ISO 965-4:2021

Overview

ISO 965-4:2021 - "ISO general purpose metric screw threads - Tolerances - Part 4" defines the limits of sizes and limit deviations for hot‑dip galvanized external metric screw threads (coarse series M8 to M64) with a basic profile per ISO 68-1. It specifies dimensional requirements for the pitch and crest diameters of external threads intended to mate with internal threads tapped to tolerance position H or G after galvanizing. The standard is focused on the special dimensional allowances and deviations introduced by hot‑dip galvanizing.

Key topics and requirements

• Scope and applicability: Coarse metric external threads M8–M64, hot‑dip galvanized, mating with internal threads tolerance position H or G after galvanizing.
• Tolerance class: Designation for external threads is 6az (see Clause 5).
• Limit deviations and sizes: Clause 6 and Table 1 define limit deviations (fundamental deviations derived from a formula dependent on pitch P and az). Clause 7 and Table 2 give the corresponding limits of sizes (major and pitch diameters), root radii and thread engagement lengths.
• Pre‑ and post‑galvanizing rules: Tolerances are applied to parts before galvanizing; after galvanizing the actual thread profile must not exceed the maximum material limits for tolerance position h and must be compatible with internal H or G threads.
• Process and performance notes:
  • Threaded products with class 6az shall be centrifuged immediately after hot‑dip galvanizing.
  • Fasteners manufactured per this standard may show reduced ultimate tensile loads (reduced stress area) compared with ISO 898-1 full loadability.
  • Do not mate external threads conforming to ISO 965-4 with internal threads specified in ISO 965-5 - this combination risks thread stripping.

Applications and users

This standard is essential for:

• Fastener manufacturers producing bolts and studs intended for post‑galvanizing use.
• Galvanizing plants and process engineers to ensure threads meet allowable limits after coating and centrifuging.
• Mechanical and structural engineers designing bolted connections that require hot‑dip galvanized protection.
• Quality inspectors and procurement teams specifying tolerances for corrosion‑protected threaded assemblies.
• Standards writers and test labs calculating nominal stress area (Annex A provides minor diameters used by ISO/TC 2).

Practical benefits include ensuring reliable thread fit after galvanizing, avoiding assembly failures, and informing tensile/stress calculations for coated fasteners.

Related standards

• ISO 965-1 (principles and basic data)
• ISO 965-5 (internal threads to mate with galvanized externals)
• ISO 68-1 (basic profile)
• ISO 262 (selected sizes for screws, bolts and nuts)
• ISO 898-1 (mechanical properties of fasteners)
• ISO 5408 (screw thread vocabulary)

Keywords: ISO 965-4:2021, hot-dip galvanized external screw threads, metric screw threads, tolerance class 6az, pitch diameter, limits of sizes, ISO 68-1, ISO 262, tolerance position H G, galvanized threads, nominal stress area.