ISO 7913:1994

Overview

ISO 7913:1994 - "Aerospace - Bolts and screws, metric - Tolerances of form and position" is an international standard that defines geometric tolerances applicable to metric bolts and screws used in aerospace construction. This second edition (1994) cancels and replaces ISO 7913:1985 and applies to nominal thread diameters from 3 mm to 24 mm. Tolerance values are expressed in millimetres and the standard is intended primarily for preparing aerospace product standards and technical drawings.

Key topics and technical requirements

• Scope: Form and position tolerances for all types of metric bolts and screws used in aerospace applications with nominal thread diameters 3–24 mm.
• Tolerance types covered (portrayals conform to ISO 1101 and ISO 2692):
  • Perpendicularity of bolt underhead bearing surface
  • Straightness of total bolt shank length
  • Run-out (radial run-out) of external flange and wrenching diameters for bihexagonal, spline and hexagonal heads
  • Run-out of width across corners for hexagonal heads
  • Position of split-pin and locking-wire holes (in thread and head)
  • Run-out of conical or cylindrical portions of pan and countersunk head bolts
  • Perpendicularity and concavity at the top of countersunk heads
• Tolerance values: Numeric limits are provided in a table (Table 2) for short/medium/long thread lengths and threaded-to-head conditions. Parameter “V” is dimensionless (mm/mm).
• Normative references: The standard references ISO 1101 (geometrical tolerancing/GD&T) and ISO 2692 (maximum material principle), so geometric dimensioning and tolerancing (GD&T) conventions apply.

Applications and who uses it

ISO 7913 is used where precise geometric control of fasteners affects assembly, fit, function or safety in aerospace systems. Typical users include:

• Aerospace designers and mechanical engineers specifying bolt/screw tolerances on drawings
• Fastener manufacturers and suppliers producing aerospace-grade bolts and screws
• Quality assurance and inspection teams applying measurement criteria for form and position
• Procurement and standards writers aligning product standards to international requirements
• Maintenance, repair and overhaul (MRO) organizations verifying replacement fasteners

Practical benefits include consistent manufacturing tolerances, improved interchangeability, reduced assembly issues, and clear inspection criteria for critical aerospace applications.

Related standards

• ISO 1101: Geometrical tolerancing - form, orientation, location and run-out (GD&T)
• ISO 2692: Geometrical tolerancing - Maximum material principle

Keywords: ISO 7913, aerospace bolts, metric bolts and screws, form and position tolerances, geometric tolerancing, GD&T, fastener tolerances, aerospace standards.