Sometimes the best solutions are deceptively simple. Take, for example, sealing the combustion chamber in an internal combustion engine. Extreme temperatures mean pretty significant expansion rates when it comes to moving components within an engine and the exacting tolerances required to make that engine work properly. It gets worse when you mix metals that have different expansion rates, which is exactly what happens when you have an aluminum piston traveling inside a cast-iron cylinder bore. Leave enough tolerance between the piston and cylinder when it's hot, and it will never seal well enough to even start when it's cold. Tighten up the tolerances enough to achieve a cold start, and the engine tears itself apart from the inside out before you can even get the car on the track.

Obviously, the answer is to use piston rings to provide proper sealing between the piston and cylinder wall. No news there; piston rings have been a part of the piston-engine formula almost since day one. But that doesn't mean you can throw a ring package haphazardly into any engine package and expect to be competitive on Saturday night. Properly choosing and installing piston rings is critical to making maximum engine power. Don't discount this fact even if you're throwing a rebuild kit into an old Chevy 350 for racing the Strictly Stock classes-it's probably even more critical then because it's so hard to make power in the lower classes.

The conventional piston configuration includes three sets of rings. The top ring is the most important. It provides the seal that is used to create compression, hold the burning-and rapidly expanding-air/fuel mixture in the combustion chamber on the power stroke, and push the burnt remains of combustion out of the chamber to complete the process. The second ring is a bit of a backup; whatever compression slips past the first ring, the second ring helps to contain. The third ring is commonly referred to as the oil ring. It is actually two scraper rings assembled with a corrugated spacer ring between them. The oil ring's job is to scrape excess oil off the cylinder walls so that it doesn't mix with the air/fuel charge.

Collectively, the rings ensure that fuel does not find its way into the crankcase (especially important if you're running alcohol, which has the unfortunate tendency to break motor oil down into useless gunk) and that the full effect of combustion is used to push the piston down and turn the crank with the full available force.

We are not talking about your stress level, although we do hope it's down and you're thinking peaceful thoughts. Ring tension is actually an important consideration when it comes to racing engines. Although a good hone job on the cylinder bores will hold a thin film of oil that does a good job of lubricating the rings as they slide along the bore, there is still a significant amount of friction involved. This is especially true because any friction created by a ring set should be multiplied by eight.

Piston rings for over-the-road cars are designed to last 100,000 miles rather than help squeeze every last bit of power out of an engine. But as a racer, you're willing to sacrifice a little durability for a chance at faster lap times. That's why low-tension rings are a virtual no-brainer. Low-tension piston rings are generally thinner than standard rings and, when properly gapped, do not exert as much pressure against the cylinder wall. You can actually feel the difference between low-tension racing rings and standard rings when you turn the crank in an assembled short-block with a wrench.

Standard rings are not a continuous circle, but have a split. When properly fitted inside a specific cylinder bore, this split causes a gap in the ring. The idea is that when the ring heats up from the radiant heat from combustion, it expands and the gap closes. (It's also impossible to get the ring into the piston groove without the split.)

Several factors determine what the gap in a cold ring should be, but generally the ring manufacturer can help with that. In order to create as much compression as possible, racing pistons move the top ring as close to the top of the piston as possible. Usually, it's around 0.150 inch from the top. For a high-quality forged aluminum racing piston, this translates to a ring gap approximately 0.018 inch. Hypereutectic pistons experience greater rates of expansion and require a larger gap. Also, if the top ring groove is farther down the body of the piston, the ring receives more protection from the heat and needs less gap. Of course, we are talking in terms of thousandths of an inch of change, but in racing that is often enough.

Finally, after grinding the ends of the ring for the proper gap, it's always a good idea to lightly file the ends to make sure there are no burrs in the ends of the ring. Use a very fine file and be sure to remove only burrs or sharp ridges; do not round the ends of the ring, or you'll reduce its ability to seal properly.

Matt Hartford of Total Seal says fitting gapless rings can actually be easier than fitting conventional rings. Total Seal is the leading manufacturer of gapless rings for racing purposes, and the design is surprisingly simple. The top ring is actually two pieces. The first looks a lot like a conventional top ring, and directly underneath it is a "secondary rail," which effectively seals the top ring. Both of these rings have gaps, which must be checked conventionally, but they are installed with the gaps 180 degrees opposite each other, creating what the engine sees as a single, "gapless" ring.

"The only thing we think is critical is that you have at least the minimum gap that we recommend per our instructions," Hartford says. "We've done testing to show whether you have 0.024 or 0.034 inch of gap. You see absolutely no difference in performance simply because of the design of the gapless ring. The secondary rail stops all the pressure from getting through the end gap. What that does is give you a safety margin. You can go a little larger on your gaps in case you were to lean out your engine or miss your tune-up. Then you don't have to worry about butting your end gap."

Total Seal
Powerhouse Products
3402 Democrat Rd.
TN  38118
MEM Engines
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