Automotive Measuring Instruments/Tools

Some service work, such as engine repair, requires very exact measurements, often in ten-thousandths (0.0001) of an inch or thousandths (0.001) of a milli meter. Accurate measurements with this kind of precision can only be made by using precise measuring devices.

Measuring tools are precise and delicate instruments. In fact, the more precise they are, the more delicate they are. They should be handled with great care. Never pry, strike, drop, or force these instruments. They may be permanently damaged.

Precision measuring instruments, especially micrometers, are extremely sensitive to rough handling. Clean them before and after every use. All measuring should be performed on parts that are at room temperature to eliminate the chance of measuring something that has contracted because it was cold or has expanded because it was hot.

GARAGE TIP -- Check measuring instruments regularly against known good equipment to ensure that they are operating properly and are capable of accurate measurement. Always refer to the appropriate material for the correct specifications before performing any service or diagnostic procedures. The close tolerances required for the proper operation of some automotive parts make using the correct specifications and taking

accurate measurements very important. Even the slightest error in measurement can be critical to the durability and operation of an engine and other systems.

--- Graduations on a typical machinist's rule. 1/8-inch scale; 1/16-inch scale; 1/32-inch scale; 1/64-inch scale

Stationary jaw--Inside jaws--Beam with main scale--Metric scale—Depth--Inch scale--Vernier scale Thumb adjuster--Adjustable jaw--Outside jaws

--- A vernier caliper. Central Tools, Inc.

Main scale 01 12345678 91 1.000"; 0.025"; 0.100"

--- Each line of the main scale equals 0.025 inch.

--- To get a final measurement, line up the vernier scale line that is exactly aligned with any line on the main scale.

--- A dial vernier caliper. Central Tools, Inc. Depth Outside Inside

Machinist's Rule

The machinist's rule looks very much like an ordinary ruler. Each edge of this basic measuring tool is divided into increments based on a different scale. As shown, a typical machinist's rule based on the Imperial system of measurement may have scales based on 1/8-, 1/16-, 1/32-, and 1/64-inch intervals. Of course, metric machinist rules are also available. Metric rules are usually divided into 0.5 mm and 1 mm increments.

Some machinist's rules may be based on decimal intervals. These are typically divided into 1/10-, 1/50-, and 1/1,000-inch increments. Decimal machinist's rules are very helpful when measuring dimensions specified in decimals; they make such measurements much easier.

Vernier Caliper

A vernier caliper is a measuring tool that can make inside, outside, or depth measurements. It’s marked in both British Imperial and metric divisions called a vernier scale. A vernier scale consists of a stationary scale and a movable scale, in this case the vernier bar to the vernier plate. The length is read from the vernier scale.

A vernier caliper has a movable scale that is parallel to a fixed scale. These precision measuring instruments are capable of measuring outside and inside diameters and most will even measure depth. Vernier calipers are available in both Imperial and metric scales. The main scale of the caliper is divided into inches; most measure up to 6 inches.

Each inch is divided into 10 parts, each equal to 0.100”. The area between the 0.100 marks is divided into four. Each of these divisions is equal to 0.025”.

The vernier scale has 25 divisions, each one representing 0.001 inch. Measurement readings are taken by combining the main and vernier scales. At all times, only one division line on the main scale will line up with a line on the vernier scale. This is the basis for accurate measurements.

To read the caliper, locate the line on the main scale that lines up with the zero (0) on the vernier scale. If the zero lined up with the 1 on the main scale, the reading would be 0.100 inch. If the zero on the vernier scale does not line up exactly with a line on the main scale, then look for a line on the vernier scale that does line up with a line on the main scale.

Dial Caliper

The dial caliper is an easier-to-use version of the vernier caliper. Imperial calipers commonly measure dimensions from 0 to 6 inches (0 to 150 mm). Metric dial calipers typically measure from 0 to 150 mm in increments of 0.02 mm. The dial caliper features a depth scale, bar scale, dial indicator, inside measurement jaws, and outside measurement jaws.

The main scale of a British Imperial dial caliper is divided into one-tenth (0.1) inch graduations. The dial indicator is divided into one-thousandth (0.001) inch graduations. Therefore, one revolution of the dial indicator needle equals one-tenth inch on the bar scale.

A metric dial caliper is similar in appearance; how ever, the bar scale is divided into 2 mm increments.

Additionally, on a metric dial caliper, one revolution of the dial indicator needle equals 2 mm.

Both English and metric dial calipers use a thumb operated roll knob for fine adjustment. When you use a dial caliper, always move the measuring jaws backward and forward to center the jaws on the object being measured. Make sure the caliper jaws lay flat on or around the object. If the jaws are tilted in any way, you won’t obtain an accurate measurement.

Although dial calipers are precision measuring instruments, they are only accurate to plus or minus two-thousandths (±0.002) of an inch. Micrometers are preferred when extremely precise measurements are desired.

Micrometers

The micrometer is used to measure linear outside and inside dimensions. Both outside and inside micrometers are calibrated and read in the same manner. Measurements on both are taken with the measuring points in contact with the surfaces being measured.

The major components and markings of a micrometer include the frame, anvil, spindle, locknut, sleeve, sleeve numbers, sleeve long line, thimble marks, thimble, and ratchet. Micrometers are calibrated in either inch or metric graduations and are available in a range of sizes. The proper procedure for measuring with an inch-graduated outside micrometer is outlined.

Most micrometers are designed to measure objects with accuracy to 0.001 (one-thousandth) inch.

Micrometers are also available to measure in 0.0001 (ten-thousandths) of an inch. This type of micrometer should be used when the specifications call for this much accuracy. Digital micrometers are also avail able. These eliminate the need to do math and still receive a precise measurement.

Reading a Metric Outside Micrometer:

The metric micrometer is read in the same manner as the inch-graduated micrometer, except the graduations are expressed in the metric system of measurement.

Readings are obtained as follows:

¦ Each number on the sleeve of the micrometer rep resents 5 millimeters (mm) or 0.005 meter (m).

¦ Each of the 10 equal spaces between each number, with index lines alternating above and below the horizontal line, represents 0.5 mm or five tenths of a mm. One revolution of the thimble changes the reading one space on the sleeve scale or 0.5 mm.

¦ The beveled edge of the thimble is divided into 50 equal divisions with every fifth line numbered: 0, 5, 10, . . . 45. Since one complete revolution of the thimble advances the spindle 0.5 mm, each graduation on the thimble is equal to one hundredth of a millimeter.

¦ As with the inch-graduated micrometer, the three separate readings are added together to obtain the total reading.

To measure small objects with an outside micrometer, open the tool and slip the object between the spindle and anvil. While holding the object against the anvil, turn the thimble with your thumb and fore finger until the spindle contacts the object. Use only enough pressure on the thimble to allow the object to just fit between the anvil and spindle. Slip the micrometer back and forth over the object until you feel a very light resistance, while at the same time rocking the tool from side to side to make certain the spindle can not be closed any further. After your final adjustment, lock the micrometer and read the measurement.

Micrometers are available in different sizes. The size is dictated by the smallest to the largest measurement it can make. Examples of these sizes are the 0-to-1-inch, 1-to-2-inch, 2-to-3-inch, and 3-to-4-inch micrometers.

Reading an Inside Micrometer--Inside micrometers are used to measure the inside diameter of a bore or hole. The tool is placed into the bore and extended until each end touches the bore's surface. If the bore is large, it might be necessary to use an extension rod to increase the micrometer's range. These extension rods come in various lengths.

To get a precise measurement, keep the anvil firmly against one side of the bore and rock the micrometer back and forth and side to side. This centers the micrometer in the bore. Make sure there is correct resistance on both ends of the tool before taking a reading.

---A digital micrometer eliminates the need to do math.

--- Major components of (A) an outside and (B) an inside micrometer. Locknut Sleeve Spindle Frame Rod point Ratchet Thimble; Thimble Anvil Insert rod here; Short handle Body Lock screw for rod Anvil (A) (B)

--- Reading a metric micrometer: (A) 10 mm plus (B) 0.5 mm plus (C) 0.01 mm equals 10.51 mm.

===

Using a Micrometer

___5 A micrometer reading of 0.50”.

___3 The graduations on the sleeve each represent 0.025”. To read a measurement on a micrometer, begin by counting the visible lines on the sleeve and multiplying them by 0.025.

___1 Micrometers can be used to measure the diameter of many different objects. By measuring the diameter of a valve stem in two places, the wear of the stem can be determined.

___4 The graduations on the thimble assembly define the area between the lines on the sleeve. The number indicated on the thimble is added to the measurement shown on the sleeve.

___6 A micrometer reading of 0.38”.

___2 Because the diameter of a valve stem is less than 1 inch, a 0-to-1-inch outside micrometer is used.

___11 Some micrometers are able to measure in 0.0001 (ten-thousandths) of an inch. Use this type of micrometer if the specifications call for this much accuracy. Note that the exact diameter of the valve stem is 0.3112 inch.

___9 To prevent the reading from changing while you move the micrometer away from the stem, use your thumb to activate the lock lever.

___7 Normally, little stem wear is evident directly below the keeper grooves. To measure the diameter of the stem at that point, close the micrometer around the stem.

___12 Most valve stem wear occurs above the valve head. The diameter here should also be measured. The difference between the diameter of the valve stem just below the keepers and just above the valve head represents the amount of valve stem wear.

___8 To get an accurate reading, slowly close the micrometer until a slight drag is felt while passing the valve in and out of the micrometer.

___10 This reading (0.311”) represents the diameter of the valve stem at the top of the wear area.

===

--- The total reading on this micrometer is 6.13mm.

--- Slip the micrometer over the object and rock it from side to side. Slip back and forth over object; Rock from side to side

--- A depth micrometer. Central Tools, Inc.

Reading a Depth Micrometer A depth micrometer is used to measure the distance between two parallel surfaces. It operates and is read in the same way as other micrometers. If a depth micrometer is used with a gauge bar, it’s important to keep both the bar and the micrometer from rocking. Any movement of either part will result in an inaccurate measurement.

GARAGE TIP -- Measurements with any micrometer will be reliable only if the micrometer is calibrated correctly. To calibrate a micrometer, close the micrometer over a micrometer standard. If the reading differs from that of the standard, the micrometer should be adjusted according to the instructions provided by the tool manufacturer.

Proper care of a micrometer is also important to ensure accurate measurements. This care includes:

¦ Always clean the micrometer before using it.

¦ Don’t touch the measuring surfaces.

¦ Store the tool properly. The spindle face should not touch the anvil face; a change in temperature might spring the micrometer.

¦ Clean the micrometer after use. Wipe it clean of any oil, dirt, or dust using a lint-free cloth.

¦ Never use the tool as a clamp or tighten the jaws too tightly around an object.

¦ Don’t drop the tool.

¦ Check the calibration weekly.

--- Parts of a telescoping gauge. Plungers Handle Lock screw

Telescoping Gauge

Telescoping gauges are used for measuring bore diameters and other clearances. They may also be called snap gauges. They are available in sizes ranging from fractions of an inch through 6 inches (150 mm). Each gauge consists of two telescoping plungers, a handle, and a lock screw. Snap gauges are normally used with an outside micrometer.

To use the telescoping gauge, insert it into the bore and loosen the lock screw. This will allow the plungers to snap against the bore. Once the plungers have expanded, tighten the lock screw. Then, remove the gauge and measure the expanse with a micrometer.

Small Hole Gauge

A small hole or ball gauge works just like a telescoping gauge. However, it’s designed to be used on small bores. After it’s placed into the bore and expanded, it’s removed and measured with a micrometer. Like the telescoping gauge, the small hole gauge consists of a lock, a handle, and an expanding end. The end expands or retracts by turning the gauge handle.

Feeler Gauge

A feeler gauge is a thin strip of metal or plastic of known and closely controlled thickness. Several of these metal strips are often assembled together as a feeler gauge set that looks like a pocket knife. The desired thickness gauge can be pivoted away from others for convenient use. A steel feeler gauge pack usually contains strips or leaves of 0.002- to 0.010-inch thickness (in steps of 0.001 inch) and leaves of 0.012- to 0.024-inch thickness (in steps of 0.002 inch). Metric feeler gauges are also available.

A feeler gauge can be used by itself to measure piston ring side clearance, piston ring end gap, connecting rod side clearance, crankshaft end play, and other distances.

Round wire feeler gauges are often used to mea sure spark plug gap. The round gauges are designed to give a better feel for the fit of the gauge in the gap.

Straightedge

A straightedge is no more than a flat bar machined to be totally flat and straight, and to be effective it must be flat and straight. Any surface that should be flat can be checked with a straightedge and feeler gauge set.

The straightedge is placed across and at angles on the surface. At any low points on the surface, a feeler gauge can be placed between the straightedge and the surface. The size gauge that fills in the gap indicates the amount of warpage or distortion.

--- Insert the ball gauge into the bore to be measured. Then expand it, lock it, and remove it. Now measure it with an outside micrometer. Small hole gauge; Outside micrometer; Hole gauge method to measure valve guide wear; Small hole gauge Valve guide; Take measurements in three locations.

--- Typical feeler gauge set. Straightedge Feeler gauge; Deck surface

--- Using a feeler gauge and precision straightedge to check for warpage.

--- A dial indicator with a highly adaptive holding fixture.

Dial Indicator

The dial indicator is calibrated in 0.001-inch (one-thousandth inch) increments. Metric dial indicators are also available. Both types are used to measure movement. Common uses of the dial indicator include measuring valve lift, journal concentricity, flywheel or brake rotor runout, gear backlash, and crankshaft end play. Dial indicators are available with various face markings and measurement ranges to accommodate many measuring tasks.

To use a dial indicator, position the indicator rod against the object to be measured. Then, push the indicator toward the work until the indicator needle travels far enough around the gauge face to permit movement to be read in either direction. Zero the indicator needle on the gauge.

Always be sure the range of the dial indicator is sufficient to allow the amount of movement required by the measuring procedure. For example, never use a 1-inch indicator on a component that will move 2 inches.

GARAGE TIP -- Metric and SAE wrenches are not interchangeable. For example, a 9/16-inch wrench is 0.02” larger than a 14-millimeter nut. If the 9/16-inch wrench is used to turn or hold a 14-millimeter nut, the wrench will probably slip. This may cause rounding of the points of the nut and possibly skinned knuckles as well.

--- A technician needs many different sets of wrenches. e.g., made by Snap-on Incorporated.

Next: Hand Tools for Automotive Use

Prev.: Automotive Fasteners