I hope you’re ready, because another Thursday is here. Let’s make it a good one. That means another week is almost over, and it’s time for Throwback Thursday. This week, I thought we would take a look back to September 2019. That’s when we had Jeff Smith take a dive into creating—and ultimately—building compression as he authored Piston Ring Gap: How Important Is It, And How To Get It Right.
He began by taking a look at why not all ring gaps are created equal. The issue that adds complexity is the engine builder must account for a given amount of heat in the cylinder. As heat is applied to a metal – like a piston ring – it will expand. Because the ring is contained in the cylinder, the initial cold gap will close as heat is applied. If enough heat is present, the ends of the ring could even touch. As soon as that happens, the ring can no longer accommodate further expansion and begins to tighten in the bore.
He went on to explain, with enough expansion, the ring can actually seize the piston. We’ve personally witnessed the upper section of a brand-new hypereutectic piston that snapped off at the oil return slots from a seized top ring, due to an insufficient end gap. Even in a mild case where the ring ends barely touch, the result can be a heavily scored cylinder wall and serious piston-ring-land damage due to this excessive load.
In the original article, he clarifies that ring-end gap specs are given as a clearance-per-inch of the bore. A normally aspirated 350 small-block Chevy with a 4.00-inch bore utilizing a 0.004-inch top-ring spec would only require a 0.016-inch top-ring end gap. But a normally aspirated 540ci big-block Chevy with a 4.50-inch bore would require a larger 0.018-inch top-ring end gap. The larger bore scribes a larger circumference ring, which means it will grow more when subjected to the same heat as a smaller diameter ring.
As one might realize, the second ring also requires a ring end gap commensurate with the engine’s design. He goes on to write, that there are also specific gaps for the second ring. If we go back to the ’70s, we can find the S-A Design book The Chevrolet Racing Engine, written by Bill Jenkins. He spec’d a tighter second-ring gap compared to the top ring. Sometime later, engine builders discovered that widening the second-ring end gap actually reduced pressure captured between the two rings.
With increased gaps on the second ring, the top ring seal is enhanced with minimal pressure present underneath the top ring. That’s the reason for the increased clearance. There’s also a spec for the minimal end gap clearance for the upper and lower rings for a three-piece oil ring set.
There is a lot more interesting and what might seem surprising information in the original article, and to learn more, you really need to check out Piston Ring Gap: How Important Is It, And How To Get It Right. Check back often, as we’ll be sure to bring you more great tech you can use.