Overview
ISO 888:2012 - Fasteners - Bolts, screws and studs - Nominal lengths and thread lengths - specifies how to define, measure and calculate the nominal lengths and thread lengths of bolts, screws and studs with ISO metric screw threads (per ISO 68‑1). The standard is intended for use in product standards, technical drawings and other documents that call out fastener sizes and thread extents.
Key topics and technical requirements
- Scope: Applies to bolts, screws and studs with ISO metric thread (ISO 68‑1). It specifies rules for length definition and thread length unless a product standard states otherwise.
- Length positioning: Defines where the nominal length l is measured for different head and end types:
- For most headed screws: from the bearing surface perpendicular to the axis to the end.
- For countersunk (flat) heads: from the top edge of the head to the end.
- For countersunk domed heads: from the theoretical intersection of the dome and head diameter.
- For headless screws, studs and rods: length is between ends (end-to-end).
- For studs: length is measured from the threaded/smooth transition on the installation side to the nut end.
- Thread length calculation: Unless otherwise specified, thread length b is calculated by formula:
- For l ≤ 125 mm: b = 2d + 6 mm
- For 125 mm < l ≤ 200 mm: b = 2d + 12 mm
- For l > 200 mm: b = 2d + 25 mm
- Note: for short screws where the smooth shank would be ≤ 0.5d, screws should be fully threaded.
- Dimensional tables: ISO 888 contains nominal length tables and recommended nominal lengths (some values shown in parentheses should be avoided if possible).
- Tolerances: Length and thread-length tolerances are specified in ISO 4759‑1. For certain drive/head types (external/internal “noble” drives vs slotted/embossed types) specific tolerance rules are referenced.
Applications and who uses it
ISO 888:2012 is used by:
- Mechanical designers and draftsmen for part drawings and fastener callouts
- Fastener manufacturers for production specifications and inspection
- Procurement and quality engineers for acceptance criteria and supplier requirements
- Standards writers and product committees when referencing fastener lengths in product standards
Practical uses include ensuring consistent measurement practice on engineering drawings, selecting fasteners with appropriate thread engagement, and avoiding ambiguous length specifications that can lead to assembly problems.
Related standards
- ISO 68‑1: ISO metric screw threads - basic profile (thread form)
- ISO 225: Fasteners - Symbols and dimension descriptions (length positioning)
- ISO 4753: Fasteners - Ends of externally threaded elements
- ISO 4759‑1: Tolerances for fasteners - Part 1 (length and thread-length tolerances)
Keywords: ISO 888:2012, fasteners, bolts, screws, studs, nominal length, thread length, ISO metric thread, ISO 68-1, thread-length calculation.
