What year did American cars start using metric bolts?
There wasn't a single year; American cars gradually adopted metric bolts beginning in the late 1970s, with widespread use by the 1990s.
The shift was driven by globalization and the auto industry's move toward ISO metric standards. While some older models and mission-critical engine components still used SAE fasteners for longer, most new American cars from the late 1980s onward employed metric fasteners for the majority of applications.
Timeline of adoption
Below are notable milestones illustrating how metric hardware entered U.S.-built vehicles over time. Note that adoption varied by manufacturer, model, and component.
- Late 1970s to early 1980s — Initial incursions: Some non-structural assemblies and imports began using metric hardware, and American suppliers started aligning with ISO standards for international parts.
- Mid-1980s — Increasing rollout: Major U.S. automakers begin integrating metric fasteners on new platforms, especially in engines and body assemblies designed for global supply chains.
- Late 1980s to early 1990s — Broad trend accelerates: Metric fasteners become common in many components across new U.S. models; some high-torque critical fasteners remain SAE in certain legacy designs or specific service applications.
- 1990s — Standardization toward ISO metric: The industry moves toward more uniform metric sizing across platforms, with most new vehicles using metric bolts for most fastener applications.
In practice, this means that while even today you may encounter SAE (inch-based) bolts in certain older vehicles, newer American-made cars from roughly the 1990s onward predominantly use metric bolts for most fasteners. Always consult the service manual or a fastener chart for precise bolt sizes on a specific model or component.
Practical implications for owners and mechanics
Understanding whether a bolt is metric or SAE matters for maintenance, repairs, and parts compatibility. Mixing fasteners or using incorrect sizes can lead to improper clamping force, thread damage, or failure. Here are some key points to keep in mind when working on American cars that may use metric bolts.
Before engaging in maintenance or repairs, verify the bolt size and thread pitch with the service documentation, a thread gauge, or a fastener reference chart. For imports and some domestics, metric sockets and wrenches are essential to avoid cross-threading and tool slippage.
How to identify metric vs SAE bolts
Note the common indicators that help distinguish metric from SAE bolts. Look at thread pitch, head markings, and the context of the vehicle model and year. Metric bolts typically use a millimeter-based pitch and are labeled with metric sizes (e.g., M8, M10), while SAE bolts use inch-based designations (e.g., 1/4", 3/8").
Additionally, some bolts carry manufacturer-specific markings or are recycled from global platforms. If in doubt, consult the vehicle's user manual or a professional mechanic to ensure you obtain the correct fastener type and torque specification.
Summary
American cars did not switch on a single date; metric bolts arrived gradually, beginning in the late 1970s and becoming widespread through the 1980s and 1990s as the industry moved toward ISO metric standardization. Today, most new U.S.-built vehicles rely on metric fasteners, though older models or certain specialty components may still use SAE sizes.
Further reading
For readers who want to dive deeper, consult automotive maintenance manuals, OEM service documentation, and credible automotive history sources on fastener standardization and the ISO metric transition in North American manufacturing.
Do American cars use metric bolts?
Yes, most new American cars exclusively use metric bolts, a trend that began in the 1980s and became dominant by the 1990s due to globalization and design standardization. Older American vehicles, especially those from before the 1980s, are almost entirely built with standard (imperial) fasteners, while many from the 80s and 90s may have a mix of both metric and standard bolts.
Why modern American cars use metric bolts
- Global standards: The increasing number of components sourced from overseas and the need to compete in foreign markets made adopting the metric system a necessity for manufacturers.
- Engineering standardization: The metric system, with its decimal dimensions, is often considered more precise and easier for engineers to work with than the imperial system.
- Design changes: Automakers did not redesign every platform or engine for the metric system all at once. Instead, as older, long-lived designs were replaced by new ones, they were transitioned to metric fasteners, leading to a period where vehicles had both types.
How to identify the fasteners
- Newer cars: You can assume that most fasteners on modern American cars are metric.
- Older cars: In vehicles from the 1980s and 1990s, it is common to find a mix of both metric and standard (SAE) bolts.
- Best practice: It is always a good idea for mechanics to have a full set of both metric and SAE tools to avoid any confusion.
When did American cars switch to metric fasteners?
The US attempted to go metric in the 70s That's when this showed up. By the 1980s it had taken hold with cars. We've all heard about "5.0s" instead of 302s, etc. The fasteners and measurements used to build the cars went metric as well.
What year did GM switch to metric?
GM first decided to switch to metric in 1973, announcing the move to its suppliers. While this was the initial decision, the transition was gradual, and cars became "primarily (if not entirely) Metric" by the mid-to-late 1990s.
- 1973: GM announces the decision to convert to the metric system and begins notifying suppliers.
- Mid-to-late 1990s: GM cars are largely metric, with measurements and fasteners being metric-based.
What year did Ford start using metric?
Ford, along with other U.S. automakers, began a transition to metric in the late 1970s following the Metric Conversion Act of 1975, but the switch was gradual and voluntary, not an immediate change. Some vehicle components began to be metric in the late 1970s, with this trend accelerating in the 1980s as models like the Ford Tempo and Taurus were introduced.
- Late 1970s: The shift began with some components, such as the design for the F-Series truck refresh starting around 1978, which included metric fasteners.
- 1980s: The transition accelerated. Major models like the Ford Tempo and Taurus, introduced in the mid-to-late 1980s, were designed with metric components and had engine displacements measured in metric units (e.g., the "5.0" liter engine).
- Gradual and voluntary process: The conversion was not a single event but a long and voluntary process mandated by the 1975 Act, which did not force companies to switch but encouraged it. This resulted in a mix of metric and customary units in vehicles for many years, a situation that continues in some areas today.
