Ring filing can be accomplished...
Ring filing can be accomplished in several ways, from using a simple hand file to hand-crank filers to more elaborate power driven units, such as the unit shown here from ProForm. All will get the job done, it's just a matter of time invested and what you're used to.
In a conventional piston-ring system, the top ring provides the main gas seal of the combustion space. While it seems simple and static in its function, the ring takes a dynamic role in sealing the piston to the cylinder bore. The ring must react to minute irregularities in the cylinder wall, maintaining a constant tension, and react to pressure differential to provide a seal. While the static tension of the ring provides contact to the cylinder wall, in operation the sealing force is produced dynamically by the pressure differential on either side of the ring. The ring must seal to the pistons ring land, and this seal must be developed under both positive pressure on the compression and power stroke, and negative pressure during the induction cycle. Both the design of the ring and the fit of the ring to the ring groove are critical in accomplishing this. Race engines use pistons with close tolerances and precision machining of the surfaces of the ring lands to help accomplish this. Rings are typically built with a torsional twist imposed on the ring by a machined bevel on the inside diameter to promote the sealing to the lands. The torsional twist promotes a linear seal contact with the ring land when under gas-induced forces.
In most ring sets, it is the top ring that defines the ring set. If you buy a pack of moly, chrome, or ductile iron rings, it is the top ring that is moly or chrome faced, or made of ductile iron. The top ring is the most highly stressed member of the ring pack, and it sees the greatest temperature and load. For it to function properly, it needs a thin film of oil on the cylinder wall to complete a seal. That brings us to the second ring.
Our .030-inch-overbored 440...
Our .030-inch-overbored 440 is fitted with SpeedPro domed racing pistons, which accept a 1/16-, 1/16-, 3/16-inch ring package. The rings are a Childs & Albert Dura-Moly set, which are .035-inch over-sized to allow for file-fitting the rings.
When discussing conventional ring sets, the second ring's primary function is oil control. The profile of the second ring is slightly tapered on the cylinder wall side, generally by about two degrees. The taper puts the lower edge of the second ring in contact with the cylinder wall, a shape that allows the ring to serve two functions in a running engine. The lower edge acts effectively as an oil scraper as the piston travels down the bore, while on the upstroke this profile serves to evenly distribute a thin layer of oil to the cylinder wall. This oil film is critical to top ring seal and longevity. Nearly all conventional ring sets are supplied with a second ring manufactured from plain gray iron.
Conventional oil rings are a three-piece assembly, consisting of two scraping rails and the expander, which fits in the middle. While it may seem as though the expander is only there to hold the two rails in position, it serves another critical purpose. The rails have virtually no outward tension on the cylinder wall; the tension is developed by the expander. Oil rings have long been available at various degrees of tension, typically referred to as standard tension or low tension. Ring manufacturers vary the tension of the oil ring by a variety of techniques, including changing the specification of the expander, or altering the radial width of the rails. Lower tension oil rings marginally reduce drag, but it is difficult for the inexperienced builder to gage at what point oil control becomes compromised. For this reason, low-tension oil ring sets are generally reserved for race applications, although OEMs have reduced oil ring tension considerably in modern production engines.
Nearly all oil ring sets relevant to our engines are 3/16-inch wide. Narrower oil ring sets are also available, notably the metric spec 3mm oil ring. There is not much to be gained with the narrower oil ring pack in the typical Mopar engine build. The oil ring is at the front line of oil control. The piston must be designed with adequate drainback to route oil from the ring back to the crankcase. This is accomplished with slots or grooves at the back of the oil groove's ring land. Effective drainback is more easily accomplished with the wider ring assembly.
The instruction sheet from...
The instruction sheet from Childs & Albert calls for .004-inch per every inch of bore size on the top ring. That works out to .0174-inch gap for our 4.350-inch bore (.004 x 4.350 = .0174). We rounded up to .018-inch for our spec. The instructions called for .003-inch of gap per every inch of bore on the second ring, giving .013-inch gap.
The rings are test-fit into...
The rings are test-fit into the cylinder bore, so we can measure the gap we were starting with. The ring needs to be squarely pushed into the bore to get an accurate reading. A ring-squaring tool can be purchased for the task, or, as shown here, an inverted piston with a second ring installed as a stop can do the job. Sometimes, out of the box, the ring's endgap may butt solid when trying to square the piston into the bore. In that situation, never force the ring in; file some material until the ring can be inserted without binding.
The endgap is measured using...
The endgap is measured using a feeler gauge. To prevent an erroneous reading from a ring that isn't seated fully, work up from a smaller feeler gauge that fits easily one thousandth at a time until the next one won't go in. Closely examine the endgap by feel with the feeler gauge, as well as by sight to judge whether the endgap is parallel.