Are car bolts metric?
In most modern passenger cars, yes—the fasteners (bolts and studs) are metric. That said, there are exceptions: older American cars and certain components still use imperial (inch-based) sizes.
This article examines why metric sizing dominates today, where imperial sizes might still appear, and how drivers and technicians can accurately identify and replace bolts to maintain safety and reliability.
Understanding the landscape of metric versus imperial fasteners
Automakers design and source fasteners to standardize across regions and models. Over the last few decades, metric sizing has become the default for most newer vehicles, especially for wheel studs, engine hardware, and suspension components. Imperial sizes persist only in a minority of older cars or certain specialty applications.
Before we dive into specific sizes and examples, it helps to know where you’re most likely to encounter metric versus imperial bolts on a typical modern car.
- Metric fasteners are common for wheel studs and lug nuts on most current cars (e.g., M12x1.5 or M14x1.5 thread sizes, with a variety of seat types depending on the wheel).
- Engine, transmission, and suspension bolts in contemporary vehicles are predominantly metric, spanning approximate sizes from M6 to M12 and beyond, with standardized grades for strength.
- Imperial (inch-based) sizes remain possible on some older models, certain American-market vehicles, or specific aftermarket or OEM parts that preserved SAE threading.
In practice, the trend across the industry is toward metric compatibility, which simplifies tooling and parts supply for global manufacturers and service networks.
Where imperial sizes may still appear
Imperial fasteners tend to crop up in older vehicles or in niche applications where a manufacturer or aftermarket supplier preserved SAE threading. If you’re working on a classic American car or a restoration project, you may encounter bolts labeled in inches rather than millimeters.
- Older American cars and some classic trucks may use inch-based threads such as 1/4"-20, 5/16"-18, or 3/8"-16 for various bolts.
- Some aftermarket components, performance parts, or restorations might use imperial fasteners for compatibility with older hardware or supplier availability.
- Brake, fuel, or hydraulic fittings can involve a mix of standards depending on the part and era; always confirm the correct size from the service manual or parts diagram.
Despite these occurrences, imperial fasteners are increasingly rare in new-car production and in OEM replacement parts. Verifying the correct standard before purchase is essential to ensure proper fit and safety.
How to identify and replace bolts safely
Correct identification is crucial to avoid cross-threading, stripping, or compromised safety. The steps below help you confirm the right fasteners and install them correctly.
- Consult the vehicle’s service manual or OEM parts catalog for the exact bolt size, length, thread pitch, and torque specification.
- Inspect the existing fastener for stamping (e.g., M12x1.5) or measure with a thread gauge and caliper if markings are unclear.
- When replacing, choose bolts of the correct grade and material (for example, metric bolts rated for appropriate strength such as 8.8 or 10.9 where specified).
- For wheels, use the proper wheel studs and lug nuts with the correct seating type and torque value; ensure the thread pitch matches the hub and wheel assembly.
- Apply lubrication or anti-seize only where specified by the manufacturer; some high-load fasteners require dry threads to achieve the correct torque.
Adhering to the right size, pitch, and torque ensures secure fastening and reduces the risk of damage or failure during operation.
Summary
Most modern cars rely on metric fasteners for engine, suspension, and wheel hardware, reflecting global standardization. Imperial (inch-based) sizes still appear in some older vehicles or specific parts, but they are less common in current production. Always verify the exact specifications with the vehicle’s documentation or a trusted parts source before replacement to ensure proper fit, torque, and safety.
