Hold the spanner wrench between the thumb and forefinger of your other hand, with your thumb resting on the sleeve. Rotate the spanner wrench to make the correction. If using a 1-inch standard, adjust until the micrometer reads exactly 1-inch. Unlock the micrometer and verify that your reading is exactly 1 inch when measuring the 1-inch standard.
Standards (measuring block references) are an absolute must, to allow you to routinely check a micrometer for calibration. The three standards, 1-, 2-, and 3-inch lengths, are shown here. These are precision-ground reference blocks and must be handled with care.
Outside micrometers feature a small hole in the sleeve for the use of a spanner wrench, which allows you to adjust the mic for calibration.
Precision measuring instruments must be kept clean. Even the slightest dust particle can alter your reading. Clean the anvil and spindle surfaces prior to every use. After cleaning with a soft pare or towel, blow dust away with your mouth. Don’t use high-pressured shop-compressed air, since the high velocity of compressed air can force dust particles into the tool’s mechanism.
A micrometer stand is highly recommended. This allows you to secure a micrometer in padded jaws, making calibration checks and various measurements without the need to hold the micrometer in one hand while trying to measure a standard or when trying to set up a bore gauge with a micrometer.
With a measuring standard in place, the spanner wrench is used to adjust the mic so that it precisely reads the standard length. Here a mic is being adjusted so that it reads exactly 1 inch while using a 1-inch standard.
Occasionally, apply a drop of precision instrument oil to the spindle where it protrudes from the frame.
Caliper Micrometer
A caliper can be used to measure the outside or the inside dimensions; for instance, the outside diameter (O.D.) of a valvestem or the inside diameter (I.D.) of a hole. Calipers are available in both dial and digital styles. A dial caliper’s gauge usually features dial marks in increments of .001 inch. The major dimension is marked on the caliper’s slide (for instance, 1 inch, 2 inches, etc.) The numbers on the slide that are visible between the caliper’s fixed and movable jaws indicate one hundredth of an inch increments (for example, the number 5 indicates .500 inch). The gauge needle further refines the measurement by showing additional increments of .001 inch. When the gauge needle begins to first move away from zero, this represents additional .001 inch of the reading. For instance, if the number on the slide indicates 5, and the gauge needle indicates 13, the reading is .513 inch.
Shown is how to measure the diameter of a pushrod. This example shows a measurement of .3013 inch. Example of a commonly available dial caliper. A dial caliper is essentially a ruler that adds increased measuring precision with the addition of a dial indicator. A dial caliper is very easy to read. This example shows a setting at .5013 inch. The initial distance is represented on the slide, where the inside of the movable jaw aligns with the marks on the slide. Each individual line on the dial represents .001 inch.
Although a traditional outside micrometer may be used to measure brake rotor thickness, a dedicated rotor thickness caliper is a better choice. The stationary anvil features a precision-ground flat surface, while the sliding arm’s contact features a pointed surface.
The rear tip of a dial caliper can be used to measure depth. Extend the tip outward by rolling the thumbwheel of the caliper, insert into the hole or groove, and gently push downward to bottom the end of the rule body against the top surface, then read the measurement on the gauge.
A dial indicator features an incremented gauge and a spring-loaded plunger. When the plunger is pushed in, the gauge reading is reduced, depicting how far the plunger moves.
Dial indicators are used in a variety of applications, which determine what type of mounting fixture is required.
Applications include (but are not limited to) measured brake rotor lateral runout, piston position relative to top-dead-center (TDC) in an engine block, measuring crankshaft thrust, measuring runout on a crankshaft, camshaft, valvestem, pushrod, etc. In each case, the dial indicator gauge must be rigidly mounted to allow only the gauge plunger to move.
Regardless of the application, the method of adjusting and reading the gauge is the same.
Let’s use checking crankshaft thrust/endplay as an example. In the case of an iron engine block, the dial indicator gauge is mounted to a mounting fixture that has a magnetic base (for an aluminum block, a fixture that bolts to an available threaded hole works). Position the dial indicator so that the plunger contacts the face of the crankshaft snout or other available flat surface. The plunger must be parallel to the crankshaft centerline and not at an angle relative to the crank. Adjust the dial indicator so that the plunger contacts the surface and creates a slight preload (about .050 inch or so). The preload is vital to make sure that the plunger remains in loaded contact with the crank at all times. Using a lever such as a screwdriver, between a main cap and crankshaft counterweight, move the crankshaft rearward as far as it will move and remove the screwdriver. Rotate the gauge dial to allow the gauge needle to read exactly zero. Then use your lever to move the crankshaft fully forward, noting the distance of movement on the gauge. Repeat this reading several times. Move the crank fully forward and re-zero the gauge, then move the crank rearward (the same reading should be seen). Typically, depending on the application, you may see .004 to .008 inch or so of crank fore/aft movement.
The use of a dial indicator is the same for all applications, whether you’re measuring for movement or runout. For example, if you’re checking pushrods for runout (warp), a special pushrod runout stand is ideal. Lay the pushrod onto the stand and position the dial indicator at the center of the pushrod. Adjust the dial indicator with a bit of preload. Slowly rotate the pushrod until the indicator gauge reads maximum or minimum. Then zero the gauge dial and slowly rotate the pushrod, noting how far from zero the needle moves. This represents the amount of runout. Generally speaking, a maximum of .0005 to .001 inch is acceptable.
When checking a brake rotor disc for runout, the dial indicator needs to be mounted rigidly to prevent the gauge housing from moving. Several methods are available including a mount with a magnetic base and adjustable-length rods to position the gauge, or a flexible/locking arm that secures to a solid area such as a strut bracket (the gauge must be mounted to a component that does not move independently, relative to the rotor). Place the dial indicator plunger 90 degrees to the rotor (straight onto the disc surface, not at an angle). The plunger should contact the rotor surface about 1 inch inboard from the disc edge. Adjust the indicator with a bit of preload (about .050 inch), and slowly rotate the rotor until the gauge reads at a
