Gapping and correctly installing piston rings is one of the most important parts of engine assembly. Here's how to do it right. 

Piston rings perform a number of duties in an engine: They seal combustion pressure and gases in the chamber, preventing them from passing by the piston and into the crankcase where they can only do harm. They stabilize the piston as it travels up and down the cylinder bore, help cool the piston by transferring heat into the block’s cylinder bore, and finally, they meter the film of oil on the bore surface during operation. Rings perform these functions under the most extreme conditions, which is why premium ring sets should always be used, with materials and designs matched to the intended application.

For even

Ring gap is absolutely critical to engine performance and longevity––and, one of our number-one tech questions. Today we explain why rings need a gap, how to do it, and some common ring-gapping mistakes.

If you’re a frequent reader of our articles here, you may have noticed that our “tech” stories tend to fall into a few major categories. Some are to let you know about new stuff that’s just hitting the market. Others are intended to give you an in-depth look at the engineering and technology behind the aftermarket products you’re selecting for your engine builds. And the rest are simply a way to answer the question: “how do I do that?”

Today, we’re looking at setting ring gap - a topic that might not exactly elevate your heart rate, but something that is absolutely critical to engine performance and longevity–and, one of our number-one tech questions. Our expert guiding us today is Wiseco’s Nickolaus DiBlasi.

Gap Insurance 

Because piston rings need to be expanded to fit over the diameter of the piston itself before they find their home in the ring grooves, they have to be split in some way in order to allow installation. At the same time, the gap between the ends of the ring needs to be kept as small as practical, for a number of reasons.

“Let’s first look at the job of the top compression ring,” DiBlasi explains. “The main objectives for the top ring are to hold in compression and pass heat from the piston to the cylinder. From that point, energy is taken away from the cylinder using water in the water jackets. Due to this fact, the top ring sees the most amount of heat.” With that in mind, it’s clear that the smaller the gap, the less opportunity there is for combustion chamber pressure to make its way past the first ring, and the more contact the ring has with the cylinder wall in order to transfer heat.

In a perfect world, piston rings that perfectly fit the bore with the ends butted together would be ideal. But as anyone who has tried to make an omelet and ended up with a ‘breakfast scramble’ can tell you, the world is an imperfect place. As the block, piston, and rings all heat up, they change dimensions ever so slightly, and at different rates. DiBlasi takes it down to the molecular level:

“With any material, heating it induces more energy into parts. As the atoms and molecules in the parts are heated, they start to move around more and the gap between them increases. The added space between atoms and molecules is the expansion that you noticed in pistons, rings, and everything else in the world. Each material is comprised of different elemental mixes, so the bond structure and spacing of atoms and molecules are different. This is the reason why you see some materials expanding at significantly different rates than each other. Case in point is aluminum pistons in an iron block. The aluminum expands significantly more.”

We accept tradeoffs in expansion rates in order to get the best compromise in material properties for each component, but that means we need to take it into account when we put all these different parts together and ask them to play nice with each other. Per DiBlasi, “Since the top ring will see a majority of the heat, they expand quite a bit. The gap that you create in the top ring is to accommodate the maximum expansion that you anticipate the top ring requiring. This is very critical to understand, as the top ring is responsible for holding in compression. There is a fine line of making sure that top ring is not bypassing compression, but not expanding so much that the ends run into each other.”

What’s the Worst that Could Happen?

“If the top ring is gapped too small, when it expands the ends will run into each other,” DiBlasi continues. “As the engine heat-cycles back and forth through its maximal temperature, the top ring expands an