Piston Ring End Gaps

Piston ring gaps are vitally important for their role in each engine application. The formula for determining ring gaps is simple, they are a factor of: 1) The material the ring is made of, 2) The type of coating the ring has and 3) how much that material expands under X amount of heat condition. X being; stock-standard, performance, fuel type, race type (short quick burst or long hard revving distances), and forced induction such as supercharged, turbocharged and/or nitrous oxide use. Each one of those applications makes more heat than the previous. More heat = more expansion of metal.

Rings expand under heat which closes the gaps up. You DON’T want the rings to expand too much and butt together or they will break because the ends have come together, have nowhere to go and will possibly break the ring lands and the top of the piston.

In retrospect, you don’t want to make the ring gaps too wide or they won't seal very well and will be down on cylinder pressure, will use oil and smoke. Take all the above carefully into consideration and FIRST you must determine how the engine is going to be used (stock-standard, performance, race, supercharged, turbocharged and/or nitrous, road race or drag race and fuel type?

Once you have determined all of the above, you can only then figure out what ring gaps to run as per the manufacturer. They have already done all the development and calculating of exactly how much the particular ring type will expand per usage. This is why they give you a choice, and why one ring size or ring type will be different than another ring type, size or brand. 

Running an engine at high RPM "most of the time" like in endurance racing and NOT like drag racing, therefore the ring gaps must be set by what they are recommend for and what they are intended for. Oval track racing is like running ‘balls to the walls’ most of the time.

The recommended gap, using a ‘ring factor’ of .0055” per inch of bore. The correct calculation of the bore factor is simple, times the bore size by the recommended ring gap factor. A 4.00" bore times .0055" = a ring gap of .022".

If you are mostly street driving, you are not going to be driving 'balls to the walls' all the time, and even when you do, it is for short bursts like when you are overtaking someone, so the heat and pressures in the cylinder is short lived and won't require as wide a gap as an engine that is running 'flat-out' most of the time. Drag racing over ¼ mile is similar because it is only running hard for 9, 10 or 11 seconds at a time, so even though pressures and temperatures are extreme, it is only for a short period of time.

Obviously supercharged, turbocharged and/or nitrous combinations are going to add to the amount of air/fuel mixture being forced through the emgine and burned, which creates a lot more heat, so this is yet another factor to take into consideration when looking at ring gaps. A supercharged, turbocharged or nitrous oxide assisted engine is going to need wider gaps because of all of the extra heat that is generated. Ring gap factors of .0065” or .0075” in mild blown or nitrous injected engines are normal.

Using the same maths for the engine example as 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 would be more than twice as much as a stock standard ring gap which is usually somewhere around .014" - .016" of an inch. If you used rings with a standard ring gap of about .015" on a blown engine or had a small shot of nitrous, it would only last 5 to 10 seconds and 'bang'.

As far as the 2nd ring goes, there are two ways of looking at it. Some say the 2nd ring needs to have a smaller gap because it doesn’t get the heat and pressure that the top ring gets so it won't expand as much. The other train of thought is to make the 2nd ring gap wider than the top ring because it lets trapped gasses and pressure out and helps to prevent “ring flutter”.

When ring flutter occurs it causes the ring sealing surfaces to 'skip' across the cylinder walls, much like your windscreen wipers ‘shudder’ across the windscreen leaving those irritating shudder marks where it lost contact with the windscreen. A piston ring behaves in a similar manner. If the ring loses it’s seal for even a nanosecond, then you can be sure that it lost pressure, and compression = horsepower!

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