; a spring bears against the head. Length specification is critical to set stroke.
- Linear guides: Shoulder bolts as linear guides in small mechanisms—slots in sliding plates with shoulder providing the bearing surface. Used in document feeders, clamping fixtures, and modular tooling.
- Standoffs and spacers: The shoulder length sets a controlled gap between two surfaces. Stacking multiple shoulder bolts creates a consistent standoff field across a panel or circuit board mount.
- Stop pins: The shoulder OD provides a positive stop surface for moving parts. Repeatability is high because the shoulder is precision ground.
Material Options
Eugene Fastener stocks shoulder bolts in two materials:
- Alloy Steel (ASTM A574): Standard choice for machine tool, industrial, and tooling applications. Black oxide finish standard. Ground shoulder provides consistent bearing fit. Not suitable for wet or corrosive environments without plating or oil.
- 18-8 Stainless Steel: For food processing equipment, washdown environments, outdoor assemblies, and chemical exposure. Same bearing tolerances as alloy; lower tensile strength at approximately 85,000–125,000 PSI. Apply anti-seize to prevent galling in stainless-to-stainless assemblies.
Metric shoulder bolts (ISO 7379) are available in the same alloy and stainless options, with shoulder diameters specified in millimeters (M6 through M20 range, with shoulder diameters from 8mm to 25mm).
Frequently Asked Questions
How is a shoulder bolt measured?
A shoulder bolt is specified by shoulder diameter and shoulder length. Shoulder length is measured from under the head to the start of the thread relief—the length of the smooth cylindrical section only. Thread diameter is determined by the shoulder diameter per ASME B18.3 and is not a separate specification for standard shoulder bolts.
What is the thread size for a 3/8" shoulder bolt?
A 3/8" shoulder bolt has a 5/16-18 thread per ASME B18.3. The thread is always smaller than the shoulder diameter. The receiving tapped hole must be sized for 5/16-18, not 3/8-16.
Can I use a shoulder bolt as a bearing pivot without a bushing?
Yes, for moderate-duty applications. Shoulder bolts are ground to tight tolerances (+0.000 / -0.0005") and can serve directly as a bearing journal in a precision-bored hole. For high-cycle, high-load, or continuously rotating applications, adding a needle bearing, bronze bushing, or polymer sleeve to the bore substantially extends service life.
What is the difference between a shoulder bolt and a stripper bolt?
They are the same fastener. “Stripper bolt” is the tooling industry term for a shoulder bolt used in die sets and injection molds to guide stripper plates. The shoulder controls travel and the spring bears against the head. Standard shoulder bolts (ASME B18.3) are interchangeable with stripper bolts in most die applications.
What clearance hole should I drill for a 1/2" shoulder bolt?
For a free fit (the most common pivot application), drill to 0.5020". For a close fit (precision location), drill to 0.5010". For a loose fit, drill to 0.5040". The shoulder is ground to +0.000 / -0.0005" so the fit is consistent if the bore is accurate.
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