The global fastening landscape is shaped by history, engineering standards, and supply‐chain realities. While nearly all countries have officially adopted the metric system, inch-based (imperial) fasteners remain deeply entrenched in U.S. aerospace and many automotive applications. Procurement managers must understand why these legacy units persist and how they affect sourcing and standardization. This whitepaper examines the historical roots, technical nuances, and practical implications of imperial vs. metric fastener usage, with a focus on aerospace and automotive industries and the challenges of a dual-standard supply chain.

Historical & Technical Roots of Imperial Fasteners

Imperial fasteners trace back to the British Empire’s standardization of weights and measures in the 19th century. Early industrial revolutions in England and the United States defined screw threads by inches and threads per inch (e.g. the Unified Thread Standard or UTS). After World War II, while much of the world moved to ISO metric threads (adopted internationally by the 1960s), U.S. military and industrial specifications (SAE/ANSIV, MIL-SPEC, and the old “Whitworth”/“BA” systems) were already well-established. Retooling factories and requalifying hardware for a new thread standard is expensive, so American aerospace (Boeing, NASA, defense contractors) and older automotive designs continued to use imperial fasteners.

Technically, metric and imperial threads share a similar 60° profile but differ in pitch and nominal sizes. For example, a 1/4″ bolt (6.35 mm) has 20 threads per inch (coarse), whereas its closest metric counterpart is M6×1.0 (6 mm diameter with a 1.0 mm pitch). These small differences mean that swapping an imperial bolt for a metric one can lead to misfits or stress concentrations. Over generations, engineers designed fuselages, engines, and chassis with specific tolerance stacks and bolt grades that match the original standard. In many U.S. designs, even today, standard part numbers and documentation refer to inch sizes, reflecting this legacy.

Global Prevalence of Imperial vs Metric

Most of the world now uses metric fasteners, but a few key regions and industries still rely on imperial sizes. The map below illustrates the global metrication timeline. The United States (black) stands out as one of the last holdouts of fully inch-based standards, alongside Myanmar and Liberia. North America’s automotive and aerospace sectors still source billions of dollars of inch-based bolts each year, whereas European, Asian, and South American manufacturers primarily supply metric hardware.

Figure: Global map of metrication by country. The USA (black) is among the few countries not to fully adopt the metric system, underscoring why inch-based fasteners remain common in U.S. aerospace and automotive industries.

In practice, U.S.-built products dominate global aerospace supply chains. Boeing, Lockheed Martin, and General Dynamics (U.S.) design with SAE and AN/MIL specifications, so even international maintenance may require inch sockets. By contrast, European aircraft (Airbus, Dassault) are predominantly metric. In automotive, American legacy platforms (classic domestic models, heavy trucks) often contain a mix: some engine and chassis bolts remain inch, even as new models switch to metric. Japanese, Korean, and European cars have been metric for decades, but when U.S. cars are designed for global markets they often use metric to ease production – yet many legacy aftermarket parts (engines, suspensions) still include inch components. Thus, globally imperial fasteners are most prevalent in U.S. manufacturing and wherever U.S. equipment is exported, creating a patchwork of needs.

Imperial Fasteners in Aerospace Applications

The aerospace industry provides clear examples of imperial dominance. Nearly all U.S. military aircraft (fighter jets, helicopters, bombers) and NASA spacecraft are assembled with inch-based bolts, nuts, and rivets. For instance, classic Boeing airliners (737, 747, 777) use Unified/NAS series hardware; even wings and fuselage panels list parts by inches. Aircraft maintenance manuals and logistic parts catalogs in the U.S. reference Federal Standard (FS) or Aerospace Standard (AS) codes for inch components. Aircraft mechanics typically carry tool sets labeled by inch sizes (e.g. 1/2″, 9/16″) for U.S. fleets, versus millimeter kits for foreign jets.

Some mixed cases occur: an American-made jet engine (e.g. Pratt & Whitney) mounted on a European airframe may bring inch bolts, while the rest of the plane (Airbus fuselage) is metric. Likewise, an engine from Europe (metric) on a U.S. airframe could lead to hybrid hardware. In global supply chains, many vendors and military specifications still standardize on imperial threads (AN, MS, NAS, MIL-SPEC). Switching a certification like MIL-SPEC-823 (“MS20470” unified-screw series) to a metric equivalent requires full recertification – a burdensome process rarely undertaken unless absolutely needed. In practice, international maintenance standards (FAA, EASA) allow either system as long as it matches the design specification, but the default is usually the original imperial spec for U.S. aircraft. As one aerospace engineer quipped: “If it’s a Boeing, grab the inch tools; if it’s a Russian, grab metric.” (Note: Russian and Chinese aircraft are fully metric by national standard.)

Imperial Fasteners in Automotive Applications

U.S. automotive history also reflects a transition from imperial to metric, but with uneven progress. Older American cars (pre-1980s) and large trucks often used SAE bolt sizes: for example, Ford pickups used 13/16″ and 7/8″ lug nuts, and many classic V8 engines had inch-diameter head bolts. By the 1970s and 1980s, U.S. automakers began a partial metrication alongside global platforms, introducing metric (M6, M10, M12, etc.) on new models. Today’s American cars and light trucks are predominantly metric, aligning with global suppliers. However, heavy vehicles and legacy models may still contain unified bolts. Agricultural and construction equipment (think of large farm tractors, earthmovers) made or used in the U.S. often retain inch threads because of the enormous cost of retooling and recertifying older designs.

Consider parts interchange: an American classic car restored in 2025 may require sourcing 1/2″ USA-made H bolts (heavy hex), which are not readily stocked by most retailers. By contrast, an imported metric part would be off-the-shelf. The mix can be frustrating: one mechanic noted replacing an air filter housing on a foreign-imported farm tractor needed both 1/2″ and 12 mm sockets. In procurement terms, the automotive industry faces a partial metrication: new designs favor metric for standardization and simplicity, but production lines still often hold dual systems to support legacy parts. Procurement managers working with mixed fleets must therefore maintain two supply pipelines.

Supply Chain and Sourcing Challenges

Maintaining both imperial and metric fasteners imposes significant supply-chain complexity. Vendor networks differ: Suppliers specializing in metric components (common in Europe/Asia) may not produce all inch sizes, and vice versa. For a U.S. firm seeking M12×1.5 bolts, domestic stockists suffice; for a European company needing UNC 7/16″-14 bolts, one often must import from U.S. stock or source from global distributors. This can extend lead times. Similarly, small or obsolete inch sizes can suffer from low availability, forcing reliance on specialty firms or longer lead times for made-to-order production.

Inventory management becomes complicated. Fasteners must be stocked and labeled to avoid mix-ups. A procurement manager often segregates two inventories: one for metric (organized in bins by M-sizes) and one for SAE/imperial (by fractional sizes). Tools and gauges likewise double up. This redundancy increases costs – two sets of wrenches, two catalogs of nuts/bolts, and separate quality certificates. Interchangeability is minimal: a 3/8″-16 bolt is not interchangeable with an M10-1.5 (different diameter and thread pitch), so part families are distinct. Even in cases where sizes nearly match (e.g. 1/4″ vs 6 mm), they cannot be mixed because a 1/4″ bolt in a 6 mm tapped hole leaves play and can shear under load.

Standardization bodies complicate matters. Aerospace fasteners often must meet MIL-Spec or AS standards (e.g. AN365 locknuts). These standards are inch-based. Metric fasteners typically meet ISO or DIN standards. Procurement must track which spec each part conforms to. When a drawing calls out a specific standard (AS, AN, MIL vs ISO 4014, DIN 6915, etc.), substitutes must be identical in material, strength class, and dimensions. Swapping to a metric standard often triggers a full requalification of the assembly (testing for clamp load, fatigue, etc.), a costly process. In regulated industries like aviation and automotive (TS16949, ISO9001 compliance), any undocumented change can break certification.

Real-world example: A major aerospace contractor found that shifting a structural panel from an AN4 (1/4″-28) bolt to an M6 bolt required redesigning the entire mating part because the head clearance and tightening torque changed. The cost of engineering review and new certification far outweighed the nominal savings of standardizing to metric components. Such stories emphasize why many companies “avoid mixing systems” to prevent assembly errors or field failures.

Barriers to Full Metrication and Procurement Risk

Complete conversion to metric fasteners in U.S.-centric industries faces steep barriers. Inertia and cost: Rewriting engineering drawings, retraining maintenance crews, and replacing legacy stock is expensive. For instance, the U.S. Navy’s attempt to metricate in the 1970s was scaled back because the supply base could not deliver metric screws in required volumes, causing delays. Converting an entire vehicle or aircraft system at once is essentially a re-design project, seldom justified by marginal efficiency gains.

Risk implications: Procurement managers must consider continuity risks. If a supplier of inch bolts goes out of business, finding a metric alternate may not be straightforward because the mating parts are imperial. During disasters or military mobilizations, mismatches in fastener type could impede repairs. Having a dual system means also doubling the risk of specification errors. In the worst case, an incorrect fastener (imperial in a metric hole or vice versa) might only be noticed under stress, leading to part failure. The infamous NASA “Mars Climate Orbiter” incident (caused by a confusion of metric vs. imperial units in navigation software) serves as a cautionary tale of the costs of unit mismatch, even though it involved force units rather than threads. In practical procurement, a mislabeled batch of screws is similarly dangerous.

Certification and compliance: Many industries have formal processes tied to unit systems. For example, automotive companies may list torque specifications in either foot-pounds (imperial) or Newton-meters (metric). Engineering change notices (ECNs) to switch a factory from inch to metric tooling require quality approvals and sometimes government permits (in defense contracting). Failure to manage these processes can result in audit findings. Procurement must therefore handle even small unit changes as high-stakes actions.

Procurement Considerations & Best Practices

For procurement teams managing both systems, careful planning is key. Consider the following practical strategies:

• Vendor Networks: Identify suppliers that stock both imperial and metric parts, or establish relationships with distributors in each system’s stronghold. For critical fasteners, qualify multiple sources (e.g. a U.S. shop for inch and a European one for metric). Maintain legacy suppliers even if usage declines, to safeguard against obsolescence.
• Lead Times & Inventory: Track lead times separately. If an inch-sized bolt has a 6-week lead time from its manufacturer and a metric equivalent has 2 weeks, factor this into production scheduling. Keep minimum stock levels high for hard-to-get sizes. Also consider “kitting” fasteners in common assemblies to simplify ordering.
• Interchangeability Management: Never mix units in the same assembly without engineering approval. Maintain clear part-number systems: one part number per size/standard. Clearly label storage bins (e.g. “SAE UNC 1/2″” vs “ISO 12 mm”) and use color coding or warning tags where confusion is possible. Conduct regular audits to ensure metric and imperial bins are not swapped.
• Tooling Compatibility: Ensure maintenance crews and assembly lines have both wrench sets. For shop operations, tools with dual markings (size in fractional and mm) can help avoid confusion, but physical compatibility is still limited. Consider procurement of torque wrenches and tools calibrated in both units. For automated assembly (robotic nut runners), software must apply the correct torque setting based on unit.
• Certification & Documentation: Maintain separate quality records for metric vs imperial specs. When ordering, specify the standard (e.g. ASTM F568M for metric vs. SAE J429 for imperial) and require material test certificates reflecting that standard. For aerospace, ensure fasteners meet AS or EN certification as needed. Document any deviation requests fully. If a metric substitute is ever justified (e.g. supply shortage), treat it as a Controlled Engineering Change with full testing.
• Training & Communication: Educate procurement and engineering teams on the differences. Label engineering drawings clearly. Emphasize in RFQs and purchase orders the unit system (“size: 5/16-18 UNC” vs “size: M8x1.25”). Internally, keep track of projects still using each system – for example, a fleet upgrade might have an “inch parts list” for vehicles built before year X.

By anticipating these factors, procurement managers mitigate risks of supply disruption, specification errors, and certification non-compliance. A dual-system strategy also allows gradual transition: new programs or parts can adopt metric where sensible, while legacy programs continue in imperial. For example, a new airplane wing design might use metric fasteners to take advantage of global sourcing, but its avionics mounts (with legacy brackets) retain inch fasteners. This “hybrid” approach is common: industries accept the overhead of managing two systems to balance modernization with reliability.

Outlook and Conclusion

Imperial fasteners persist not out of technical superiority, but out of industrial momentum. U.S. aerospace and much of the automotive aftermarket still rely on unified threads because decades of engineering and manufacturing are built around them. Globally, this means procurement teams face challenges in sourcing, inventory, and compliance whenever inch-based components are involved. While metric fasteners offer advantages of global standardization and decimal convenience, fully displacing imperial hardware carries high conversion costs and risks.

Procurement managers should view dual-system handling as part of everyday reality. Embrace best practices: segment inventories, qualify versatile suppliers, and maintain thorough documentation. By understanding that imperial vs metric fasteners is fundamentally a supply-chain issue as much as an engineering one, managers can make informed choices. Ignoring legacy systems, or forcing abrupt metrication, could disrupt production and maintenance. Instead, careful alignment of design specifications, vendor capabilities, and logistical planning is the optimal path. In a global market, both systems coexist – and smart procurement acknowledges and plans for this fact.