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File Fitting Pistons - Step By Step

File-Fitting Pistons for Max Performance

Building a motor isn't difficult, but it certainly calls for a wide range of tools and requires certain knowledge that isn't always evident. Unfortunately, thereare so many components involved when piecing a motor together that most buildup stories tend to be more of an overview rather than a detailed step-by-step process, so you tend to miss the little things.

Take file-fitting piston rings. The main advantage is that it promotes a better combustion seal by setting exact tolerances, which helps to reduce blow-by and, more importantly, prevents the rings from butting up against one another and scoring the cylinder walls. A healthy set of rings is essential to generating power. Without them, there's nothing to hold back the pressure during the compression cycle, and while it isn't hard to figure out, let's just say that you won't be going anywhere anytime in any hurry.

This is not to imply that file-fitting pistons is a requirement, especially considering many aftermarket manufacturers offer complete file-fit ring packages. But it is a good idea to check them for every build, as it'll ensure proper clearances for your particular application. So while your average rebuild certainly doesn't demand it, following this method will ensure maximum performance out of every build. Bear in mind, when we say "maximum," that file-fitting rings won't necessarily translate into increased horsepower; however, by improving the motor's efficiency, you'll be taking full advantage of the power your mill can generate.


We began by cleaning each ring with a paper towel saturated with lacquer thinner. Once they dried, we made sure there was no remaining lint and checked for any irregularities, like sharp edges.

Next we placed the ring into the bore with a squaring tool and measured the initial end gap. We dropped the ring 1 inch into the bore and leveled it with the deck. (Or you can use a flat-top piston by turning it upside down to square it.)

Using a feeler gauge, we measured the current end gap so we'd know how much to take out. If the rings are too large, don't force them into the bore; instead slowly start cutting the ends in stages until they can fit.

When grinding the end gap, you only have to do one side. It may take four or five tries, so grind slowly. While commercial-grade piston-ring filers are available (and generally faster), we used this one from Powerhouse Products. At only $65, it's affordable and gets the job done.

After filing them to size, we used a fine file to clean the edges and smooth out the sharp ends in a downward motion. It's important to note that the hardness of the ring determines how much you'll need to file. A cast ring won't leave an edge, whereas a tool-steel and stainless ring will. Then we repositioned each ring in the bore to ensure they measured to spec.

To place the oil ring, which is a three-piece configuration, we spaced the end gaps approximately 70-90 degrees away from one another, simply to make sure they never intersect. Always work from the bottom ring up, with the oil ring, then the second ring, followed by the first (top) ring. It's best to spiral the rings like a coil spring, then to pull it up and walk it around in a circle. We used the base of a small flat-blade to make sure we didn't scuff the piston.

Make sure all the rings rotate freely. For the second and top rings, we placed them 180 degrees apart (arrows). When it comes to determining end gaps, it'll be contingent on the bore size and the material of the top ring--and if it's a nitrous motor, the amount of nitrous being sprayed will come into play. Even though ring packs come with guidelines, engine builders tend to have their own preferences. As an example, in an SBC with a 4.060 bore, QMP likes to gap a nitrous motor with a 0.035-inch top and 0.030 inch on the second ring. These gaps are larger, so they won't butt together under pressure. On a similar motor with a blower, the end gap will be less, at 0.032 inch on top and 0.028 inch on the bottom. With nitrous, the initial hit isn't as forgiving, and the larger gap helps prevent the ring from forcing into itself.


Arias Pistons
Gardena, CA 90249
Powerhouse Products
Memphis, TN 38118
Chatsworth, CA

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