How to Choose Piston Rings and Gap Them Correctly

Why is piston ring selection and endgap crucial for proper break-in, power, and longevity?

Piston ring selection and ring endgap play a vital role in engine life and power. The first few minutes of an engine’s run set the stage for power, efficiency, and longevity. Ideally, every engine build would include a dyno pull for proper break-in, but many builders cannot afford the added expense. On a dyno, the engine builder can load the engine and push the throttle to seat the piston rings and bearings, a task that’s more difficult on the road.

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However, engine break-in is just one part of the story. The process starts with piston and ring selection. Before diving into the different types of piston rings, it's important to understand their function. Piston rings seal the cylinder and control oil, but they also carry heat from the piston to the cylinder wall through direct contact. As piston rings have become thinner to reduce friction and increase efficiency, heat transfer has become even more crucial, making it harder on pistons.

If you're planning high-performance driving, selecting the right piston ring is just as important as choosing the piston itself. Your selection depends on how the engine will be used. There are three main piston types: cast, hypereutectic, and forged. Cast pistons, once common in most cars and trucks, are considered old-school. Hypereutectic pistons, made with high silicon content, offer increased durability at a similar cost to cast pistons. Forged pistons are highly durable and ideal for high-performance applications but require greater piston-to-cylinder wall clearances due to their aggressive expansion rate, which leads to cold-start noise.

When selecting piston rings, the first consideration is ring width, which depends on the engine's intended use. Thinner piston rings are better for racing or fuel economy challenges, as they create less friction and use less energy. Wider rings, however, are more suitable for weekend cruisers and daily drivers due to their better wear resistance and ability to carry greater loads, providing longer life.

Historically, standard piston ring packages included 5/64-inch top and secondary compression rings with a 3/16-inch oil ring. These dimensions were necessary to provide enough pressure for proper sealing but also created significant friction, especially from the oil rings. This friction reduced engine power and efficiency.

To improve this, automotive engineers developed thinner piston rings to reduce internal friction. By the s, factory engines commonly used a 1.5mm/1.5mm/3.0mm piston ring package, which significantly reduced friction. With thinner rings, the radial tension required for sealing against the cylinder wall is reduced because the total surface area of the ring in contact with the cylinder wall is smaller, allowing the same load with less friction.

By reducing radial tension on the cylinder wall with thin piston rings, friction generated as the ring moves against the wall is minimized. This leads to improved power by lowering internal friction across multiple cylinders. Thinner piston rings also provide better sealing, reducing blow-by and preventing lost power. As a result, more cylinder pressure (heat energy) is retained above the pistons, boosting overall power. The aftermarket performance industry continues to offer a wider variety of piston and ring combinations.

For performance applications, selecting the right piston configuration and compression ratio is crucial, along with choosing an optimized ring package. The process begins with selecting the appropriate ring material. Once the material is chosen, ring widths and types can be determined. Carbon steel, for example, is more malleable than traditional cast iron, capable of withstanding higher temperatures without losing temper, and is better equipped to handle detonation. Cast iron, by its very nature is brittle and not as strong as hardened forged steel. Hardened steel top rings perform so well that even automakers are using more of them these days in production engines for durability. Steel makes more sense if you're planning boost, nitrous, or excessive amounts of compression because it tolerates the extremes better than iron.

Chrome-faced rings were once popular but have fallen out of favor among engine builders. The issue with these rings was their extreme hardness, making them difficult to break in. Additionally, they didn't handle detonation well, so it's generally recommended to avoid using them.

Some rings feature a tough plasma molybdenum coating, along with gas nitriding for added durability. Steel-nitrided top rings are a solid choice for performance applications, though they can be costly. For those on a budget, Dongya offers a variety of affordable ring options. Hardened rings are ideal for street performance engines, offering an upgrade over traditional cast iron by incorporating magnesium into the grey iron to improve ductility. Ductile iron is more flexible and less likely to break under stress, making it a great choice for top rings when cost is a concern. Dongya also offers ductile iron rings with a plasma molybdenum face coating for better compatibility with iron cylinder walls.

The plasma process involves spraying the ring with an alloy powder containing Chromium, Molybdenum, and Nickel, which is then melted and applied to the ring face under extreme heat. This method improves adhesion, reduces the risk of flaking, and ensures faster break-in and better cylinder sealing.

The second piston ring doesn't face the same heat and pressure as the top ring, and Dongya recommends their Plasma Moly ring package, which combines a carbon steel nitride top ring with a ductile iron second ring.

CB Tech Advisor Rich Burgess has been doing top-end work on his old Airhead Beemer. Now it’s time for rings.

If you are looking for more details, kindly visit motorcycle piston ring.

There is more to understand about piston rings than you might realize—some of the ins and outs, or should I say ups and downs—with a spin (they actually rotate at a fair clip when the engine is running).

A major part of any engine rebuild involves a look at the pistons and rings. We kind of have two worlds here. One is the iron liner crowd, which includes most vintage bikes and Harleys. The other group would include most modern Euro and Asian brands, which have induction-hardened or hard-coated bores. A subset here would be chrome plated bores (never use chrome rings in them or you get an almost instant seizure). 

The right piston rings are also absolutely necessary in the iron liner group. I know this because I once used a high-performance piston and ring combo in my Harley. The rings had a narrow face width (about half a “Hastings” type ring) and a hard alloy construction. These guys wore the bore to the point it need an overbore in less than 15,000 kilometres. The conventional Hastings ring replacements I used have left the bores in spec 50,000-km later. The reason: the narrow Wiseco X-rings were meant to fit a hardened bore—my experiment was not the best idea ever. You see, nitride treated bores don’t wear much; the idea is to just do a light (plateau) hone, change out the rings and you can carry on. In soft iron they were acting as little cutting tools, although power was very good while they lasted!

The piston rings gap is also very important, with the main concern being that the gap never closes up completely when the engine gets hot: that would be one of those catastrophic “fails.” The coefficient of linear thermal expansion of steel (depending on the alloy) is about 0. inches per degree F. So a ring on a piston that has a circumference of 10 inches (3.5-in. diameter) would grow 0. per degree F. A piston temperature of 450F above ambient would mean the gap would close by about .003. 

The second concern is getting a good seal but the gap needed varies with temperature and bore size. Keep in mind, more power = more heat! Gap closest to zero = instant destruction. Read the instructions!

I know folks that had bad luck with Keith Black pistons and I know why, they did not read the instructions in the box! KB’s require a lot more gap than normal on the top ring. This is because the top ring groove is higher on the piston closer to the heat (one of the features that helps make more power). The instructions are interesting. They say, for racing use an even bigger gap. This seems counter intuitive but they seal well enough once hot.

My latest engine adventure involved a Deves ring set for my Beemer (Nikasil bores). These are good quality aftermarket rings but a little different than the original equipment rings. First, the compression rings required a lot of grinding to get an adequate gap when test fitted in the bores. Lucky for me Paul Shore over at Motorrad Performance in Turner Valley, Alberta loaned me his ring grinding gizmo. It works great to remove a lot of material while keeping the gap square—hard to do with a file. A small fine file is still needed to do small chamfers on all ground edges. As is my usual habit I bought one so that I won’t need to borrow again, they come in around a hundred bucks. 

The second issue got me concerned enough to contact Deves (they are in California). Carefully mounted on the piston, well-oiled and using a good ring compressor it was difficult to compress them enough to go into the bores. Once “tapped” in (I like to be able to push the piston in by hand), they seemed a bit too tight. Well live and learn. 

Deves #1 FAQ: The oil rings fit way tight, what’s up? Their answer is, yes the tight fit is normal. Once the engine gets warm they relax and all is good. There’s something about Swedish Steel and its character. After turning the engine over with the kicker everything feels normal.

So having done the rebuild of the top end I now have a good engine, great paint, and new tires. It’s almost done. A few details (where the devil lives) and while not a showpiece it will be a very nice “rider.”  Well, a new exhaust would be nice. 

by Rich Burgess Canadian Biker Issue #313

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