First-time engine builders and those working on mass-produced powerplants commonly use off-the-shelf pregapped piston rings. However, high-performance engine builders seem to spend a lot of time fiddling with ring gaps. Why? Because the piston ring is charged with the critical task of sealing the cylinder, and the endgap of the ring is the biggest leak in that seal. While it doesn't take an engineering degree to recognize that the ring endgap represents a leak, the question is just how much horsepower is really blowing out though those little gaps? File-fitting the rings to a prescribed endgap specification has become standard practice with high-performance engines, and we have a hunch that these experts aren't just wasting their time.
Ring endgap in a running engine doesn't remain constant, because the heat of combustion and friction cause the ring to expand and contract. As the ring expands, the endgap narrows to something less than the installed measurement. The characteristic closing of the ring gaps during operation is generally a good thing unless the endgap closes entirely or the ring ends butt together, which can cause numerous problems, from scuffing of the piston or bore wall to loss of ring seal, destructive engine failure, or grenaded pistons. Standard pregapped piston rings come packaged for a specific bore size with gaps conservatively set on the wide side to minimize the potential for destructive endgap butting. For popular applications, oversized file-fit rings are available that allow the engine builder to custom-tailor the ring endgap.
File FitsFile-fitting rings is a fairly simple procedure. It's simply a matter of squarely inserting each ring into the engine's bore, measuring the endgap with a feeler gauge, and adjusting the gap by filing or grinding until the desired gap is achieved. There are a variety of tools that can be used to open the ring endgap, from simple hand files, to inexpensive hand-operated cutting wheels, to more elaborate precision electric ring grinders. Whatever the method used, the goal is to create a square and parallel gap with the desired clearance, which requires a reasonable amount of skill and care. Typically, each ring is sized and assigned to the specific bore in which it will be run, accounting for any minor variations in actual bore sizing, which can affect the endgap.
Ring manufacturers provide various endgap recommendations, which vary according to application. Generally, as the heat and load expectation becomes greater, such as with nitrous or supercharging, the ring endgap recommendation gets wider. This is to compensate for the increased heat and expansion the ring will be subjected to in these applications.
Conventional wisdom says that top rings should be set to the tightest endgap under the anticipated operating conditions that doesn't cause butting to produce the best power. Clearly, there is a balancing act involved in deciding what endgap will provide optimal power without the danger of going too tight. Piston design can also play a role in the amount of expansion the ring will see. Ring placement, land design, and the piston's material and thermal conductivity all affect the operating endgap. Unless you have the resources to test and try specific endgap settings for a particular combination-a luxury few can afford-it's best to stick to the ring maker's recommendations. As noted, piston design can play a role as well, and ultimately, if the piston maker has specific recommendations on endgap, these should take precedence when gapping rings.
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.
Pregapped RingsWhile theory and conjecture make for nice bench-racing conversations, what better or more ambitious way to test the effects of ring endgap on power output than to run the various configurations on the dyno and let the numbers do the talking? Our test engine was a Vortec-headed 350 small-block we built several months ago and have used for a few dyno-tests since. The bottom end in this mild 350 featured conventional pregapped rings in the budget-priced mail-order short-block. We had recently run this engine in a camshaft lobe separation angle test session establishing a baseline with the pregapped rings. The engine was making a respectable 434 hp. So good, in fact, that the crew at our dyno facility was skeptical that there was much room to the upside with a ring change.
After the teardown, we noted that the engine's ring gaps were very much on the wide side, swallowing a 0.030-inch feeler gauge with ease. Some of the clearance was no doubt gained in the nearly 100 dyno pulls this engine has been subjected to. Despite the wide ring gap, this little Chevy was delivering strong power without any visible signs of excessive blow-by, so we had to wonder how this test would turn out.
Peak hp: 434.7 hp @ 5,700 rpmPeak torque: 450 lb-ft @ 4,400 rpmAverage hp (3,000-6,000 rpm): 363.3 hpAverage torque (3,000-6,000 rpm): 425.6 lb-ftCranking compression: 160 psiAverage leakdown: Not recordedPeak volumetric efficiency: Not recorded
File-Fit RingsThe pregapped rings in our test engine were much wider than any custom-engine builder would let out the door, so we broke out our trusty hand-crank ring file (from Summit Racing Equipment) to custom-fit a fresh set. The rings we chose were Total Seal's Classic set, a conventional file-fit ductile-iron/moly ring package. In accordance with the recommendations from Total Seal, the top rings were gapped with 0.0045 inch of clearance per 1 inch of cylinder bore diameter, for a total of 0.018 inch, and the second ring was gapped to 0.014 inch, per the recommendation of 0.0035 inch per inch of bore diameter. The rings were changed without any additional cylinder-wall prep, and the engine was reassembled for another turn at the dyno. After a short cycle to seat the rings, the numbers were in, and they were impressive, eclipsing the power turned in by the pregapped rings. Most notable were the higher average power and torque numbers showing the engine was stronger across the board. So there's something to this ring-gapping stuff after all.
We spun the engine over for a cranking compression check, and found an average gain of 5 psi per cylinder, indicating the cylinders were sealing measurably better and making use of the increased seal to produce more power. We also hooked up a leakdown tester and ran a leakage check of every cylinder. The average leakage was 12 percent, which was higher than we expected, but air escaping the ports indicated that the valve sealing on our Vortec heads was less than optimal. Unfortunately, we did not perform a leakdown test with the original ring set, but the amount getting past the valves remains a constant compared to the amount escaping past the rings.
Peak hp: 441 hp @ 5,700 rpmPeak torque: 459.2 lb-ft @ 4,500 rpmAverage hp (3,000-6,000 rpm): 370 hpAverage torque (3,000-6,000 rpm): 433.9 lb-ftCranking compression: 165 psiAverage leakdown: 12 percentPeak volumetric efficiency: 94.8 percent
Gapless Top RingsThe Total Seal Gapless Top ring set uses a two-piece ring assembly in order to close the flow path through the ring gap. Like any other ring, each ring segment has a gap, and is filed to an end-clearance specification to prevent butting. The endgap specs on Total Seal's rings are wider than on conventional rings, since the rings will contain more heat and pressure. The wide clearance provides plenty of insurance against butting, while the rail segment closes the gap to ensure a gas seal. That theory makes sense to us, but would the dyno results bear it out?
We tore the 350 down for yet another re-ring to find out. In a couple of hours we had the engine back up on the dyno and it was showtime. The numbers didn't disappoint, with the engine now cracking the 450hp level and again showing a gain in power across the board. Impressive! Interestingly, volumetric efficiency was up across the board as well, indicating that the engine was taking in more air as a result of the ring change. Though it is often overlooked, ring-seal efficiency is just as relevant on the induction stroke as it is in holding gas pressure in the power stroke. The better the seal on the intake stroke, the stronger the pull on the air/fuel mixture entering the cylinder. Subsequent leakdown testing showed a marked improvement in cylinder leakage rate, essentially minimizing the leakage past the rings, though we still registered leakage as a result of the inadequate valve sealing. In other tests we have seen total recorded cylinder leakage of 1 percent with the Total Seal rings.
Peak hp: 451 hp @ 5,600 rpmPeak torque: 465 lb-ft @ 4,500 rpmAverage hp (3,000-6,000 rpm): 375 hpAverage torque (3,000-6,000 rpm): 440 lb-ftCranking compression: 185 psiAverage leakdown: 7 percentPeak volumetric efficiency: 96.4 percent
