EngineLabs

EngineLabs’ Tool Of The Month — Summit Racing Pro Dial Bore Gauge

When it comes to engine building, your tools are critical. A good tool will make your task easier while also allowing you to perform your task more accurately than with a substandard tool. While it’s true that there have been engines slapped together with a socket set purchased from K-Mart while waiting for your personal pan pizza from Little Caesars to come out of the oven, that’s not the way we like to do things around here.

We are firmly of the belief that you should purchase the best tools you can afford, in order to do the job right. Conversely, we also don’t feel that you need to run out and buy the absolute most expensive tool out there, just because it’s the most expensive, either. There exists a middle ground of quality and price, and those are the tools we hope to discuss in this new Tool of the Month series. First up is the Summit Racing Pro Dial Bore Gauge, P/N: 900041-01.

What Is A Dial Bore Gauge?

Before we look at the Summit Racing Pro Dial Bore Gauge itself, let’s first talk about what, exactly, a dial bore gauge is and what it does. A dial bore gauge is used to measure the inner diameter of a hole — be it a cylinder bore, lifter bore, cam bore, main bore, rod bore, etc. However, unlike an inside micrometer, the dial bore gauge doesn’t give you an absolute measurement. Rather, the design of the tool gives you the variance from a standard reference measurement.

This might sound more complicated on paper, but as you’ll see in the following article, it actually eliminates several math-related steps (reducing the chances for errors in arithmetic) and gives you your clearances via direct mechanical comparison of two measurements. With a little practice, you can get reliable, repeatable measurements down to “half a tenth” — that is half a ten-thousandth (.00005) of an inch.

The Summit Racing Pro Dial Bore Gauge has an extremely high-resolution dial gauge. Each hash mark represents .0001 (one ten-thousandth) inch, so reading to half that — .00005 or five hundred-thousandths of an inch — is relatively simple.

Traditionally, a dial bore gauge will be available in multiple size ranges, which is dictated by the measuring head’s size and design. While there are split-ball designs available, for this article we’ll be focusing on the more common anvil-and-ball design. Since this particular tool is aimed at engine work, it has been designed for the range of 2.00- to 6.00-inch inner diameters, which covers most of the measuring required in a modern V8 engine application.

Other size ranges, such as 0.700 to 1.400 inches, and 1.400 to 2.400 inches are also commonly available. Besides size ranges, there are also ranges of precision available. Cheaper gauges will read in increments of .0005 inch, which generally allows a skilled user to read to about .0002-inch of precision. More precise tools will have a dial with .0001-inch graduations, which will allow an average user to read to .00005-inch of precision. The Summit Racing Pro Dial Bore Gauge has the more precise .0001-inch graduations.

Setting Up and Measuring With A Dial Bore Gauge

As we mentioned earlier, the dial bore gauge is a comparative measurement tool as opposed to an absolute measurement tool. That means that you will need a reference standard upon which to zero the tool. There are a number of ways to do this, but they usually involve the use of an appropriately sized micrometer as the standard.

In the case of measuring main bearing clearances, it’s a simple matter of measuring the crankshaft journal with a micrometer, locking the micrometer at that measurement, and then mounting the micrometer in a vise of some kind (either a micrometer stand or a bench vise). Then, based on the measurement of the micrometer, you will set up the dial bore gauge with the appropriate anvil configuration.

The kit comes with everything you need to measure bores from 2.000 inches up to 6.000 inches. The wheels on the anvil head allows for stable, repeatable measurements without risking any scratches on the surfaces being measured.

Say your journal size mics at 2.7503 inches. You would choose the 2.6-inch anvil and add the 0.100 and .050 shims to get a nominal 2.75-inch measurement range. Then, you take the gauge and set it in-between the micrometer jaws at an angle and rock it over center. As you pass through the perfect 90-degree point, you will notice the dial gauge will stop and reverse direction — that is your zero point.

To zero the gauge, you then loosen the bezel locking screw and rotate the bezel so that the highest reading on the dial face as you rock the tool through the measurement arc lines up with the zero. In our example provided, the tool is now zeroed at 2.7503 inches. To properly measure your main clearance, you would use the tool in the same method (rocking over in the bore) of your mains, which need to be torqued to spec, with the bearings in place.

Here, we're looking at the process of measuring the piston-to-wall clearance. On the left, the piston is measured with a micrometer at the measuring point specified by the manufacturer. In the center, we then zero the bore gauge in the micrometer as the reference measurement. On the right, we then rock the gauge through the cylinder to determine clearance. Note, when the reference measurement is smaller than the measured bore, the dial will spin backwards. The reading of 5.7 is actually the inverse (4.3) meaning a piston-to-wall clearance of .0043 inches.

Assuming you are adhering to the rule of thumb for clearances (.001 inch per inch of journal diameter), you will be shooting for a nominal clearance of .00275 inch in this scenario. Lots of numbers, I know, but this is where the dial bore gauge shows its worth. As you rock the gauge through each main bore, it will read the actual bearing clearance on the dial. Reading .00275 inch on the dial is a straightforward affair, and will tell you immediately whether you have too much, not enough, or the perfect amount of clearance.

Precisely measuring your bore size is a similarly easy process. If you want to easily know the exact bore size, you can set your micrometer to the nominal bore size and use that to set your zero point, or as we’ve seen in some high-end shops, set the gauge on a known-size bore standard. Then, it’s just a matter of rocking the gauge through the bore to get your reading.

You can also measure at multiple depths and radial angles in the bore to get an idea of the bores’ roundness and cylindricity. Additionally, you can easily find piston-to-wall clearance using the same method, but by using the measurement of the pistons as the zero point of the gauge. While there are a number of ways to measure bearing clearances and inside diameters, at the end of the day, the dial bore gauge offers the best combination of accuracy, precision, speed, and ease of use — at least in our opinion.

To measure bearing clearances, first you use the micrometer to measure the journal diameter, This can be the main journals or rod journals. Here, we're checking the mains. Then, once again, we zero the gauge on the micrometer before rocking the tool through the torqued down mains with the bearings installed. Here you can see a measurement of .0022-inch of clearance, which is right in line with the rule of thumb clearance guide (2.240 x .001 = .0022 clearance).

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About the author

Greg Acosta

Greg has spent eighteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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