LO 5-1 List and explain the factors requiring technicians’ skills related to precision measurements

LO 5-01: List and explain the factors requiring technicians’ skills related to precision measurements.

Introduction

Efficient and reliable engine operation depends on the use of precision, machined parts correctly assembled to function together. Precision measurement and machining generally refer to dimensional control of parts and components beyond the capabilities of ordinary, non-precision measuring instruments, such as the effort or “feel” for how much something is tightened and dimensional accuracies achieved by tape measures and rulers. Instead, precision measurement and machining techniques concern dimensions represented by thousandths of an inch or millimeters FIGURE 1. Assembly of components using precision techniques enables duplication of precise fits and operating clearances involving assembly techniques (such as precise control of clamping or tightening force). Examples of precision fits and clearances between rotating or moving parts include components where lubricating oil circulates between crankshafts, camshafts, and their associated bearings. Valve stem-to-guide clearance must allow movement but minimize clearance where oil can collect and burn, which can convert it to an abrasive particle. Pistons reciprocating in cylinders are constructed with operating clearances that prevent loss of compression and combustion pressure yet prevent piston-to-cylinder seizure and allow for an oil film to lubricate the cylinder walls. In other engine locations, precision machining of parts is necessary to create a fit between parts that lock components together or form a seal, such as between a cylinder liner and a block or piston pins and pistons. Incorrect fits and clearances can have consequences ranging from premature wear and noise to catastrophic failure, defined as a direct failure damaging more than 50% of engine parts or consequential damage resulting from a failure.

Accurate control of dimensions for engine components is also necessary to enable components to be used interchangeably. To produce and replace components ranging from cylinder heads and piston rings to bearings or cylinder sleeves requires precise duplication of component dimensions during production and installation. Today’s engines are produced with ever-decreasing tolerances for variations from exact dimensional standards. Efforts to minimize manufacturing tolerances are driven by the perception that precise control of component dimensions improves quality, reliability, and durability.

FIGURE 05-1: Measuring the backlash of a cam gear with a precision dial indicator.

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LO 5-01: List and explain the factors requiring technicians’ skills related to precision measurements.

With increasingly precise dimensional controls used by engines and subcomponents, technicians need to be skilled in the correct selection and use of precision tools and measurement more than ever before. Skills like these are necessary in order to precisely measure components to determine serviceability—a term describing the process of inspecting and evaluating a part to make a correct recommendation for replacement, repair, or return of a component to service as it is. Technicians are also responsible for service procedures that may involve measuring and cutting parts with great accuracy, such as installing and adjusting cylinder liner heights or when replacing valve seats and poppet valves in cylinder heads. Measuring runout of components such as a flywheel housing or the amount of lift provided by a camshaft lobe are examples of other, almost countless service procedures requiring precision measurement FIGURE 2. When not performing these tasks, technicians may need to make specific service recommendations or perform inspections on machined components after machining repairs are completed. To do this, technicians should be guided by a correct understanding of how these processes are performed and how to inspect serviced or rebuilt components to evaluate whether they are suitable for installation. Machining cylinder heads containing replaceable valve guides and seats or refinishing block decks, crankshafts, cylinder bores, and liner counterbores are just a few examples of the frequent requirement for precision measurement skills. Technicians also need to inspect for hidden damage to components caused by failures not easily visible, such as components that are warped, twisted, or bent. Precision inspection tools are used in these instances, and technicians should know how to select and use the appropriate instruments.

FIGURE 05-2: Adjusting valve clearances and injector plunger travel are service procedures requiring precision measurement techniques.

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