Everything You Need to Know About Piston Rings – Jeff Smith @Hemmings

Everything You Need to Know About Piston Rings – Jeff Smith @Hemmings


Everybody wants more power, and that attention has usually been paid to the romance items like cylinder heads, intake manifolds, carburetors, and camshafts. While those aspects of the engine are still essential for moving air, more engine builders are now scrutinizing the combustion space and making sure that all that air and fuel you worked so hard to get into the cylinder actually contributes to shoving the piston down instead of leaking past the rings.

Are thinner piston rings really better?

New technology now calls for not only thinner rings as viewed from the side, but also reduced radial thickness—as viewed from the top or bottom. Newer rings like those for LS engines take advantage of this. A narrow radial-wall thickness allows the ring to conform better to cylinder wall irregularities. This reduces blow-by and improves efficiency.

In the muscle car days of the ’60s and ’70s, production top and second piston rings measured 5⁄64-inch, and this remained the standard for decades. But with the coming of the modern engine era with powerplants like the GM LS, Ford modular V-8, and the Chrysler Gen III Hemi, piston rings began to slim down for many excellent reasons. If you don’t retain anything else from this story, just remember that thinner is better.

To get an idea of the benefits of slender ring packages, let’s start with some basic concepts. A thick piston ring, like the older 5⁄64-inch designs, presents a very wide contact face to the cylinder wall. This requires significant internal pressure exerted by the ring, called radial tension, to help seal the ring to the cylinder wall. The people at Total Seal have invested in an expensive machine that measures this tension and expresses this tension in units of pound-force (lb-f). Simply stated, this is the amount of force in pounds exerted against the cylinder wall after the ring is squeezed into the cylinder. This lb-f number is not a torque number (expressed as pound-feet or lb-ft) so don’t be confused. Nor is pound-force a sliding friction number, though clearly it is directly related to the friction generated as the piston and ring package move up and down in the cylinder

Piston rings are available in a wide variety of thicknesses.

Before we get into the actual numbers, it’s important to understand why a thicker ring must exert a greater force. This force is directly proportional to the ring face area that contacts the cylinder wall. This might be best explained by using the comparison of two different shoes. When walking on damp grass, it is easier to navigate the surface in a typical flat shoe. However, if the point of the heel is narrowed, as in a high heel shoe, the situation changes: the wearer’s gait is changed and the force of the heel is concentrated in a much smaller area, which easily presses the heel into the soft ground.

A wider piston ring must use a much greater radial tension to apply sufficient load to the cylinder wall to help seal the ring against cylinder pressure. With a thinner design like a 1.0-mm top ring for example, its static radial tension can be substantially reduced because the area of the ring face contacting the cylinder wall is far less than the larger 5⁄64-inch ring.

Piston ring radial tension, sliding friction, and oil control

Oil rings generate the most amount of friction as evidenced in our radial tension chart. However, Total Seal tells us they can build a 3⁄16-inch oil ring with improved radial tension numbers. The Summit GPX ring package we’re using for a 4.030-inch bore small-block Chevy 355 uses a 3⁄16-inch oil ring producing only 15 lb-f. Compared to a “standard” 3⁄16-inch oil ring’s 20 lb-f rating, the GPX offers a 25 percent reduction in radial tension yet can still deliver the expected oil control for street use.

Again, this radial tension is not the same thing as sliding friction, like that which might be measured with a fish scale pulling a piston with rings up a cylinder wall. But these radial tension loads are still proportional to sliding friction. As a practical example, we’ve installed 4.010-inch LS pistons using a ring package with 1.5 mm top rings, 1.5 mm second rings, and 3.0 mm oil rings into a bore and then pushed the pistons in using mere thumb pressure. But similar bore-size engine using 5⁄64-inch top and second rings and standard tension 3⁄16-inch oil rings demand a hefty hit with a hammer handle to drive the piston into the bore. The difference is the amount of friction produced by the different ring packages. Another way to measure this friction would be to use a digital torque wrench to gauge the friction required to rotate all eight pistons.

A typical small-block Chevy with 5⁄64-inch ring package might require a torque reading of 20 to 25 ft-lb but an LS engine with a 1.0-mm ring package with a similar bore and stroke may require 8 to 10 ft-lbs less torque. At 5,252 rpm, 10 lb-ft of an engine’s torque output is equal to 10 hp. This is not free horsepower because thinner ring packages do cost more and may require new pistons, but other than cost, there are no negatives to this approach.

As an additional benefit, thinner rings also allow the move to higher quality ring materials. As an example, budget ring packages costing $50 most often use grey cast iron that’s rather weak and brittle. Upgrading to a ductile iron will more than double its tensile strength. Plus, many high-quality thinner rings are now made using steel alloys with high-tech face coatings to further reduce friction while improving cylinder pressure sealing capability

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