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.
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 and contracts. If the gaps are too small, the top ring ends will run into itself. Since the ring has nowhere to expand at this point, the outward force applied to the cylinder increases. This added force introduces even more heat and the ring expands further. Once that happens, catastrophic failure will occur rapidly as it is a continuous cycle of more heat, more outward pressure, and nowhere for the ring to expand into.”
If that sounds bad, it is.
“The increased force to the cylinders starts to hold the piston tighter to the cylinder wall,” DiBlasi explains. “This added resistance, in conjunction with heat starts to soften the piston. It first will start to pull on the ring grooves and lands, spacing them further and further apart. Next, the lands grow excessively in diameter due to the heat they are seeing, as the rings can no longer adequately pull enough heat away from the piston into the water jackets. At this point, the top land, perpendicular to the pin axis, will be flattened out as it grows with excessive heat. In extreme cases, the top of the piston is ripped completely off the part.”
Clearly, that’s not something you want happening. Per DiBlasi, “Many piston failures occur due to this reason, and more often than not when we are diagnosing the problem we see that the rings were either not gapped enough, or not gapped at all.”
Read the Instructions…
The best way to prevent this kind of runaway destruction is, surprisingly enough, to pay attention to the information provided with your new Wiseco pistons and rings. “We have outlined our recommendations that are included with pistons. These are also available in our catalogs and website,” DiBlasi adds.
The process begins with inserting the ring into the bore, and making sure it is square using a squaring tool to align it properly. A feeler gauge inserted into the ring gap will show you the existing dimension, and from there you can make your adjustments. For a street engine, multiplying your bore size by 0.004in will give you the top ring gap you are looking for.
- 004 x 4.00in bore = 0.016 inch ring gap
For high performance engines, the multiplier changes to add more clearance, but the math stays the same:
- Modified or Nitrous Oxide - 0.005in x 4.00in bore = 0.020 inch ring gap
- High Performance Racing - .0055in x 4.00in bore = 0.022 inch ring gap
- Racing with Nitrous/Turbo - 0.006in x 4.00in bore = 0.024 inch ring gap
- Racing Blower/Supercharger - 0.007 x 4.00in bore = 0.028 inch ring gap
For the second ring, the process is the same, but with a slightly different gap, based on application:
- Street - 0.005in x bore size
- Modified or Nitrous Oxide - 0.0055in x bore size
- High Performance Racing - 0.0053in x bore size
- Racing with Nitrous/Turbo - 0.0057in x bore size
- Racing Blower/Supercharger - 0.0063in x bore size
The reason for these variable specifications is that different types of engines put radically different heat and pressure loads on the ring package. DiBlasi explains, saying, “Let’s look at an LS3 engines that many of our customers have, including myself. If you are building a fun street naturally aspirated LS3 you can expect about 525 horsepower out from it. The heat the rings will see will be very similar to what they are in factory form. Let’s take another person with an LS3 that drops in a new Eaton 2650 positive displacement blower. They can make about 1,100 horsepower out of the same bore, stroke, and general engine combination as the N/A guy. With over double the power output it will significantly increase the cylinder pressure and heat that rings will see. The ring end gaps will need to be substantially larger than the engine with 525 horsepower.”
What it all comes down to is the same thing we mentioned at the start - heat. The more heat, the more expansion in the ring material, and the more clearance that is required. “Forced induction engines are adding significantly more cylinder pressure than a naturally aspirated engine. The added air and fuel in forced induction engines acts as additional displacement in the same space as the naturally aspirated variant,” says DiBlasi. “That added cylinder pressure is added heat. Since heat is the driving force behind end gaps, the hotter cylinders require more end gaps. Similarly, two naturally aspirated engines with different compression ratios will, too. If one engine is 9:1 and the other is 14:1, the higher compression version will need the larger of the two gaps.”
For more in-depth info on filing rings, check out this story!
Wiseco’s gap recommendations are intended to get you into that “Goldilocks zone” - not too big, and not too small. DiBlasi counsels, “If the top ring end gap is correct, it will end up the smallest amount of ring end gap when fully expanded, and not run into itself. That way it holds in the highest amount of compression in the engine for maximum power output.”
Moved and Seconded
Most of the engines you’ll encounter utilize a ring package with a second compression ring below the first, to provide additional sealing and another path for heat to escape the piston into the cylinder bore, and from there into the coolant jacket and eventually the atmosphere. As shown above, the desired ring gap for the second ring is often, but not always, greater than the top ring.
“In OEM applications the second ring is tighter than the top ring,” DiBlasi explains. “It sees less heat, and its purpose is to control oil and be a secondary compression ring for any pressure that gets past the top ring. Since OEM applications are designed to an exact power output, heat, lifecycle, and emission compliance, they run a tighter second ring end gap.”
The situation changes once engine modifications come into the picture. Per DiBlasi, “We recommend a larger second ring end gap than the top ring for performance and racing engines since they see a wider range of power and heat than OEM. A forced induction engine may see the same power output as when it was in stock form daily driving, but at the track sees 2-3 times the power output it was originally designed for.”
There’s a clear technical reason for that extra clearance - “We like to see the second ring end gap .001-.002 larger than the top ring so it does not lock in any compression that gets past the top ring,” says DiBlasi. “If combustion gasses go past the top ring and the second ring gap is less than the top ring, the gas will not pass the second ring. The gas will be forced to return back to the top ring and press up on the bottom of the top ring. That will lift on the bottom side of the top ring, reducing its seal even further.”
Providing a way to control the pressure between the first and second rings is the key to allowing both rings to seal the way they are designed to; because a little bit of ‘blow-by’ is inevitable, dealing with it is necessary. “Since the name of the game is keeping as much combustion pressure above the top ring as possible, keeping it seated is required for maximum power. By allowing the second ring to freely let any excessive compression gases keep going, the top ring stays seated. The gas simply goes past it, cools slightly, and enters through the oil drain backs into the bottom of the piston. Your PCV system should be doing its job at that point forward,” DiBlasi explains.
The design of the piston can also play a role in managing the pressure between the first and second rings in order to improve ring seal. DiBlasi says, “In most cases, we build a groove in between the top and second ring on the second ring land. This groove increases the volume of area between the top and second rings. The increase in volume helps lower the pressure of any gases that end up there.”
Theory into Practice
Now that we’ve covered the “why” of piston ring gapping, it’s time to put that knowledge to work. We asked DiBlasi for the main issues people run into, and he gave us some very practical advice, saying, “There are several mistakes people make when setting ring end gaps. I myself have done it in the past, so it happens to everyone. I will also address ring install problems in general…”
- Mistake #1 - Not setting ring gaps at all.“Some people think that the rings are pre-gapped and ready to drop in. Ring manufacturers have a pretty larger tolerance when it comes to the gaps right out of the box. Always place each ring in the cylinder you are going to set it for and measure. Keep that ring designated for the cylinder the entire time. Sometimes your cylinders are a few .0001in’s different from each other, so you will want to be consistent. I personally get zip lock bags and number each bag with a cylinder number. Those ring stay in those bags with the correct cylinder they belong in until I am ready to gap them, then go back into the bag until I am ready to install them one by one.”
- Mistake #2 - Not reading the suggested ring end gaps provided by the piston manufacturer.“Many ring suppliers only provide data on OEM ring gaps. Follow the ring gap instructions for the pistons you are using. Never use OEM ring end gap specs, as the material and applications are wildly different. We often get calls about people setting ring gaps to OEM specs which can be as little as 1/3 the amount of ring gap that’s actually required.”
- Mistake #3 - Putting rings in upside down or in the wrong location.“99% of every top and second ring set has a specific orientation. There are bevels, twist, coatings, and various other things that make rings directional. There is typically writing or a dot on all first and second rings that indicate which side is up. In terms of location, some pistons have the same thickness top and second rings. Pay attention to the shape and what each ring does so you know where it goes.”
- Mistake #4 - Gapping gaffes.DiBlasi breaks this down even farther, saying, “This is going to be a boring process with a lot of filing, counting, placing in the bore, measuring, and repeating.” It’s easy to get lazy or careless, but working cautiously and paying attention during this process is critical.
- Filing rings before initially measuring end gaps.“Some people assume the rings are already too small and start filing away without a baseline measurement. Always measure first.”
- Count your turns if using a manual ring filer.“This will help you understand how much material removed each turn on the filer equates to.”
- Not being aware of the material of the ring you are filing.“A steel ring is going to need significantly more turns on a ring filer to remove material than a cast ring. A steel ring might require 30 turns on the filer to yield .005in of gap increase, where a cast ring requires 10 turns. I made this mistake 10 years ago and filed an extra .020 from a cast ring and had to buy another ring…”
- Not squaring your end gaps off.“Make sure your ring end gaps are square and perpendicular to the cylinder wall surface. You do not want a taper, as the measurement of the end gap will not be correct.”
- Not taking your time.“This process is boring and requires back and forth measuring so you can be precise. Take your time and start filing little at a time. Once you are a few cylinders in, you will have an idea of how much each ring requires as far as turns.”