Gap has to be checked with...
Gap has to be checked with the ring in the cylinder in which it will be run. It's easiest to slide the ring in at an angle with the gap on top, and then rotate it down.
Standard rings are not a continuous circle, but have a split. When properly fitted inside a specific cylinder bore, this split causes a gap in the ring. The idea is that when the ring heats up from the radiant heat from combustion, it expands and the gap closes. (It's also impossible to get the ring into the piston groove without the split.)
Several factors determine what the gap in a cold ring should be, but generally the ring manufacturer can help with that. In order to create as much compression as possible, racing pistons move the top ring as close to the top of the piston as possible. Usually, it's around 0.150 inch from the top. For a high-quality forged aluminum racing piston, this translates to a ring gap approximately 0.018 inch. Hypereutectic pistons experience greater rates of expansion and require a larger gap. Also, if the top ring groove is farther down the body of the piston, the ring receives more protection from the heat and needs less gap. Of course, we are talking in terms of thousandths of an inch of change, but in racing that is often enough.
Finally, after grinding the ends of the ring for the proper gap, it's always a good idea to lightly file the ends to make sure there are no burrs in the ends of the ring. Use a very fine file and be sure to remove only burrs or sharp ridges; do not round the ends of the ring, or you'll reduce its ability to seal properly.
Total Seal's gapless ring...
Total Seal's gapless ring technology uses two rings with the gaps 180 degrees to each other to effectively seal off the combustion chamber. The top of the two rings is actually "L"-shaped, and the smaller ring underneath sits in the pocket of the "L." This keeps the design from being significantly thicker than a conventional top ring.
Matt Hartford of Total Seal says fitting gapless rings can actually be easier than fitting conventional rings. Total Seal is the leading manufacturer of gapless rings for racing purposes, and the design is surprisingly simple. The top ring is actually two pieces. The first looks a lot like a conventional top ring, and directly underneath it is a "secondary rail," which effectively seals the top ring. Both of these rings have gaps, which must be checked conventionally, but they are installed with the gaps 180 degrees opposite each other, creating what the engine sees as a single, "gapless" ring.
"The only thing we think is critical is that you have at least the minimum gap that we recommend per our instructions," Hartford says. "We've done testing to show whether you have 0.024 or 0.034 inch of gap. You see absolutely no difference in performance simply because of the design of the gapless ring. The secondary rail stops all the pressure from getting through the end gap. What that does is give you a safety margin. You can go a little larger on your gaps in case you were to lean out your engine or miss your tune-up. Then you don't have to worry about butting your end gap."