Sealing Piston Rings - Some Things About Rings

Ask any serious engine builder what makes a good engine a great one, and the answer likely will be "sealing." While the average guy thinks of sealing as either what kind of valve cover gaskets to use or putting in the rear main seal just right, the concept of sealing referred to here is another subject entirely-sealing the combustion space. Why is it important? To operate at peak efficiency, gas pressure in the cylinder has to be captured; a loss of combustion pressure is a direct loss of potential power. The rings have the job of providing the seal between the piston and cylinder wall, and a complicated job it is. A well-sealed engine will make the most of combustion pressure's ability to exert its force on the piston during the power stroke, but that's only part of the story. A good seal will help draw harder on the intake stroke, taking in a greater charge, and it will provide for more effective compression of the charge during the piston's upward travel on the compression stroke.

An effective gas seal is critical, but the duties of the ring package go further than that. The rings need to provide an effective oil seal, separating the oil-washed crankcase environment from combustion side of the cylinder. We all know that oil contamination in the combustion space fouls plugs, but oil upstairs also dramatically reduces the detonation tolerance of the mixture. The oil ring pack and second compression ring should provide the required oil control, without adding excessive friction.

Ring Materials
The casual engine enthusiast tends to think of ring materials in terms of cast-iron, moly, or chrome rings. Though these are the most common terms of differentiation between ring types, these terms actually refer to surface processes, rather than the ring material itself. There are basically two ring materials that are of concern to the vast majority of engine builders: plain gray iron rings and ductile iron rings. Plain iron rings have very little elasticity and will snap when subjected to excessive stress. This ring material is quite brittle and soft, however, due to the material's natural porosity it offers reasonably good oil retention, and in a moderately stressed engine will survive just fine. Ductile iron is many times tougher than plain iron, and, as the name implies, has greatly enhanced ductility as compared with plain iron. A ductile iron ring is far more elastic and is much more capable of enduring in a highly stressed environment. It will bend and distort rather than fracture. There are other materials, such as steel alloys and stainless steel rings available from some manufacturers, favored for specialized applications. However, for the vast majority of Mopar engine builds, the ring material choice is almost always ductile iron or plain gray cast iron. For anything but a mild or stock build, it's worth stepping up to the ductile ring.