Precision Measurement, Tools & Fasteners
Key Takeaways
- A standard metric micrometer thimble has 50 divisions, each representing 0.01 mm, and a full turn of the thimble moves the spindle 0.5 mm.
- A dial indicator must be preloaded by 1.27 to 2.54 mm (0.050 to 0.100 inches) to ensure it measures both positive and negative runout.
- Torque-to-yield (TTY) cylinder head bolts stretch permanently beyond their elastic limit when tightened to specification and must never be reused.
- Metric bolt grade markings represent tensile strength (e.g., Class 10.9 indicates 1000 MPa tensile strength), whereas Imperial grades are determined by adding 2 to the number of radial slash marks on the bolt head.
- Thread chasers are designed to reshape and clean damaged threads without removing metal, whereas taps and dies cut new threads and can weaken existing fasteners.
Precision Measurement, Tools & Fasteners
Modern automotive repair requires tolerances measured in thousandths of a millimeter or ten-thousandths of an inch. Technicians must be proficient in operating precision measuring instruments, using torque tools correctly, and identifying and repairing structural fasteners. Accurate diagnostic measurement prevents catastrophic component failures in engines, drivetrains, and chassis systems.
Precision Measuring Instruments
Outside, Inside, and Depth Micrometers
The micrometer is the standard tool for measuring component dimensions such as crankshaft journals, cylinder bores, and valve guide wear.
- Metric Micrometer Reading: A standard metric micrometer has a resolution of 0.01 mm. The sleeve (barrel) features a horizontal index line. Graduations above the index line represent whole millimeters (1.0 mm), while graduations below represent half-millimeters (0.5 mm). The rotating thimble has 50 divisions around its circumference. Since one complete rotation of the thimble moves the spindle exactly 0.5 mm, each individual thimble division represents 0.01 mm (0.5 mm / 50 = 0.01 mm). To read, sum the visible sleeve millimeter markings, the half-millimeter marking (if visible), and the thimble graduation aligned with the sleeve index line.
- Imperial Micrometer Reading: A standard imperial micrometer has a resolution of 0.001 inches. The sleeve has 40 graduations per inch. Since 1.0 inch divided by 40 is 0.025 inches, each tick mark represents 0.025 inches. Numerical markings are placed every four lines (0.100 inches). The thimble features 25 divisions. A complete rotation of the thimble moves the spindle 0.025 inches, meaning each thimble line represents 0.001 inches.
- Calibration and Use: Micrometers must be calibrated regularly using a precision gauge block or standard rod. When taking measurements, always use the ratchet stop or friction thimble at the end of the thimble. This prevents over-tightening the spindle against the component, which would deflect the tool frame and yield a false small reading.
Dial Indicators
Dial indicators measure movement or runout in components, such as brake rotor lateral runout, crankshaft endplay, steering knuckle play, and differential ring gear backlash. The tool features a spring-loaded plunger connected to a needle on a circular dial.
- Preloading: To ensure accurate measurements, the technician must mount the dial indicator firmly and push the plunger tip against the component surface, compressing it by a distance of 1.27 to 2.54 mm (0.050 to 0.100 inches) before locking the bracket and zeroing the dial. This "preload" ensures that the plunger remains in contact with the surface even if the component moves away from the indicator during testing, allowing the dial to measure both positive and negative deviations.
Plastigage
Plastigage is a plastic thread used to measure the oil clearance between engine crankshaft journals and bearing shells. The procedure is as follows:
- Thoroughly clean all oil from the journal and bearing surfaces.
- Place a small strip of Plastigage thread across the journal.
- Install the bearing cap and torque the fasteners to specification. Do not rotate the crankshaft; doing so will smear the Plastigage and ruin the reading.
- Remove the bearing cap. The Plastigage will be flattened.
- Match the width of the flattened Plastigage at its widest point to the graduated scale printed on the Plastigage envelope. The scale indicates clearance in thousandths of an inch or millimeters.
Feeler Gauges
Feeler gauges consist of precision-ground steel blades of specific thicknesses. They are used to measure clearance gaps, such as piston ring end gap, valve guide-to-stem clearance, spark plug gap, and pump rotor clearances.
Torque Wrenches and Fasteners
Proper clamping force is essential to prevent joint separation and bolt breakage.
Torque Wrench Maintenance
Technicians utilize click-type, dial-type, and digital electronic torque wrenches. The click-type wrench uses an internal calibrated coil spring under tension.
- Storage: When storing a click-type torque wrench, the technician must dial the handle back to its lowest marked setting on the scale (never unwind it completely). Storing the wrench under high tension compresses the internal spring over time, fatiguing the metal and causing the wrench to under-torque fasteners. Storing it completely loose can cause internal components to shift, throwing off calibration.
Torque-to-Yield (TTY) Fasteners
Torque-to-Yield (TTY) bolts are designed to stretch past their elastic limit into their plastic deformation zone when tightened. This stretch provides a highly uniform clamping force across joints subject to high thermal expansion, such as aluminum cylinder heads on cast-iron blocks.
- Single-Use Rule: Because TTY bolts deform plastically, they are permanently stretched and weakened. They must never be reused. Reusing a TTY bolt will lead to bolt breakage or low clamping force, causing blown head gaskets.
- Tightening Procedure: TTY fasteners are tightened using a multi-step torque-plus-angle method. A torque wrench is used to achieve a low torque baseline (e.g., 40 N·m), followed by a specific degree of rotation (e.g., 90 degrees plus an additional 90 degrees) measured using a torque angle gauge.
Fastener Grading and Identification
| Metric Class | Tensile Strength | Yield Strength | Radial Slashes (Imperial) | Imperial Grade | Tensile Strength |
|---|---|---|---|---|---|
| Class 8.8 | 800 MPa | 640 MPa | 3 Slashes | Grade 5 | 120,000 PSI |
| Class 10.9 | 1040 MPa | 940 MPa | 6 Slashes | Grade 8 | 150,000 PSI |
| Class 12.9 | 1220 MPa | 1100 MPa | None | Grade 2 (Low carbon) | 60,000 PSI |
To identify an Imperial bolt, add 2 to the number of radial lines on the head. A bolt with 3 lines is Grade 5; a bolt with 6 lines is Grade 8. Metric bolts display numerical stamps (e.g., 8.8, 10.9) directly on the head.
Thread Repair and Maintenance
When internal threads in aluminum components (like cylinder head bolt holes or spark plug holes) strip out, technicians use Heli-Coils (wire thread inserts) to restore the hole. The hole is drilled out, tapped with a special Heli-Coil tap, and the wire insert is threaded in, restoring the original thread size and pitch.
For dirty or damaged threads:
- Thread Chaser: A specialized tool designed to clean and reshape existing threads. It rolls the metal back into shape without removing material, preserving thread strength.
- Taps and Dies: Cutting tools designed to cut new threads. Using a tap or die to clean existing threads removes metal, weakening the thread engagement and reducing the maximum torque the joint can support.
On a standard metric micrometer, one complete rotation of the thimble moves the spindle by what distance across the sleeve index line?
Why must a dial indicator be preloaded by 1.27 to 2.54 mm (0.050 to 0.100 inches) before it is locked in place and zeroed to measure brake rotor runout?
Which of the following describes the correct storage procedure for a click-type torque wrench to preserve its calibration over time?