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SAE J370 (Cancelled): Bolt and Capscrew Sizes for Construction and Industrial Machinery
The SAE J370 standard, originally issued in 1968 and cancelled in April 2006, prescribed a set of preferred bolt and capscrew diameters for use in construction and industrial machinery. The goal was to improve serviceability by encouraging designers to choose common sizes with Unified Coarse (UNC) threads that could be easily serviced with standard tools. Although no longer active, its legacy offers valuable lessons about balancing performance with maintainability in heavy equipment design.
Overview of SAE J370
SAE J370 was a recommended practice that listed eight bolt and capscrew diameters from 3/8 inch to 1-1/4 inch, all using the UNC thread series. The selection was intended to cover a wide range of proof loads while minimizing the number of distinct tools required for field service. The standard acknowledged that some applications might require other sizes or fine threads, but it strongly preferred the convenience of UNC threads for the listed diameters.
| Diameter (in) | Thread Series | Proof Load Range |
|---|---|---|
| 3/8 | UNC | From 1043 kg (2300 lb) |
| 1/2 | UNC | — |
| 5/8 | UNC | — |
| 7/8 | UNC | — |
| 1 | UNC | — |
| 1-1/8 | UNC | — |
| 1-1/4 | UNC | Upward at convenient intervals |
The standard also noted that diameters above 1-1/4 inch should increase in 1/4-inch increments, and below 1/2 inch should decrease in 1/16-inch increments, extending the logic of common sizes to both ends of the range.
Rationale for Cancellation
The official rationale states that the scope and purpose were not in unison with the content, and the reason for having this type of document was no longer valid. Over time, machinery design became more diverse and application-specific; a simple list of preferred sizes could not adequately address the varying strength, fatigue, and packaging requirements of modern equipment. The SAE Fasteners Committee therefore withdrew the standard in 2006.
⚠️ Cancelled Standard: SAE J370 is no longer maintained or endorsed by SAE International. Designers should rely on current fastener standards and application-specific analysis rather than this historical list.
Lessons for Designers and FAQ
The core intent of SAE J370—improving serviceability by using common fastener sizes—remains relevant. However, the cancellation highlights that fastener selection must be a holistic decision, balancing ease of maintenance with engineering requirements.
🛠️ Engineering Design Insight: Standardization on common fastener sizes reduces inventory costs, simplifies training, and speeds up field repairs. But a one-size-fits-all list may not suit every application. Always verify that selected fasteners provide adequate strength for the worst-case load scenario while still enabling practical service access. Periodically review internal standard lists to ensure they align with current industry practice and supply chain realities.
Frequently Asked Questions
What were the preferred bolt sizes under SAE J370?
The standard recommended eight diameters: 3/8, 1/2, 5/8, 7/8, 1, 1-1/8, and 1-1/4 inches, all with UNC threads. Sizes above 1-1/4 inch were to increase in 1/4-inch increments.
Why did SAE cancel J370 in 2006?
According to the rationale, the scope and purpose were not aligned with the content, and the need for such a list was no longer valid. Modern machinery designs often require application-specific fastener selection rather than a one-size-fits-all list.
Should designers still use the sizes from SAE J370?
Not necessarily. While the listed sizes are common and serviceable, the standard is cancelled and no longer represents a current SAE recommendation. Designers should follow current fastener standards and verify suitability for their specific application.
How can serviceability be improved without sacrificing strength?
Select proven common sizes and UNC threads where appropriate, but confirm that the chosen fastener can handle required loads through proper grade selection and torque specifications. Consider tool clearance, access for maintenance, and standardization across machine families.
