By: Marlan Davis

Tech Editor's Note: Rings are round, square, tapered, flat, slanted, and above all, precision made. They are required to seal approximately 1,800 psi of combustion pressure that is trying to slip by the piston. All of this has to be done while moving at a speed of about 3,500 feet per minute. That is a distance of about two-thirds of a mile. While this is happening, throw in a boat load of heat as the ring tries to shed any lubricant that comes its way. It is also trying to keep the lubricant out of the combustion chamber. Therefore mentioned describes the piston ring's environment and job description.

 

There is no one piston ring design or piston ring material that can do this job in different engines at different power levels. A piston ring set that may work well in your Street Stock engine making 350 hp at6,500 rpm would be out of place in a modified engine making 650 hp at7,800 rpm. The Street Stock engine may not have the extensive and expensive block machine work and cylinder treatment of the Modified engine. Rpm, oil control, compression, and horsepower all have effects on the required ring material and design.

 

Horsepower in an engine is directly related to several facets in ring design. The top ring's primary function is to seal the combustion pressures. However, it must do this without pushing so hard on the cylinder wall as to cause excessive wear. Low-tension rings are available that can turn reduced friction into power. These same low-tension rings might not seal properly or live very long unless some rather heroic measures are taken to make sure the cylinder walls are round and true when the engine is running.

 

At this point, I'm going to let Marlan Davis explain piston rings. Marlan is the senior technical editor at Hot Rod, a sister magazine to Stock Car Racing. The following is from an article he wrote for Car Craft magazine, another Primedia publication. I have read it, and I must agree with what he has written. --Sleepy Gomez

 

A gapless Total Seal ring is used in this low-compression piston. No, really, the piston has a cutaway to show the manner in which the ring fits together. Photo courtesy of Total Seal

 

Piston rings perform a number of important functions. They seal the gap between the piston and cylinder wall to prevent combustion gases from blowing into the crankcase. They stabilize the piston as it travels up and down in the bore. They help cool the piston by transferring heat into the engine block, and they scrape oil off the cylinder walls. That's a tall order, and in recent years the theory on how to make rings best carry out these tasks has undergone revision.

 

Old-school thinking followed a brute force approach: Make everything as rigid as possible to force the rings into contact with the walls. Today, the trend in current production and racing engines is toward a more flexible ring package that better conforms to the cylinder wall. Back during the muscle car days, most production engines used a5/64-5/64-3/16-inch package. The 1/16-1/16-3/16-inch packages were for all-out racing. These days, Detroit automakers and many racers are gravitating toward even thinner "metric" rings. Standard-tension oil rings have been replaced by low-tension rings. Many of the new ring packages feature reduced radial wall thickness. Besides decreasing friction, this makes for a more stable package--assuming the piston rings, piston profile, and cylinder wall finish take advantage of these improvements. In the custom piston world, most build-to-order pistons can be ordered for reduced radial thickness rings; otherwise, spacer stock can be used to convert conventional pistons.