Piston Rings - Special Rings, Special Service!

Piston rings seal the combustion chamber from the cylinder crankcase in order to prevent combustion gases (or blow-by) penetrating the crankcase and to prevent the lubricating oil being sprayed around in the crankcase from penetrating the combustion chamber. In addition, the piston rings dissipate the heat from the piston to the cylinder and ensure the lubricating film is evenly distributed. Therefore the main requirements for piston rings are high resistance to wear and corrosion and a low drop in elasticity at high temperatures.

Perfect base materials, correct geometry and highly specialized coatings require a modern production facility. In order to satisfy the high requirements for piston rings, the basic material used in Federal-Mogul piston rings comprises high-quality lamellar or spherolithic cast materials. The basic material is selected from a wide range according to application and fuel quality. We are able to assist your choice of the correct material for your specific application: click HERE to know more.


For small bore MAN engines (26 to 50), a new type of piston ring has been introduced into the market which supersedes the CL grooves version.
The Port on Plane (POP) version grants a constant pressure drop across the top ring even if worn out. This effect is obtained by the replacement of CL grooves made by a number of ports milled on the lower side of the piston ring (see picture) which increase the bypass area as the ring wears, instead of reducing it.
The new version has been developed after the observation of premature wear out of the controlled leakage (CL) grooves causing increased wear on the top ring and on the liner.


Thermally induced stress leading to hard ring-liner contact and/or unintended blow-by in large-bore engines has been a well-known problem for many years. An effective way to avoid such problems is to use a gas-tight (GT) lock on the piston ring, while controlling the blow-by leakage through grooves (CPR Controlled Pressure Relief grooves) on the ring's periphery, as shown in the picture.
Normally CPR grooves are introduced together with Cermet coating, in order to reduce the wear and ensure that CL grooves are not prematurely worn out.

In order to improve the wear resistance and tribological properties of the rings, different coatings are used on running surfaces.


Piston Ring Base Materials

RM5 is a flake graphite cast iron specifically developed as a 2-stroke engines' piston rings base material because of its morphology and its matrix' constituents.
Formerly used on all the piston ring grooves, nowadays, with different coatings and shapes, RM5 is the standard base material for all piston rings, except for the first groove made by Darcast.


For modern high performance applications, Daros has developed the Darcast™ compacted graphite iron (CGI), which offers high strength combined with minimum wear.
It is nowadays used only on the first groove piston ring, as that's the most stressed ring due to its position close to the combustion chamber.




Piston Ring Coatings

Cermet represent an important family of thermally sprayed wear-resistant coatings. Cermets are applied to the running face of the ring for highly loaded applications and consist of a mixture of metallic and ceramic compounds. The Cermet coatings prolong the lifetime of the ring package significantly, as shown in the graph. Daros offers a variety of different Cermet coatings. All Cermets are produced in-house with state-of-the-art plasma-spraying equipment. The coatings are always thoroughly tested in Daros' laboratory, ensuring world-leading performance.


Alucoat™ is a soft running-in coating applied to the ring's running face (arc-sprayed) in order to let the ring seal properly within the first few hours of operation (during shop-testing or after overhaul for marine engines). 
Another function of the running-in coating is to gently finish the running surface on the cylinder liner during the first few hundred hours after installation of a new liner. The running-in coating is applied directly to the base material or in addition to a wear-resistant coating.



In order to make the bottom ring plane more resistant to wear, hard chromium is sometimes applied on the ring plane as well as the piston groove, in order to increase the TBO. Hard chromium is a well-tested technology providing excellent resistance to ring groove wear.
By hard-chrome plating the lower face, the surface structure becomes smooth and prevents the abrasive particles (ex. Catalytic fines) from being trapped, thereby reducing the ring groove wear.