Internal combustion engines — Piston rings — Part 4: General specifications
1Key Takeaways
This document specifies the general characteristics of piston rings for reciprocating internal combustion engines for road vehicles and other applications (the individual dimensional criteria for these rings are given in the relevant International Standards). It also provides a system for ring coding, designation, and …
2Expert Interpretation
In-depth interpretation of the latest ISO 6621-4:2024 international standard for piston rings, covering the general technical requirements for the diameter range of 30-200mm. Analyze the piston ring coding system, design specifications, surface treatment process (chrome plating/spraying/nitriding) and mechanical property calculation standards, and provide selection guidelines and implementation suggestions for compression rings/oil rings.
Analysis of the core content of ISO 6621-4:2024
As the general specification part of the internal combustion engine piston ring series standard, ISO 6621-4:2024 fourth edition replaces the 2015 edition, mainly updating the unification of the terminology system and the revision of the side groove dimensions. The standard applies to piston rings for reciprocating internal combustion engines, covering the nominal diameter range of 30-200mm.
Piston ring coding system and marking specifications
Chapter 4 of the standard establishes a complete piston ring coding system, which uses alphanumeric combinations to indicate ring characteristics:
- Basic ring type code: such as R (rectangular ring), KB (symmetrical barrel tapered ring)
- Surface treatment code: CR2 (chrome plating ≥ 0.1mm), SC1F (spray embedded design)
- Special structure code: NH1 (anti-rotation inner groove), KG (thinning edge at the opening)
| Code type | Typical example | Technical meaning |
|---|---|---|
| Material code | MC11 | Grey cast iron non-heat treated material |
| Pressure grade | PNM | Medium nominal unit pressure (1.49N/mm²) |
| Coating type | CR3 | Chrome plating thickness ≥0.15mm |
Key mechanical properties calculation specifications
Chapter 7 specifies the corrected calculation method for tangential force (Ft) and radial force (Fd) in detail, and the following correction factors need to be considered:
- Material correction factor: According to the difference in elastic modulus (cast iron 100GN/m², steel 210GN/m²)
- Structural characteristic coefficient: the inner chamfer (KI) is taken as 0.98, the chrome plating layer (CR2) is taken as 0.88
- Free clearance ratio correction: the m/(d1-a1) ratio is used for adjustment (see Table 4)
Calculation case: ISO 6622-1 B-95×2.5-MC53/MR CR2 IW piston ring
Total correction factor = 1.6 (material) × 0.75 (clearance ratio) × 0.88 (chrome plating) × 0.78 (inner step) = 0.824
Final Ft = 15.2N±20%, Fd = 32.8N±20%
Surface treatment technical requirements
| Type of treatment | Thickness requirement (mm) | Hardness standard | Edge treatment |
|---|---|---|---|
| Chrome plating (CR2) | ≥0.1 | ≥800HV0.1 | Openings require chamfering/radius (s3≤0.4mm) |
| Spraying (SC3) | ≥0.15 | According to manufacturer's specifications | Semi-embedded design h7=0.1-0.5mm |
| Nitriding (NT090) | Surface ≥0.09 | - | s3≤0.35mm at the opening |
Special process requirements
- Chrome plating design: three types: full surface (CRF), semi-embedded (CRE), embedded (CRF)
- Phosphating treatment: PO (minimum 0.002mm) is used for rust prevention, PR (maximum 0.002mm) is used for temporary protection
- PVD coating: PC005 represents 0.005mm thickness, which can be stacked in multiple layers
Implementation suggestions and industry applications
Selection guide:
- Chrome plating or spraying rings (CR3/SC3) are preferred for high temperature conditions
- Steel oil rings need to be nitrided (NT series) to improve wear resistance
- The original factory marking code system should be checked during maintenance and replacement
Key points for quality control:
- The coating thickness is measured by eddy current or X-ray
- Tangential force detection needs to be carried out under a standard environment of 23±5℃
- The original orientation of the ring needs to be maintained in the packaging (TOP marking is unified)