ISO 6621-2:2020

Overview - ISO 6621-2:2020 (Piston rings - Inspection measuring principles)

ISO 6621-2:2020 specifies the measuring principles used to inspect piston rings for reciprocating internal combustion engines (rings up to and including 200 mm diameter). The document defines how to measure geometric, dimensional and performance-related characteristics of rings - including ring width, radial wall thickness, free and closed gaps, forces, ovality, surface texture and coating depth. It can also be applied to piston rings for compressors operating under analogous conditions. The standard references GPS surface texture standards (ISO 4287, ISO 4288) and the ISO 6621 series vocabulary and ring-type standards.

Key technical topics and requirements

The standard defines measurement methods and units (millimetres, micrometres, Newtons, degrees) for many ring characteristics, for example:

• Ring geometry

  • Ring width (h1, h3), radial wall thickness (a1)
  • Keystone angle, land width/offset, twist, obliqueness, helix
  • Ovality, point deflection, free flatness, unevenness

• Gaps and forces

  • Total free gap (m, p) and closed gap (s)
  • Tangential force (Ft) and diametral force (Fd) - measured in Newtons

• Peripheral surface features

  • Taper, barrel (t2, t3), circumferential waviness, light tightness (percentage)
  • Surface roughness parameters (Ra, Rz) per ISO 4287/4288

• Materials and coatings

  • Plating/coating thickness, nitrided case depth (hardness ≥ 700 HV0.1)

• General measurement conditions

  • Rings normally rest on a reference plane in the free/open state; some checks require the ring closed in a gauge of nominal bore size
  • Instruments shall have sufficient resolution (measurement resolution not to exceed 10% of the applicable tolerance)
  • Specific procedures are given for single-piece vs multi-piece rings and for rings with features such as internal notches

Practical applications and who uses this standard

ISO 6621-2 is used by:

• Engine and compressor designers specifying ring geometry and performance
• Piston ring manufacturers for quality control and incoming inspection
• Metrology and test laboratories performing dimensional and surface inspections
• OEMs, aftermarket suppliers and quality assurance teams ensuring ring conformity
• R&D teams evaluating ring designs (taper, barrel, twist, coatings, nitriding)

Benefits include consistent ring measurement practice, clearer specification of functional properties (sealing, oil control, mechanical fit), and reduced ambiguity in supplier/manufacturer communications.

Related standards

• ISO 6621-1 (Vocabulary)
• Other parts of ISO 6621 (Part 1–5 cover types, materials and dimensional tables)
• ISO 6624 series (keystone rings - cast iron/steel variants)
• ISO 4287, ISO 4288 (surface texture measurement)

Keywords: ISO 6621-2:2020, piston rings, inspection measuring principles, ring measurement, tangential force, diametral force, free gap, ovality, surface roughness, plating thickness, nitrided case depth.