Automotive Machining. Mike Mavrigian
until the ratchet stop disengages by one click.
Always store a micrometer in a protective case when not in use. These are delicate, precision instruments and require care when handling or storing.
“Digital” micrometers are also available; they feature an easy-to-read measurement number in a display window, eliminating the need to read the traditional mark lines. However, it is absolutely critical that either style must be routinely kept in calibration.
This reading shows one mark on the sleeve greater than the number 2, which translates into .225 inch. The number 2 indicates .200 inch, with one additional line visible at the edge of the thimble. Because each line on the sleeve indicates .025 inch, the measurement shown here is .225 inch.
This digital readout micrometer must be properly calibrated. The reading is displayed in the viewing window. The mic can also be read referencing the marks on the sleeve and thimble. Close-up of a digital mic window. The lock on this model is a knurled rotating knob instead of a toggle lever.
Measuring piston skirt diameters requires having outside micrometers in the range of the pistons that you deal with. When measuring a piston skirt diameter, always measure at the skirt location specified by the piston maker, since a slight taper may exist from the ring area to the skirt bottom.
Calibrating a Micrometer
A micrometer should be checked regularly for calibration because calibration can be affected by wear or damage (dropping it, etc.). To check calibration, first verify that the anvil and spindle surfaces are clean. For a zero-to-1-inch micrometer, insert a clean sheet of paper between the anvil and spindle and close the gap to capture the paper, then pull the paper out. Carefully clean the surfaces but avoid leaving lint on the surfaces. Next, you need a “standard” (also called a checking block), which is a length of steel that has been precision ground to an exact length. Standards are readily available, and many micrometer kits include a set of standards in various lengths. Both the micrometer and standard must be absolutely clean and both must be at room temperature. Using a checking standard, capture the standard between the anvil and spindle. For example, if using a 1-inch standard, and the reading is not exactly 1-inch, the gauge must be recalibrated. Insert the standard onto the micrometer and tighten the micrometer to “feel” (don’t overtighten with force). Make sure that the micrometer’s anvil and spindle are mated squarely onto the standard (not cocked). Engage the micrometer’s lock to prevent it from moving. Hold the knurled thimble with one hand. Insert the spanner wrench (included with the micrometer) into the small hole in the micrometer sleeve. 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