Overview
ISO 6698:1989 specifies the screw thread profile, limits and tolerances for threads used to assemble freewheels on bicycle hubs. The standard adopts the ISO basic thread profile while using the inch pitch of the traditional British Standard Cycle (BSC) freewheel thread to ensure satisfactory interchangeability. It defines base dimensions in millimetres for the 1 3/8 in × 24 t.p.i. (1.375 in × 24 tpi) thread and prescribes verification by gauges made to ISO 1502.
Key topics and technical requirements
- Thread form: Uses the ISO basic thread profile as defined in ISO 68, applied to freewheel/hub threads.
- Nominal size and pitch: 1.375 in × 24 tpi (inch pitch retained to match BSC 1.37 × 24 tpi).
- Base dimensions (mm) for 1.375 × 24 tpi:
- Major (external) diameter: 34.925 mm
- Pitch (flank) diameter: 34.238 mm
- Minor (internal) diameter: 33.779 mm
- Tolerance class: Thread tolerances are in class 6 in accordance with ISO 965‑1; example designation: 1,375‑24 6H/6g (hub/freewheel tolerance positions).
- Pitch and flank tolerances:
- Pitch tolerance = 0.005 mm
- Flank angle tolerance: +0°15′ (full-flank profiles) or +0°16′ (truncated-flank profiles)
- Gauging and verification: Gauges and limit checks follow ISO 1502; the standard includes dimensional limits for “go” and “no-go” gauges, ring and plug gauges and worn/nominal gauge conditions.
- Interchangeability note: The ISO profile’s crest truncation produces a flank-diameter difference of about 0.003 mm compared with the BSC 1.37×24 tpi. In rare worst-case tolerance combinations this could cause tight assembly at the outer diameter, but practical trials indicate the risk is very low.
Applications and users
ISO 6698:1989 is intended for:
- Bicycle manufacturers and component suppliers specifying freewheel-to-hub interfaces
- Hub and freewheel designers, mechanical engineers and OEMs ensuring interchangeability with BSC threads
- Quality control, metrology and inspection teams using ISO 1502 gauges for acceptance testing
- Aftermarket spare-parts suppliers, repair workshops and standards committees concerned with thread compatibility and replacement parts
Practical uses include production tolerancing, specification of inspection gauges, parts interchangeability assessment and design validation for freewheel assemblies.
Related standards
- ISO 68 – Basic profile for metric screw threads (profile definition)
- ISO 965‑1 – Thread tolerances and classes (principles and fundamental deviations)
- ISO 1502 – Verification by limit gauges (thread gauge design and use)
- British Standard Cycle (BSC) thread practice - historical reference for existing bicycle freewheel threads
Keywords: ISO 6698, bicycle freewheel threads, bicycle hub threads, 1 3/8 × 24 tpi, BSC thread, screw thread tolerances, ISO thread profile, ISO 1502 gauges.
