US Bolts VS Metric Bolts

Fundamental differences in standard systems

1. Measurement basis
American bolts (UNF/UNC) are based on imperial units (inches), while metric bolts (such as ISO 7380) are based on millimeters. For example, 1/4-20 UNC (American) represents a coarse thread with a diameter of 1/4 inch and 20 threads per inch, while M6×1.0 (metric) represents a thread with a diameter of 6mm and a pitch of 1mm.

2. Thread angle and tooth type
Both use a 60° thread angle, but the top and bottom of the American thread are flat (UN thread), while the top of the metric thread is flat and the bottom is rounded (ISO thread). This subtle difference leads to differences in stress distribution and fatigue life between the two.

3. Standardization organization
Taking hexagonal bolts as an example, American follows the ASME B18 series of standards, while the metric system is based on ISO 898 and DIN 931 DIN 933 and other standards. For example, the strength grade markings of American bolts (such as SAE Grade 5 and Grade 8) and the metric performance grades (such as Grade 8.8 and Grade 10.9) use different evaluation systems.

Comparison of key parameters

1. Size identification system

2. Correspondence between strength levels

3. Tool compatibility
Wrench interface: The US system uses a fractional system (such as 9/16″), and the metric system uses a millimeter system (such as 14mm). Mixing them will cause edge wear.
Thread repair: US systems require UNC/UNF taps, and metric systems require M series taps. They cannot be used interchangeably.

Selection logic in engineering applications

1. Regional preference
The North American market is dominated by US bolts (AN/MS standards are common in the automotive and aviation fields);
Europe and Asia mostly use the metric system (DIN standards are commonly used in German industrial equipment).

2. Differences in dynamic load scenarios
US UNF fine-pitch threads (such as 1/2-20 UNF) are more suitable for high-frequency vibration environments (such as engine blocks);
Metric fine-pitch threads (such as M12×1.25) have a smaller pitch and better sealing in precision instruments.

3. Repair and replacement strategy
Approximate size substitution: M6≈1/4″ (actual diameter 6mm=0.236″, slightly less than 0.25″ of 1/4″), tolerance needs to be evaluated;
Equivalent strength substitution: SAE Grade 5 (≈8.8) can be temporarily interchangeable, but key parts must be strictly matched;
Thread remediation plan: Do not force M6×1.0 taps (theoretical diameter difference 0.014″) into damaged threaded holes of US 1/4-20 bolts.

US bolts and metric bolts are the two major mainstream standard systems in the field of global mechanical engineering. The difference is not only reflected in the size unit, but also related to design concepts, manufacturing processes and engineering applications. Understanding the difference between the two is crucial for equipment maintenance, cross-border project collaboration and supply chain management.