Automotive Machining. Mike Mavrigian
rock the dial bore gauge back and forth as well as side to side in the micrometer. Zero the bore gauge dial to zero at the minimum reading found while in the micrometer.
When the bore gauge is then inserted into the bore to be measured, you are able to see if the bore diameter based on the reference in the gauge dial. When measuring a bore, rock the bore gauge back and forth as well as side to side to obtain a precise reading. When the gauge needle reaches a stopping point and then starts to move the opposite direction, rock the gauge to reach the point where the needle stops, immediately before it starts to move in the opposite direction. For example, if the bore reading is .002 inch less than the zero mark, the bore is .002 inch too small. If the dial gauge reads .0017 inch greater than the dial’s zero mark, you know that the bore is currently .0017 inch too large. When you know what the bore diameter is, you can then correct by enlarging the bore by boring or honing. If the bore is too large, you can correct (depending on the specific application) by sleeving a cylinder and then honing to size, installing a bushing in a lifter bore and honing to size, or by moving to larger-diameter pistons or lifters, etc.).
Prior to and during align honing a block’s main bore, a bore gauge is used to monitor bore diameter. Here a measurement check is made at twelve and six o’clock. Main bore measurement is also checked at three and nine o’clock and diagonally to check not only diameter but also to check for runout.
In essence, you’re setting up the dial bore gauge for a target bore diameter, then using the gauge to see how much the existing bore deviates from the target diameter.
Setup Tip
Although setting up a bore gauge sounds simple enough, the tricky part is in obtaining a precise point in the micrometer. Basically, you need three hands, because it’s difficult to keep the gauge properly centered on the micrometer while holding each tool by hand. You can mount the micrometer on a bench vise, leaving both hands free to manipulate the gauge, but that gets tricky, because overtightening the bench vise even slightly can cause the micrometer to distort.
A better solution is to use a specialty tool that safely secures the micrometer. An example is Goodson’s Micrometer Stand (MIC-FIX). This serves as a third hand, freeing both hands to hold the bore gauge. This stand features rubber jaws that secure the micrometer without damaging it.
Another option is to secure the bore gauge itself with a dial bore gauge setting fixture. Goodson’s bore gauge setting fixture accepts most popular gauges including Mitutoyo, Phase II, Fowler, Peacock, and others. This fixture does not work with a Sunnen bore gauge, but Sunnen offers an appropriate fixture for its gauges.
Mounting a Bore Gauge with a Setting Fixture
Instead of using a C-frame style micrometer, a bore gauge can be set up using a dial bore gauge setting fixture, such as Goodson’s DBG-FIX. This heavy-solid milled steel base accepts a standard (three are included, one for the 2- to 3-inch range, 3- to 4-inch range, and 4- to 5-inch range). First adjust the end opposite the adjustable spindle to provide a secure pocket for the end of the bore gauge. An anvil is featured against which one side of the bore gauge rests. Two opposing adjustable set screws, one on each side, are then adjusted to capture the sides of the bore gauge. Adjust the two opposing set screws equidistant from the center anvil. Don’t tighten these set screws against the bore gauge. Rather, adjust the set screws to provide .005- to .10-inch clearance. This provides proper alignment of the bore gauge when laced in the fixture, which in turn provides proper alignment to the fixture’s spindle and standard.
Although a bore gauge can be set up by using a micrometer (with the micrometer adjusted to the desired bore diameter), a bore gauge setting fixture eases the task, eliminating the need to hold an outside micrometer. A selection of standards are provided for the 2- to 3-inch range, 3- to 4-inch range, and 4- to 5-inch range, with final adjustment via the built-in micrometer. The bore gauge rests at the pocket end of the setting fixture. The threaded set screws on either side are adjusted an equal distance from the center anvil seen here. This centers the gauge. Do not adjust the set screws tightly against the gauge. Allow .005- to .010-inch clearance at each side of the gauge.
Insert a standard to the fixture, based on the target bore diameter. Adjust the mic thimble to achieve your precise target diameter. When the fixture is set up, you’re ready to adjust your bore gauge
Choose the bore gauge extension that’s appropriate for your target bore diameter. The short end seen here inserts into the gauge. To alter the length of the extension as needed, install a supplied spacer washer to the short end of the extension. Here I installed a .100-inch-thick washer. Gauge kits commonly include spacers at .100-, .050-, and .020-inch thick.
Install the appropriate standard (for example, if your target for the bore gauge needs to be set up to check a 3.500-inch bore, install the 2-inch standard that allows setting to a 3- to 4-inch range. Then adjust the fixture’s micrometer spindle to obtain a fixed setting at 3.500-inch.
Set up your bore gauge with the appropriate extension and washers for your desired bore diameter. Place the bore gauge in the setting stand. If it is too long to fit, change to a shorter extension or fewer washers. If too short, go with a longer extension or add washers as needed. Adjust the bore gauge so that you have a bit of preload on the gauge when placed in the fixture. Rock the bore gauge toward and away from the fixture’s micrometer. With the gauge held at its minimum reading, adjust the gauge bezel to obtain zero on the gauge. The bore gauge is now ready to check the target bore.
Insert the extension to the gauge. Install the threaded collar onto the gauge to secure the extension. The side of the gauge opposite the extension features two rollers that guide the gauge in the bore. The center of the gauge features a spring-loaded plunger that presses against the bore wall. As the plunger is depressed or extended, this movement is seen on the tool’s dial indicator.
A valveguide bore gauge is required to measure the inside diameter of a guide. Measurements should be taken at the bottom, middle, and top areas of each guide. Here a split-ball gauge is inserted into the guide. The gauge is allowed to relax and expand inside the guide, then tightened to maintain the diameter.
With the bore gauge installed onto the setting fixture, gently rock the gauge forward and rearward, noting the minimum indicator reading. Holding the gauge steady at this minimum reading, zero the dial indicator’s bezel so that the needle rests at the zero mark. You now have a zero reference for your target bore diameter. Insert the gauge into the bore (the bore must be clean and free of oil and contamination) and gently rock the gauge in opposite directions, noting the minimum needle reading (where the needle runs to a point and begins to reverse direction). Note the reading on the dial indicator to see how far the bore measures, under or over your target diameter, indicating how far the bore is undersized or oversized relative to the target diameter.
A depth gauge is a micrometer-style tool that features a flat base and allows the