If you’ve been following the EngineLabs Tool of the Month series you’ve noticed a recurring theme of “Even though there might be other ways to do it, why not use the proper tool for the job.” This month’s tool falls right in line with that sentiment. The Summit Racing adjustable piston squaring tool is deceptively simple with a surprising amount of features.
First, let’s look at the problem needing to be solved. When you are file-fitting (or “gapping”) piston, you are dealing in thousandths of an inch on clearance. The basic geometry of a cylinder says that the cylinder bore size is the smallest at exactly 90 degrees to the axis of the bore. Since that happens to be the plane the piston operates in, that is the measurement we are concerned with.
Since we measure the piston ring gap with the ring in the cylinder, in order to compress it to its actual working diameter, it’s crucial to ensure that the piston ring is sitting exactly perpendicular to the bore in the cylinder. As it gets away from 90 degrees, it not only strays from being a circle (becoming an oval) but, changes the ring gap.
Since the ring gap is a critical dimension when building an engine, it’s crucial that you are measuring it (and setting it) properly. Hence the development of tools specifically to ensure the ring is square in the bore.
There are a number of ways that the ring has been “squared up” in the bore throughout history. All of them rely on the fact that the bore is ideally square to the deck of the block. Some people will use a caliper’s depth gauge to measure around points of the ring in the bore, and others will use a piston for the bore size, to squarely push the ring into the bore, using the top ring on the piston as the depth stop.
While the inverted piston has the right idea, there are several drawbacks to that technique. The first is the chance of damaging the ring on the piston or the piston itself. The second is that a loose ring isn’t a rigid, exact shoulder for the piston to index on. Finally, it allows no access to the underside of the ring to ensure that it’s flush against the top of the piston.
The Summit ring squaring tool addresses all of those issues, along with being adjustable for different bore sizes. The tool is available in two size ranges: 2.950 to 3.940 inches and 3.750 to 4.650 inches, the latter of which is shown here. That second size range should cover everything we currently have in the shop. The tool is precision machined, offering a flat, square surface to index on the deck, as well as a flat, square underside to butt the top of the rings flat against, ensuring they are truly square in the bore.
Another feature of the ring squaring tool, as opposed to an upside-down piston, is that there is space designed into the tool to allow you to get your fingers into the bore while the tool is in place, and ensure that the rings are flat against the bottom of the tool. Not only is the process easy, but you know you’re assured to have your rings perfectly square in the bore.