Piston rings hold a bit of a strange place in the engine building world. Circle Track works with lots of engine builders on different engine projects. And almost every engine builder we talk to admits that filing piston rings is the job he enjoys least out of the entire build process. There's no other way to cut it, the process of file-fitting piston rings is tedious, boring, repetitive and-we'll say it again-boring.
But, on the other hand, we've also noticed that engine builders are willing to invest more money than ever in a good set of rings. And that's because a set of well-made, low-tension rings can make power. A high-quality set of rings that are made to exact tolerances and properly ground can reduce the amount of friction produced as they move up and down the cylinder walls while still maintaining good oil control and longevity.
The function of a set of piston rings is to seal the gap between the piston and cylinder wall. Since the piston is constructed from aluminum and the cylinder bore of iron, they will have different expansion rates from the heat of combustion. The ring must also be able to adapt for these changes as well.
On almost all stock car engines, a package of three rings is used to seal the piston to the cylinder wall. The top ring requires a special construction since it is in direct contact with the flame from combustion and sees the most heat. Most modern racing rings for the Saturday night level use a top ring constructed with a moly coating to help it live in such a harsh environment. But just because it's popular, that doesn't mean that a moly face is the absolute best option for every situation. Wiseco, for example, sells the popular "GFX" ring package with its racing pistons that uses a stainless steel gas nitrided top ring that works extremely well.
A high quality ring pack,...
A high quality ring pack, like this GFX package from Wiseco, can unleash power by reducing friction as the rings slide up and down the cylinder bores, but properly gapping any ring set is still critical.
The top ring's primary purpose is to seal the combustion chamber so that no gas (or as little as possible) makes it past the piston and into the crankcase. To achieve this, the ring must be precisely gapped so that once the engine is at operating temperature the ring has expanded to the point that the gap has nearly disappeared. Normally, the ring manufacturer will offer guidelines as to how large the gap should be based on several criteria such as the engine's compression ratio, how far down the piston the top ring is located, piston diameter and other factors. It all relates to heat-the more heat the ring sees, the greater the cold gap should be. So the gap for a street rebuild will be less than for a stock car racing motor, and that gap will be less than a ring specified for a supercharged drag racing engine.
For instance, we recently gapped the rings while rebuilding the Circle Track dyno mule. We're using a set of Wiseco 4.030-inch forged racing pistons and a set of GFX rings Wiseco provided to go with them. For circle track racing with these rings Wiseco recommends the cold gap be the diameter of the cylinder bore multiplied by 0.0050 for the top ring and 0.0057 for the second. That equals a top ring gap of 0.022-inch and 0.023 for the second.
The reason the second ring gap is larger is because in racing the purpose of the second ring is primarily for oil control and not to seal the combustion chamber. Any high-pressure combustion gasses that make it past the first ring and get trapped by the second can cause the top ring to flutter, which can significantly affect power production. So the larger gap with the second ring helps allow any gasses to move on past and not get trapped between the first and second rings.
The second ring is typically constructed of a softer material than the first because it is protected from the intense heat of combustion by the top ring. The softer material also helps the ring seal to the cylinder wall better for improved oil control. On most economy-level ring packs the second ring features a taper so that only the bottom portion of the outside edge of the ring comes into contact with the cylinder wall. This helps reduce friction as the ring moves up and down the cylinder bore and also helps it to function better as an oil scraper. But the face of the second ring will wear with engine use, and it must be replaced when the taper is gone so that the entire face of the second ring is in contact with the cylinder wall.
A more advanced design for the second ring is a Napier face. A Napier-style piston ring has a notch or ridge on the bottom of the ring so that the outside edge of the ring is narrower than the inside (which sits inside the piston ring land). This gives the ring extra rigidity inside the ring land where it is most important, but reduces that thickness at the cylinder wall where a thinner ring means less friction. Just a few years ago, Napier-style rings were only used in high-end dry sump race engine applications, but lately they have become more affordable and are used in all types of race engines. The GFX ring package we are using for the dyno mule features a Napier second ring, and Wiseco recommends it for all types of wet- and dry-sump engine packages these days.
Lowest on the piston is the oil ring, which is actually two very thin rings sandwiching an expander ring. The oil ring's only job is to scrape oil off of the cylinder walls and dump it back into the oil pan. Many pistons also use oiling ports off of the land for the oil ring to direct some oil toward the wristpin.
Many street-oriented ring packs come pre-gapped, but that's not precise enough for racing. High end piston rings require the engine builder to gap them specifically to the cylinder bore it will be going into. In general, it is better to gap your rings too large rather than too small. A ring gap that's too big will only suffer from poor control of oil and combustion gasses, but a ring that is gapped too tightly can lead to major engine problems. If the ends of any ring are allowed to butt together, that will lead to scuffing the cylinder walls, and possibly a broken ring or even a busted piston. So you obviously want to avoid that as you tackle the job of prepping piston rings for an engine build.
1 A Napier-style ring is...
1 A Napier-style ring is a great option for a second ring in a performance engine. Here, you can see the notch on the underside of the ring. It reduces the surface area of the portion of the ring that touches the cylinder wall to provide good oil control while reducing friction.
2 Piston rings are more complex...
2 Piston rings are more complex than simple stamped rings of steel. Most are designed to work a certain way. This GFX ring pack from Wiseco has an “N” stamped on both the first and second rings to mark the top side of the ring. If you accidentally install them upside down, be prepared to burn plenty of oil.
3 Before you start gapping...
3 Before you start gapping your new rings, take a moment to make sure they are properly matched for the pistons you will be using. The rings should slide easily into the appropriate ring land but still have a tight fit. The radial thickness (the distance from the inside to the outside edges) should be matched to the depth of the land so that the entire ring will slide into the groove, but the land should be so deep that it absolutely swallows the ring with room to spare. Inserting the ring into the land backward like you see here is an easy and quick way to check proper fitment.
4 Here’s a shot to help you...
4 Here’s a shot to help you visualize how a performance ring package can free up horsepower. On top is the 0.047 inch GFX top ring, while underneath it is the 5⁄64-inch top ring for a stock Chevrolet 350. At 0.078-inch thick, the stock ring is nearly twice as thick as the GFX ring.
5 The only other tool you...
5 The only other tool you will need is a feeler gauge, which can be had at practically any auto parts store in America. We’re gapping the rings for Circle Track’s dyno mule engine exactly to the specs Wiseco provided with the rings: 0.022 for the top ring and 0.023 for the second. Oil ring gap isn’t critical as long as it is at least 0.010-inch.
6 The rings to be gapped...
6 The rings to be gapped fit on the grinder like you see here. Wiseco recommends grinding only one side of the ring. That way to keep a factory edge to make sure the other one remains square. Push the end of the ring against the side of the grinding wheel with only moderate pressure and spin the wheel counterclockwise so it grinds from the outside edge of the ring toward the inside. This is important to keep the coating on the face of the ring from flaking off near the ground edge.
7 This is what you want to...
7 This is what you want to avoid. This ring was not ground evenly, so the ends are no longer butt square. This will lead to blow-by and a loss of combustion pressure.
8 Use a honing stone to lightly...
8 Use a honing stone to lightly deburr the edges after grinding. You only want to knock off any burrs, not round the edges of the ring.
9 The easiest way to get...
9 The easiest way to get the ring into the cylinder bore is to slide it in vertically and then spin the ring so that it is fully seated into the bore. Check the ring in the cylinder bore it will be used in on the final build.
10 In order to properly measure...
10 In order to properly measure the gap, the ring must be perfectly square to the bore. You can buy an expensive ring squaring tool, but the easiest way to do it is to install a ring on the second ring land of your piston and push it into the bore upside down until the second ring contacts the deck. The ring you are checking must be in the bore shallow enough that the top of the piston can contact the ring all the way around and push it further into the bore.
11 Now use your feeler gauge...
11 Now use your feeler gauge to check the gap. You can’t put material back onto the ring, so you want to sneak up on the final gap size in several steps. Here, you can see the gap is only 0.014-inch. We’ve got a way to go to get to 0.022, but we’ll get there slowly to make sure we don’t overshoot the mark. Once you get the gap correct, place the ring in an envelope marked with its correct cylinder until you are ready to install it on the piston and the piston in that hole.
12 You can install rings...
12 You can install rings on the piston by hand, but if you aren’t careful you run the risk of twisting the ring or breaking it as well as scratching up the piston. A simple ring installation tool like this one is an easy and cheap way to make sure your rings are installed properly while stressing them as little as possible.
13 Here’s a shot of one of...
13 Here’s a shot of one of the oil rings sitting on the expander rail. Notice how the expander rail has tabs that fit on the inside radius of the oil ring. To make these fit correctly inside the piston’s ring land, install the expander rail first and then the two oil ring rails.