EN 26157-3:1991

Overview

EN 26157-3:1991 (identical with ISO 6157-3:1988) is a CEN standard that defines limits for surface discontinuities (surface defects) on high-strength fasteners. It specifically covers bolts, screws and studs for special requirements, establishing acceptance criteria and inspection guidance to ensure integrity and reliability of threaded fasteners used in critical applications. The standard also includes an annex with a sampling plan for inspection procedures.

Key Topics and Requirements

• Scope of application
  • Applies to fasteners with nominal thread diameters of 5 mm and larger.
  • Covers product grades A and B.
  • Intended for nominal lengths 1–10 d (or longer when specified or agreed).
• Property classes
  • Directly applicable to property class 12.9.
  • Also applicable to property classes 8.8, 9.8 and 10.9 when specified or agreed between parties.
• Surface discontinuities
  • Sets limits for types and sizes of surface defects (cracks, laps, seams, folds, cold shuts, etc.) that may affect fastener performance.
  • Defines accept/reject criteria based on severity and location of discontinuities.
• Inspection and sampling
  • Includes an annex with a sampling plan to guide batch inspection and quality control.
• Reference harmony
  • Harmonized with ISO 6157-3:1988, ensuring international alignment on defect classification and acceptance.

Applications

• Manufacturers use EN 26157-3:1991 to set production quality controls, define acceptable surface conditions, and reduce risk of in-service failure for high-strength fasteners.
• Quality and inspection engineers implement the sampling plan and acceptance criteria during incoming inspection and final inspection.
• Purchasing and procurement teams reference the standard in technical specifications and purchase orders to ensure supplied bolts, screws and studs meet agreed surface quality.
• Design and maintenance professionals rely on these limits when specifying fasteners for safety- or performance-critical assemblies (e.g., heavy machinery, transportation, energy infrastructure) that require high-strength property classes.

Related Standards

• ISO 6157-3:1988 - the international standard identical in content.
• Other parts of the EN 26157 series (surface discontinuities for different product groups) - consult for washers, nuts, rivets, or general fastener requirements.

This standard is essential for anyone specifying, producing, inspecting or procuring high-strength bolts, screws and studs where surface integrity directly affects safety and performance. Keywords: EN 26157-3:1991, fasteners, surface discontinuities, bolts screws studs, property class 12.9, thread diameter ≥5 mm, sampling plan.