Are ring gaps really THAT important?

Ring gaps are simple; they are a factor of A) the material of the ring, B) the type of coating the ring has, and C) how much that material expands under X heat conditions. X being; stock, performance, fuel type, race type (short burst or long hard hauls), and forced induction / nitrous use. Each one of those makes more heat than the previous. More heat = more expansion.

Rings expand under heat which closes the gaps up. You NEVER want the rings to expand too far and butt together or they will break, or possibly even break the top of the piston off! And in retrospect, you do not want to make the gaps too wide or they won't seal very well (make cylinder pressure) and it will use oil and smoke. So with that you FIRST have to determine HOW the engine is going to be used (stock, performance, race, supercharged or nitrous, long haul such as road race or short haul such as drag race), and of course fuel type because different fuels produce different BTU’s of heat & pressure. Once that is determined, THEN you can figure what gaps to run PER THE MANUFACTURER. They have already done the calculating of how much that particular ring will expand per usage for how much the ring size is, and the material type and coating type of the ring. This is why they give you choices, and why one ring size (or type) will be different than another ring type, size or brand. 

Running balls to the wall "most of the time" would be more like endurance racing (road racing) and NOT like drag racing, so you would set the gaps by what they recommend for how you are going to be running that engine. Circle track & road racing is also more like what running balls to the wall “most of the time” would be like, so I would say to gap them at a ring factor of the recommended .0055” per inch of bore. A bore factor is simple. You simply times the bore size by the recommended ring gap factor. A 4.030" bore times .0055" = a ring gap of .022". Now obviously if you are street driving, you are NOT going to be driving balls to the wall "most of the time", and even when you do, it is for short bursts such as getting onto the freeway or passing someone, so the heat and pressures in the cylinder is short lived and won't require as wide of a gap as an engine that is running balls-out all of the time. 1/4 mile drag racing is similar in "most" cases in that it is only running hard for 8, 9, 10 or 11 seconds at a time. 1/8th mile drag racing is even shorter yet, so even though pressures and temperatures are extreme, it is only for a short period of time. Obviously power adders such as blowers or nitrous (or both) are going to add to the amount of fuel being burned which creates more heat, so this is yet another factor to take into consideration when looking at ring gaps. A blower or nitrous engine is going to need wider gaps because of all of the extra heat coming from burning that extra fuel. Ring gap factors of .0065 or .0075 in a mild blown or nitrous injected engine are very common, and on more serious engines can get as high as .0080 or more! Using the same math for the engine example I gave above, this would make a ring gap of .026" on the small side and as much as .035" on the more extreme side on a 4.030" bore. That's more than twice as much as a stock ring gap which is usually somewhere around .014" - .016". If you used out of the box rings that came with a stock gap of about .0015" on an engine that was blown or had a small shot of nitrous (100 - 150 HP), it would last about 5,4,3,2,1 kaboom before the gaps butted together and most likely brake the top of the piston off above the ring land.

As far as the 2nd ring goes, there are 2 schools of thought there. Most will say the 2nd ring needs to have a smaller gap because it doesn’t see the heat & pressure that the top ring sees so it won't expand as much. That seems to make sense and has been what 99.9% everyone has done for decades.

The other train of thought is to make the 2nd ring gap wider than the top ring because it lets trapped gasses & pressure “out” and helps to prevent “ring flutter”. When ring flutter happens it causes the ring sealing surface to skip across the cylinder wall, much like your windshield wipers skip across the windshield making that annoying noise when there isn’t enough rain on it. The wiper leaves skippy marks where it lost contact with the windshield. Well, imagine a ring doing the same thing. If it loses its seal for even a nanosecond, then it means it lost pressure too. Cylinder pressure = horsepower, so if you lose ring seal via ring flutter, you are losing power. Now, I am not sure just how true the ring flutter theory is, but VERY reputable & respectable places that do a LOT of dyno testing, such as Reher-Morrison, have found that widening the 2nd ring gap prevents ring flutter and thus makes more power, or rather… PREVENTS the LOSS of power. Those guys are NOT dummies, so if I was going to believe anyone on a subject like this, unless I personally did a BUNCH of dyno testing myself, (which I'm not willing to do at this point just to find that out), they would be the one’s I'd trust.
 

Here is a question I get from time to time so I thought I'd make a Tech Tip out of it:

"If a ring manufacturer calls for .004" per inch of bore for stock, .0045" for high-performance street, .005" for pro-street / drag race, .0055 circle track / road race, .0065" moderate blown gas, and .0070" blown gas race or nitrous race only. Where should I set the gap for an engine I am going to run balls to the wall most of the time, but no supercharging of any kind? And why not just set them at the biggest spec (.007") and not worry about it? And then; should the 2nd ring gap be bigger or smaller than the top ring?"

Well, if you set the gaps too wide, your engine will use oil and might smoke a bit because the rings aren't getting enough heat to expand them enough to close the gaps down to where they "should be", such as when the nitrous is flowing or when under higher boost conditions. 

No, you won't break the top of the piston off by butting the rings, but it also won't make it's power making potential either because the rings aren't sealing as good as they could be. Remember.... the key to horsepower is "cylinder pressure". Raise the cylinder pressure and you'll raise horsepower. If the engine isn't sealing very good because your gaps are too big, then it's power making potential is going to suffer as well, so you really want to run the gaps that are right for how you are going to be running the engine. Obviously, too narrow of a gap will cause the most damage, but too wide has it's set backs as well. 

Here is a typical chart for setting ring gaps for various types of engines and conditions. This chart is from Total-Seal brand rings. Not to be confused with "gapless" rings. Total Seal is just the brand name, and yes, they do make gapless rings as well, but this chart is for their standard type of rings. You can click on the image to enlarge it so you can read it.

In this chart they use a "ring gap factor" calculation to determine what the gap should be, which simply means; you times your cylinder bore size by the ring gap factor number. For instance, let's say you have a mild blown small block street engine with typical forged pistons. Looking at this particular chart, it says the ring gap factor is ".005", so if you had a block that has a 4.030" bore, you would take 4.030 and times it by .005 to get your recommended gap. 

4.030 X .005 = .020"

If it were a healthy sized big block with a 4.500" bore, it would be:

4.500" X .005 = .0225" or basically .023" if you rounded it up. 

Here is a look at another chart from another piston ring manufacturer and what they suggest running for different set-ups. This one is from Speed-Pro piston rings. They show gaps for 2 different types of rings they make so they can use one chart for both types. Their standard cast rings, and their Plasma Moly / Iron Ductile rings. Again, I made this image clickable so it will enlarge. 

If you liked this tech tip then please be sure to share it with your fellow gear head friends, and on your favorite car forums by copying the URL at the top of the page and sharing it with them. Thanks! 

Just a visual for you; this is a hypereutectic piston that broke the top portion above the ring land off because the customer built the engine himself and didn't know to follow the ring gap factor guide to open-up the gaps to the recommended clearance so this kind of a disaster wouldn't happen. In a nutshell, Hypereutectic pistons, especially Keith Black brand, have a design that throws heat back up to the top of the compression ring. That extra heat expands the ring material more... and what happens when metal gets hot? It expands. So the standard ring gaps that he had, ended up butting together, which resulted in this scenario, so it's VERY important to pay attention to instructions for the products you're using, and to set the clearances and tolerances accordingly. 

With this, I am not saying Keith Black hypereutectic pistons are bad. I'm just saying what they do, which is exactly what they say in their own instructions that come with the pistons. if you follow their guidelines and set the ring gaps accordingly, you shouldn't have a problem like this occurring.