ISO 6624-4:2016

Overview

ISO 6624-4:2016 - "Internal combustion engines - Piston rings - Part 4: Half keystone rings made of steel" specifies the essential dimensional features and common characteristics of steel half-keystone piston rings used in reciprocating internal combustion engines (road vehicles and other applications). The standard covers three ring types: HK (straight faced), HKB (barrel faced) and HKBA (asymmetrical barrel faced) for nominal diameters 50 mm to 160 mm. It defines designation formats, measurement methods (including calculation of gauge width h - Annex A), and compatible materials, surface treatments and coatings.

Key technical topics and requirements

• Ring types and geometry: Definitions, reference planes and illustrations for HK, HKB and HKBA half keystone rings (7° angle) including how to designate each ring in ISO format (example: Piston ring ISO 6624-4 HK - 60 × 1,2 - MC62/CR2).
• Dimensional tables and tolerances: Detailed tables list nominal widths, radial wall thicknesses, measurement widths and tolerances, and recommended gauge values for each ring type.
• Materials and subclasses: Examples in the standard include CrSi alloyed steels and martensitic steels (e.g., MC62, MC65) - used to specify material subclass in ring designation.
• Surface treatments and coatings: Defined codes and minimum thicknesses for chromium plating (CR codes), spray coatings (SC codes), PVD coatings (PC codes), and nitriding (NB/NT codes). Notes flag restrictions (e.g., NT nitriding not recommended for HKBA rings).
• Nitrided case depth: Specified minimum case depths for peripheral and face surfaces with code-based values (NB/NT series).
• Force factors and corrections: Tangential and diametral force data for ring types (Table 9) and multiplier correction factors when coatings or additional features are present (Table 8). Force corrections for geometric ratios follow ISO 6621-4.
• Measurement guidance: Annex A provides the calculation method for the measurement/gauge width h.

Practical applications and users

ISO 6624-4 is intended for:

• Engine designers and powertrain engineers specifying seal and wear performance in reciprocating engines.
• Piston ring manufacturers and OEM suppliers producing half keystone rings to ensure interchangeability and consistent performance.
• Materials and surface-treatment engineers selecting nitriding/PVD/plating options and thicknesses.
• Quality, inspection and test laboratories verifying dimensional conformity and force characteristics.
• Aftermarket and repair suppliers needing standardised ring designations and replacement specifications.

Use of ISO 6624-4 helps ensure reliable piston sealing, predictable friction/wear behavior, and compatibility across manufacturers and applications.

Related standards

• ISO 6621-4 - Internal combustion engines - Piston rings - General specifications (normative reference)
• Other parts of ISO 6624 series: Part 1 (keystone cast iron), Part 2 (half keystone cast iron), Part 3 (keystone steel)
• Developed by ISO/TC 22, Subcommittee SC 34 (propulsion, powertrain and powertrain fluids)

Keywords: ISO 6624-4, piston rings, half keystone rings, steel piston rings, HK HKB HKBA, nitriding, PVD, chromium plating, internal combustion engines, piston ring dimensions.