left over from use can easily contaminate not only the engine that you’re working on, but any other engines or equipment in the same room. Whether or not you use Scotch-Brite pads during initial cleaning, never have them anywhere near the engine during assembly.
A Roloc bristle head operated with a pneumatic die grinder can be used to remove deposits from various external surfaces.
Precision measurement tools are a necessity for both machining and assembly procedures. In this chapter I discuss common precision measurement tools and systems that are used on a routine basis in any engine machining shop.
A micrometer is a measurement tool available in various formats, including an outside micrometer, used to measure thickness, length, or diameter; an inside micrometer, used to measure inside diameter of a hole; and depth micrometers, used to measure hole depth.
An outside micrometer features a C-clamp style frame. At the far end is a flat-faced anvil. Facing the anvil at the opposite end of the frame is a flat-faced spindle. The anvil is in a fixed position on the frame. The spindle moves as the micrometer is adjusted. The sleeve features incremented index marks and is stationary. The thimble at the far end of the grip rotates and features additional marks. A lock lever is provided to secure the adjustment in a locked position when required.
An inch-format micrometer features an internal screw drive that features 40 threads per inch. One complete revolution of the screw moves the thread 1/40, which is equal to .025 inch. Each mark on the sleeve represents .025 inch. The beveled face of the thimble has 25 equally spaced lines that each represents .001 inch.
The primary components of an outside micrometer are labeled here, including the frame, stationary anvil, sliding spindle, stationary index marked sleeve, and rotating thimble. Micrometers feature a lock that allows you to lock a measurement for reference. This micrometer features a toggle lever lock. A micrometer set typically includes ranges of 0 to 1 inch, 1 to 2 inches, 2 to 3 inches, and 3 to 4 inches. Also included are three calibration standards and a spanner wrench for adjusting micrometer calibration.
When reading a micrometer, first note the larger lines and numbers on the sleeve, and then look at the smaller lines between the numbered lines. As an example, if the number 3 is visible on the sleeve, that means that you are measuring at least .300 inch. Let’s say that three smaller lines after the number 3 are also visible. You now know that the measurement is at least .375 inch (remember that each line indicates .025 inch, so seeing three of these lines indicates .075 inch).
Next, note the lines on the moving thimble to see which thimble line matches up to the last visible line on the sleeve. As an example, let’s say that the thimble line is identified as number 8. Because each thimble line indicates a space of .001 inch, that means that you have an additional .008 inch. Because the previously noted measurement was .375 inch, when you add the spindle sleeve mark of 8, the final measurement is .383 inch.
This sample reading shows the number-2 line flush with the chamfered edge of the thimble, with the zero line of the thimble aligned with the horizontal sleeve line, indicating exactly .200 inch.
A first-time user of a micrometer will likely be intimidated, but with a bit of practice, reading a micrometer becomes much easier.
When measuring a part with a micrometer, adhere to one rule: go slowly. Avoid spinning or twirling the thimble. Work slowly and carefully. Adjust the micrometer to almost the desired opening by rolling the thimble along your hand; avoid twirling it. Place the micrometer onto the part, holding it firmly with one hand. Use your sense of feel to make sure that the axis of the micrometer is perpendicular to the measured surface. Don’t rock the micrometer; use your sense of feel. Close the micrometer using the ratchet knob to close the micrometer until the spindle is almost touching the part. Then gently close the micrometer spindle 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
