ISO 6621-5:2013 specifies those quality aspects that are capable of definition but not normally found on a drawing specification.

It is applicable to the following: single-piece piston rings of grey cast iron or steel; multi-piece piston rings (oil control rings) consisting of cast iron parts and spring components; single-piece and multi-piece oil control rings of steel, i.e. oil control rings in the form of strip steel components or steel segments (rails) with spring expander components.

In addition to specifying certain of the limits of acceptance relating to inspection measuring principles (covered by ISO 6621-2), ISO 6621-5:2013 also covers those features for which no recognized quantitative measurement procedures exist and which are only checked visually with normal eyesight (spectacles if worn normally) and without magnification. Such features (superficial defects) are additional to the standard tolerances of ring width, radial wall thickness, and closed gap.

ISO 6621-5:2013 does not establish acceptable quality levels (AQL), it being left to manufacturer and client to decide the appropriate levels jointly. In this case, the recommendations of ISO 2859 are followed.

ISO 6621-5:2013 specifies the quality requirements of piston rings for reciprocating internal combustion engines for road vehicles and other applications. It is applicable to all such rings of a nominal diameter from 30 mm up to and including 200 mm.