Metric vs. Inch Screw Size Conversion Guide
When repairing imported equipment or mixing parts from different countries, screw thread confusion is common. Understanding the difference between metric and inch standards prevents stripped threads and wasted time.
Metric (ISO) vs. Inch (UNC/UNF) Standards
Metric screws are designated by diameter in millimeters (e.g., M6) plus pitch in mm (e.g., M6×1.0). Inch screws use a diameter number or fraction plus TPI — threads per inch (e.g., 1/4-20 means 1/4-inch diameter, 20 TPI). The US primarily uses inch standards; most of the world uses metric. Japan, Korea, and Europe use metric (JIS/ISO).
Why You Cannot Mix Metric and Inch Screws
Even when outer diameters are close, the thread pitch differs. Forcing a 1/4" bolt into an M6 hole (or vice versa) will initially seem to work but will cross-thread and strip the fastener. Always verify both diameter and pitch before substituting.
TPI to Pitch Conversion
To convert TPI to pitch in millimeters: Pitch (mm) = 25.4 ÷ TPI. For example, 1/4-20 UNC: pitch = 25.4 ÷ 20 = 1.27 mm. M6 pitch = 1.0 mm — similar diameter but noticeably different pitch.
Frequently Asked Questions
Use a thread pitch gauge to check pitch and a digital caliper to measure outer diameter. For inch screws, a thread counting gauge gives TPI directly. If the screw is metric, the pitch will be a round number like 0.5, 0.75, 1.0, or 1.25 mm.
Most modern bicycles worldwide use metric screws (M5 for brake bolts and stem bolts, M6 for other components). Older American bikes may use inch screws. Japanese-brand bikes (Shimano components) use metric exclusively.
Coarse threads (UNC for inch; standard pitch for metric) are easier to assemble quickly and more resistant to cross-threading. Fine threads (UNF; fine pitch metric) offer higher clamping force per torque and better resistance to loosening under vibration.