LO 5-3 Identify and describe common instruments and techniques to make linear precision measurements

1. Linear Measurement

Linear Measurement

Nearly all measurements common in shop practices involve measurements of length, which is the straight-line distance between two points. Because the measurement is a straight line, these measurements are more accurately called linear measurements . Linear measurements are so numerous and of such importance that a multitude of measuring tools are available for the purpose of obtaining them. Non-precision linear measuring tools include rulers and tape measures. For engine-related service work, commonly used precision linear measuring tools include:

• Dial- and digital-type sliding calipers and vernier calipers
• Micrometers—inside, outside, and depth type
• Telescoping gauges
• Thickness gauges (more commonly called feeler blades )
• Gauge blocks

1.1. Video: Precision Measuring Tools

1.2. Gauges Versus Measuring Tools

Gauges Versus Measuring Tools Before examining the construction, operation, and applications for the above-listed tools, it is helpful to distinguish between a gauge and a measuring tool. Measuring tools determine actual dimensional sizes, physical quantities, and distances or volume, and the results are reported numerically FIGURE 3. For example, the thickness of textbook paper is 0.1 mm or 0.0039". Gauges, however, compare a dimension to a designated standard. For example, when determining whether a cap screw for a cylinder head is deformed by stretching, the length of the cap screw could be measured with an accurate ruler. The measured length is then compared with specifications to check its serviceability. Alternatively, a gauge comparing acceptable limits for the cylinder cap screw length is also used. The gauge would not produce a measurable quantity for length, but in this case, a gauge can more quickly determine whether the bolt is deformed and stretched beyond acceptable limits FIGURE 4.   FIGURE 05-4: An example of a gauge to measure head bolt length. This go-no-go gauge is used to determine whether a cap screw is excessively distorted or reusable.

FIGURE 05-3: This tool measures valve spring length and spring tension. Results are reported numerically on a dial (pressure or weight at specific spring length) or on a graduated scale with markings indicating length.

1.3. Sliding Calipers

Sliding Calipers

After simple rulers, various types of calipers are the frequently used measuring tools to make linear measurements of outside, inside, and depth dimensions. Sliding-type calipers range in size from 6–12" in length (15–30 cm) and have markings on graduated scales with either SI or USCS scales. The most common type of caliper that engine technicians use is a sliding caliper, which has two pairs of jaws attached to a frame or bar resembling a steel ruler FIGURE 5. While calipers can measure either inside or outside dimensions, the most versatile have a pair of jaws to measure both inside and outside dimensions, and they have a depth bar attached to the movable section that extends from the end of the caliper bar FIGURE 6. Jaw tips have knifelike edges shaped to make inside or internal measurements more precise on threaded fasteners or round objects. Only one each of the pairs of jaws is movable and slides along the graduated scale marked or stamped into the bar. The opposite jaw is fixed and does not move. A knurled roller, operated with a thumb, moves the sliding jaws along the bar. This allows the caliper to be operated with only one hand. A locking screw on the sliding jaw also allows the operator to lock the movable jaw after taking a measurement.

FIGURE 05-5: A sliding vernier-type caliper with a dial indicator instead of a vernier scale.

FIGURE 05-6: Note the sliding caliper measurement methods for determining inside, outside, and depth dimensions.

To use a sliding caliper, the movable jaw is adjusted to allow its tips to be placed inside or over the work piece FIGURE 7. The thumb roller is turned until the moveable jaw contacts the work with moderate force. The locking screw is tightened before removing the caliper from the work piece to read the measurement. A similar procedure is used to take a depth measurement. If sliding calipers are used to take depth measurements, the bar rests squarely on the upper part of the work surface, such as a ledge around a hole. The depth bar is lowered until it contacts the lower work surface requiring measurement and the adjustment is locked FIGURE 8.

FIGURE 05-7: Using the tips of the sliding caliper jaws to measure inside and outside dimensions.

FIGURE 05-8: Using a sliding caliper to measure the protrusion of a replaceable valve guide from the machined surface of the cylinder head.

To read the dimension, technicians must first carefully note which main scale on the caliper bar is used to measure inside, outside, and depth dimensions. The bar may have the letters “IN” and “OUT” on the upper or lower side of the main scales stamped into the caliper bar or use two separate pointers on the movable jaw for each main scale. The edge of the movable jaw or other marker indicates a dimension on the main scale of the ruler-like bar.

1.4. Video: Precision Measuring Tools: Vernier Calipers

1.5. Anim: Verniers

1.6. Vernier Calipers

Vernier Calipers Vernier calipers are a type of sliding caliper that make more precise measurements than a simple sliding caliper with a single main graduated scale. Like a simple caliper, vernier calipers are also constructed with a fixed, movable jaw and a bar (also called a beam or frame ) displaying the main graduated measuring scale. What is unique to a vernier caliper is the addition of a second graduated scale positioned on the moving jaw frame. Finer graduations or markings on the second scale, called the vernier scale , are used to calculate work piece dimensions by combining its smaller, higher-resolution readings with main scale graduations FIGURE 9. Measuring devices with higher resolution means a smaller, more accurate measurement is made having more significant digits or numbers. Vernier scales are also on micrometers and other measuring tools to enable a device to achieve extra decimal points in its measurement. To do this, divisions or markings on the graduated vernier scale are made at intervals of one or two divisions shorter than main scale divisions. This means the vernier scale will provide tenths or hundredths of a measuring unit for additional resolution. If the caliper measures in inches, the main scale divisions are typically tenths of inches, but finer resolution vernier calipers can have 20 divisions for each inch FIGURE 10. Metric scales use 1 mm divisions on the main scale with 0.5 or smaller divisions on the vernier scale FIGURE 11. When the movable jaw is adjusted while taking an inside, outside, or depth measurement, the vernier scale moves with the moveable jaw. This means that when the caliper jaws are moved, a graduation line on the vernier scale will align with a single line on the main scale but all other lines are slightly misaligned. To then calculate a measurement, the main scale graduation is added to the vernier scale graduation marking that aligns with the main scale FIGURE 12.

FIGURE 05-9: On short sliding calipers, the vernier scale is made shorter than one main scale division. Longer calipers use two main scale divisions to equal one vernier scale division.

FIGURE 05-10: A sliding-type vernier caliper with vernier scales in 1/10". The main scale reading of 0.6" is added to the vernier scale of 0.05" to sum to 0.65" diameter.

FIGURE 05-11: The reading of 16.15 mm measurement is read by adding the mark at line for the main scale graduation to the 0.15 mm mark where the vernier and main scale graduations align.

FIGURE 05-12: The vernier scale is a device that is used for gaining an extra decimal point in the accuracy of a measurement.

When the vernier scale on the caliper frame is replaced by a dial indicator or digital display, technically it is no longer a vernier caliper because it does not use a vernier-type scale. When a dial is used, rather than adding a main scale reading to a vernier scale, the main scale reading is instead added to the measurement on the dial or dial indicator FIGURE 13. To measure the dimensions of the work piece, technicians must: Locate on the vernier scale where the zero position aligns with the main scale (11.9 in the illustration) plus an unknown distance measured by the vernier. Identify the line on the vernier scale that matches most closely to a line on the main scale. In this illustration, the main and vernier scales align at the number 0.03, which has divisions measured in hundredths. Add the two dimensions together. In this case 11.9 + 0.03 = 12.12 units.

FIGURE 05-13: The reading of 16.13 mm is read by adding the main scale graduation mark to the reading on the dial scale.

1.7. Micrometers

Micrometers

To make even smaller, more accurate measurements than a vernier-type sliding caliper, a micrometer can be used. Like sliding calipers, micrometers can measure inside, outside, or depth FIGURE 14. This means that while the construction of external-, internal-, and depth-type micrometers changes considerably, the basic measuring mechanism remains the same. A standard mechanical-type micrometer has a scale capable of measuring linear distances with a resolution to the nearest one-thousandth of an inch or millimeters, which is written as 0.001" (0.002 mm) or smaller. The displays on many easier-to-use digital electronic micrometers, ranging up to 4" (100 mm), can provide resolution to 0.00005", with twice the accuracy of mechanical micrometers (according to manufacturers). Technicians should know how to write and express these numbers:

• One-thousandth of an inch or mm is expressed as 0.001
• One hundredth of either measure is 0.01
• One tenth is 0.1
• One ten thousandth is 0.00001
• Half a thousandth is 0.00005

FIGURE 05-14: Micrometers are used to make linear measurements of inside, outside, and depth dimensions.

FIGURE 05-15: Common terms used for the parts of an outside-type micrometer.

Micrometer Construction

A frequently used mechanical micrometer is an outside micrometer, which can measure the diameter of round objects, such as a valve stem or crankshaft journal, and the thickness of flat objects like shims FIGURE 15.

Construction of the outside micrometer consists of the following parts:

• The C-shaped frame, which is either forged, cast, or machined steel. A large open hole or multiple holes in the frame help minimize measurement error due to heat-related expansion of the frame.
• The spindle, which is the round, precision-machined critical component of the tool that moves toward the work piece when the thimble is turned. On the spindle are precision threads cut externally into the hardened shaft. The thread pitch, or the number of threads per inch on the micrometer, are the key to determining how far the spindle moves per rotation of the thimble. This distance is different for metric and USCM micrometers. A thread pitch of 40 threads to the inch will move the spindle one-fortieth (or 0.025) of an inch with each turn. A metric micrometer has a thread pitch 0.5 mm between each thread, which advances the spindle 0.5 mm per spindle rotation.
• The anvil, which is pressed into the frame opposite the spindle and serves as a stop for the spindle. It is usually adjustable to enable “zeroing,” or calibrating the micrometer.
• The sleeve, which is also called a barrel , is fixed to the frame and has a graduated scale to make linear measurements. Inside the sleeve is a nut with internal threads that engage the spindle threads.
• The thimble, which is the rotating component that is turned with the thumb. The thimble has graduated markings that are used with markings on the sleeve to measure an object.
• The ratchet-type torque-limiting stop, which is optional, limits the amount of torque applied to the thimble to prevent over-squeezing of an object in the spindle. Typically, the torque limiter will slip and not turn the thimble further when a calibrated torque is reached.
• A locking mechanism, which, after the object is measured, a lever or lock nut is tightened to hold the spindle measurement to allow it to be viewed more easily.

Microcontroller-based electronic tools are available that enable the transmission and storing of measurement data through a wireless data collection system.

Micrometer Range
A popular mechanical micrometer used by engine technicians measures a range up to 4" or approximately 100 mm; however, the selection of micrometer frame size is made so that the travel between the spindle and the anvil is no more than one inch or ∼25 mm. This travel distance or movement is called the micrometer range . For example, the range of a 1" micrometer is from 0–1". A 2" micrometer has a range from 1–2". Extensions can be added to the spindles of larger micrometer frames when measuring depth, inside, or outside dimensions.

To take measurements, the work is held between the spindle and the anvil with the left hand while the thimble is turned with the thumb and index finger of the right hand. A torque-limiting ratchet mechanism prevents distortion of measurements caused by over-tightening of the spindle. Before reading the final linear measurement, the micrometer adjustment is locked in place.

1.8. Video: Precision Measuring Tools: Micrometers

1.9. Video: Using an Outside Micrometer

1.10. Reading Micrometers Graduated in Inches

Reading Micrometers Graduated in Inches

Objects and work pieces measurable by a sliding caliper are measured more accurately using the precision screw adjustment in a micrometer. Micrometers, measuring in inches, operate with a spindle thread pitch of 40 threads per inch and move the spindle 1/40 of an inch, or 0.025", with each turn. A micrometer measuring in inches will require turning the thimble 40 times to move an inch. In contrast, metric micrometers have threads spaced 0.5 mm apart for each rotation. This means it will take 50 turns to move the spindle a normal distance of 25 mm.

Two graduated scales and a vernier scale are used to calculate linear measurements of the most precise micrometers. The first scale is the reading line, also called the index line , on the micrometer barrel or sleeve that has 40 equal divisions designated by vertical lines that correspond to the number of threads on the spindle. This means each vertical line division represents 1/40 or 0.025 of an inch of spindle travel and every longer fourth line designates tenths of an inch, also expressed as hundreds of thousandths. For example, the major line marked “1” represents 0.100", the next major line marked “2” represents 0.200", etc. Smaller lines (between the major lines) represent 0.01", 0.02", etc. On the rotating thimble is the second graduated scale. Its bevelled edge has 25 divisions, with each consecutive graduation line designating 0.001" movement of the spindle. Each mark indicates spindle movement equal to 1/25 of 0.025". One complete rotation represents 0.025 of an inch of spindle movement. A vernier-like scale is used as an option on micrometers providing finer resolution.

To read a non-vernier micrometer in thousandths, first note the largest major division uncovered by the thimble on the index or reading line. Next, multiply the number of smaller vertical divisions observed after the larger major division visible on the sleeve by 0.025". Add these previous two numbers to the smaller line graduations of thousandths of an inch shown by the thimble on the sleeve. The sleeves index line will align with the thimble’s divisions FIGURE 16. The “2” line on the sleeve is visible, representing 0.200" (hundreds of thousandths or one-tenth). The number of lines between “2” and the thimble edge, each representing 0.025", is 1 × 0.025" = 0.025". The number of lines on the thimble with each line representing 0.001" is 16 × 0.001" = 0.016"; therefore, the micrometer reading is 0.241". An alternate method of calculating dimension is to think of the various measuring units as change returned after making a purchase with a large-denomination bill. Count the numbered, large measuring units on the sleeve as dollars, the smaller vertical lines on the sleeve as quarters, and the divisions on the thimble as pennies. Add up the change and place a decimal point instead of a dollar symbol in front of the number. In the above number, the change is 2 dollars and 41 cents. Examine the measurements in FIGURE 17 to further practice calculating measurements.

FIGURE 05-16: Steps to calculate a precision measurement of an outside micrometer measuring in inches.

FIGURE 05-17: Examples of micrometer readings and calculation of measurement in inches.

FIGURE 05-18: How to read a sliding-type vernier micrometer.

A vernier-type micrometer uses a third scale around the sleeve providing 1/10,000" (0.0001") resolution. A typical vernier inch scale has 11 parallel lines that are the same length as 10 graduation lines on the thimble. This means the difference between the lines on the sleeve and the thimble is one-tenth of a thousandth of an inch. To read the vernier scale, first lock the micrometer after measuring the object and remove the micrometer from the work piece and observe the thousandths reading. Next, determine which of the lines on the vernier scale align with a line on the thimble. To calculate the reading, examine FIGURE 18.

1.11. Anim: STD Micrometer

1.12. Reading Micrometers Graduated in Millimeters

Reading Micrometers Graduated in Millimeters

Because the distance between threads of the metric spindle screw is one-half millimeter (0.5 mm), one complete rotation of the thimble moves the spindle 0.5 mm distance. The reading line on the sleeve is graduated in divisions of 1.0 mm. This means two rotations of the thimble are required to move the spindle 1.0 mm. Every fifth millimeter graduation on the reading or index line is a major division numbered from 0–25. Smaller divisions between each major are each typically 0.025 mm.

The bevelled edge of the thimble is graduated in 50 divisions, with every fifth line numbered as a major division from 0–50. Because one revolution of the thimble moves the spindle 0.5 mm, each thimble graduation equals 1/50 of 0.5 mm or 0.01 mm.

To take a measurement, the numbers on the sleeve and the thimble are added together. This means first observing the number of millimeters and half millimeters uncovered by the sleeve. Next, add the sleeve numbers to the number of hundredths of a millimeter represented by the thimble graduations (which align with the measuring line on the sleeve). See FIGURE 19 to calculate this measurement.

FIGURE 05-19: How to read and calculate a linear measurement made on a metric-type micrometer.

FIGURE 05-20: An outside micrometer is used to make precise measurements of many precision-machined engine components, such as the diameter of this piston.

When a vernier scale is added to a micrometer, an additional fourth significant digit can be added to the measurement’s resolution. The vernier scale has a range of 0.01 mm and is graduated in 10 increments, each of which represents 0.001 mm vernier scale divisions, and are typically numbered every 0.002 mm. To calculate this number, FIGURE 20 shows several micrometers where the number on the thimble lines up with a number on the vernier. Essentially, the numbers on the sleeve are added to the thimble division numbers, which are in turn added to the line designated by the vernier scale aligning with the thimble.

Inside and Depth Micrometers
The inside micrometer is read the same way as an outside micrometer. A depth micrometer, however, reverses the order of the numbers on the sleeve. As the spindle extends from the micrometer, the numbers uncovered by the thimble will increase.

Adjusting and Caring for Micrometers
Looseness or free play in a micrometer spindle is corrected by tightening an adjusting nut inside the barrel or sleeve. A small purpose-made wrench, supplied with the micrometer, is placed around the nut located at the end of the sleeve. The nut is tightened only enough to remove looseness in threads between the nut and the spindle. To adjust the zero reading, the micrometer measuring surfaces are cleaned, and the micrometer is gently closed with the ratchet torque-limiting stop, if equipped. A screwdriver is used to turn either the anvil or sometimes the sleeve until its zero-graduation line aligns with the zero line on the thimble. Precision instrument oil can be applied to the spindle threads and the spindle to help maintain free-running performance. Micrometers should be wiped clean and stored in a case when not in use.

1.13. Anim: Metric Micrometer

1.14. Telescoping Gauges

Telescoping Gauges Telescoping gauges are linear measuring devices that can measure holes or internal diameters. These devices are used with micrometers, which make the final numerical measurement. The head of a T-shaped telescoping gauge is spring loaded and locked by its handle to hold it in its most compact position. When unlocked with finger force, spring force expands the heads across the hole. The gauge is rocked slightly while slowly tightening the handle to ensure that the maximum dimension is obtained. Once the measurement is made, the gauge locks and the dimension is transferred for measurement by a micrometer with the correct range. Telescoping gauges can also be set and locked to a specific size inside an external micrometer or other gauge to validate whether a hole or internal diameter has the correct dimensions. Telescoping gauges vary in size, with a set often packaged together measuring from an inch to 6" in diameter FIGURE 21.

FIGURE 05-21: The handle of the telescoping gauge is slowly rocked while tightening the handle on the lock. The dimension measured by the gauge is recorded by a micrometer.

1.15. Video: Precision Measuring Tools: Telescoping Gauges

1.16. Thickness—Feeler Gauges

Thickness—Feeler Gauges

Feeler gauges, commonly called thickness gauges , are linear measuring tools used to precisely measure the dimensions of small gaps or between two parts FIGURE 22. Feeler gauges are some of the most essential measuring tools owned by technicians. Several types are in most technicians’ toolboxes and are used for a variety of tasks, such as adjusting valve clearances, measuring piston ring gap clearances, determining geartrain backlash or turbocharger bearing clearances, or even adjusting sensor air gaps FIGURE 23.

FIGURE 05-22: Feeler gauges are available in a variety of lengths and thicknesses and may be bent at an angle.

FIGURE 05-23: Measuring a piston ring’s end gap with a feeler blade.

Feeler gauge blades are available in various lengths and may be flat or bent at an angle to enable them to be more easily used for a specific purpose. Metric and imperial measurement thicknesses are available in a stack of blades, and the thickness of each gauge is stamped into the base of the blade. A slight drag of a feeler blade indicates that the measured dimension is identical to the blade thickness. A spring scale can be hooked to the blade, in some circumstances, to verify whether the amount of drag is correct, since the perception of the correct amount of blade drag, or “feel,” can vary between technicians. Thickness blades may need to be put together to measure a large or unusual gap dimension. To understand the use of a set of thickness gauges, a common application is described in SKILL DRILL 5-1.

1.17. Video: Precision Measuring Tools: Feeler Gauges

1.18. Video: Using a Feeler Gauge

1.19. SKILL DRILL 05-1  Verifying Valve Clearances

To verify that valve clearances of an engine are within specifications, use the following guidelines: STEP 1 Rotate the engine to a position where the valve is fully closed. STEP 2 Select a feeler blade that is the same thickness as the recommended clearance. STEP 3 Insert the feeler blade squarely into the clearance between the rocker lever and the top of the valve stem. Do not force the blade into a smaller clearance than the blade thickness. Instead, select a thinner blade to measure the clearance. STEP 4 When the blade is inserted into the clearance at a right angle to the valve stem, pull the blade from between the valve stem and the rocker lever while noting the amount of drag. STEP 5 The drag encountered by the feeler blade having the same thickness as the valve clearance should correspond closely to the resistance of pulling the feeler blade from between the middle pages of a thick textbook. STEP 6 Observe and record your results. Analyze how close the valve clearances are to specifications.

1.20. Gauge Blocks

Gauge Blocks
Gauge blocks are precision-machined pieces of aluminum or steel used to determine whether a dimension’s thickness or width is within specification FIGURE 24. For example, injector height, on some injectors, is adjusted with gauge blocks to obtain a safe plunger travel distance FIGURE 25. The thickness or width of the gauge block measures small distances to adjust the clearances of components or to check for excessive wear between parts that are under 6" (150 mm). They can also be used to check the size of a piston ring groove, check the accuracy of a micrometer, or calibrate a dial bore gauge. Gauge block dimensions are usually stamped on the gauge as either a metric or an imperial measurement.

FIGURE 05-24: Notches in the end of the camshafts are aligned to be parallel using this gauge block.

FIGURE 05-25: A gauge block used to adjust the height of an electronic unit injector (EUI).

1.21. Dial Indicators

Dial Indicators One of the most versatile precision measuring tools used by technicians is a dial indicator FIGURE 26. Dial indicators are incorporated into a variety of precision measuring tools. Its measurement speed, ease of use, and accuracy make it a popular choice for precision measurement. Digital electronic- and mechanical-type indicators are both available. A mechanical-type dial indicator converts linear movement of a spring-loaded plunger in contact with an object into rotary movement of a pointer against the background of a dial gauge. The plunger of a quality gauge has a contact tip that is typically threaded into the plunger to enable interchangeability with tips of any length or shape that are better suited to gauge the object under measurement. For example, a sharp needle-like plunger tip may be used to make contact between surface threads while a larger, round roller or mushroom-shaped tip is better used to glide over a shaft surface to prevent it from marking its finish or detecting tiny but irrelevant surface imperfections.

FIGURE 05-26: Parts of a dial indicator.

Construction and Use of Dial Indicators
Inside the dial indicator, the plunger moves a toothed rack that transmits motion to a pinion gear and then through a series of gears that eventually turn the pointer around the dial face. This gear arrangement magnifies the smallest movement of the plunger to rotate the dial pointer around the dial face. Common graduations of the dial face are available for reading in inches, with increments from 0.001", 0.0005", 0.00025", and 0.0001". For metric, graduations are available in 0.01 mm, 0.002 mm, and 0.001 mm increments. To prevent errors between metric and inch measurements, dial faces are commonly painted yellow to measure metric dimensions while inch measurements are white.

FIGURE 05-27: Most dial indicators are balanced, which means there are two sets of numbers or + and – indicators on either side of the zero graduation to indicate the direction of plunger travel.

Dial indicators can be constructed with a second, smaller pointer on the dial face. In what is called a balanced gauge , the second, smaller pointer marks the number of revolutions for each rotation of the larger dial pointer FIGURE 27. Positive and negative symbols are on either side of the zero graduations to indicate the direction of plunger movement. This helps prevent measurement error and extends the plunger’s travel distance. A second type of dial indicator, called a continuous reading gauge , uses a revolution counter for the second, smaller dial. A revolution counter requires multiplying the distance of the total dial sweep times the number of rotations, plus adding the distance of the last incomplete rotation. The continuous-type measurement extends the measuring distance even further than a balanced gauge but may not have the same precision. Dials may be built with a rubber dust guard around the dial to seal out dust and dirt but generally use a metal outer ring, called a bezel , with two small markers and a method to turn the dial face to the zero graduation mark. Turning the bezel aligns the pointer position. These features help technicians accurately measure pointer travel when the gauge is moved around to measure a variety of objects. Magnetic bases are used to locate a dial indicator on a work piece. A half-turn-type knob that changes the strength of the magnetic field enables the magnetic base to securely clamp onto or release from a steel alloy or iron work piece.

1.22. Video: Precision Measuring Tools: Dial Gauges

1.23. Video: Using a Dial Indicator