The question of optimal second-ring gap is a subject open to debate. The second ring can physically tolerate a significantly tighter clearance than the top ring, since it is exposed to less heat and thus expands less. As a result, second-ring endgaps were traditionally set considerably tighter than the top rings. In recent years, some ring manufacturers have begun recommending running wider gaps in the second ring. The theory is that a wider second gap allows an escape path for any high-pressure gasses caught between the compression rings, which can cause the top ring to unload or flutter at high rpm. Some engine builders embrace this idea, while others dismiss it. We haven't made up our minds on this one, though we wouldn't recommend going against a specific ring maker's instructions.
Closing The GapAll piston rings, whether top, second, or oil control, must be open at the ends to allow them to be installed into the ring grooves of the piston. More importantly, the open end also allows the ring to act as a spring. The radial tension of the ring presses outwardly on the cylinder walls, providing a seal (with the aid of the gas forces), and compensating for small diameter changes in the cylinder bore.
As both the cylinder and ring wear, a piston ring expands radially outward under the ring's tension, allowing the ring to adjust to the bore shape for a continued seal throughout the service life of the engine. As a conventional ring continues to wear, those carefully set ring gaps start to open up, increasing leakage past them.
Gapless rings have been around for a long time, and one of their biggest proponents these days is Total Seal, which introduced its Gapless Top ring sets several years ago. With Total Seal's unique design, the top ring is actually a two-piece assembly-a main ring section similar to a conventional ring is machined with a recess to accept a second rail. The rail installed into the recess of the main ring section staggers the gaps between the two and effectively eliminates any endgap in the ring assembly.
With Total Seal's Gapless Top rings, the overlapping gaps of the two ring segments remain closed even as bore and ring wear increase the end clearance of the individual segments. It sounds like a valid concept, but the question remains-just how much of a difference in real-world horsepower is up for grabs in that little flow path at the endgap of a ring?
To conduct a test of the Total Seal Gapless Top ring theory, we took an average cast-iron-headed street engine making about 430 hp and tested three ring configurations: a set of pregapped rings, a set of hand-gapped file-fit rings, and a set of Gapless Top rings. To eliminate variables, we gapped both the file fit and Gapless Top rings prior to the test and swapped them in a marathon one-day dyno session.
The results speak for themselves, but one of the most significant things we learned is that even though the area open to leakage at the piston-ring endgap seems small, it can have a significant effect on output. Our initial baseline was performed with the rings having seen some hard dyno duty not unlike a season's dragstrip time or extended street mileage. We don't know exactly what the original installed gap was with the pregapped rings when the engine was built, but we can be certain that it was something less than the 0.030 inch we found when the engine was torn down. With enough dyno time, our carefully file-fit set would also no doubt eventually open up that large or more. With Total Seal's Gapless Top rings, the endgap will remain closed even as the rings wear and the individual ring segment's gaps open up. Overall, we picked up an average of 12 hp and 15 lb-ft of torque by changing from wide-gapped rings to the Gapless Top rings in this rel-atively mild street engine. That's something to think about.