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Halderman ch033 lecture


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Halderman ch033 lecture

  2. 2. Objectives <ul><li>The student should be able to: </li></ul><ul><ul><li>Prepare for Engine Repair (A1) ASE certification test content area “C” (Engine Block Diagnosis and Repair). </li></ul></ul><ul><ul><li>Describe the purpose and function of pistons, rings, and connecting rods. </li></ul></ul><ul><ul><li>Explain how pistons and rods are constructed and what to look for during an inspection. </li></ul></ul>
  3. 3. Objectives <ul><li>The student should be able to: </li></ul><ul><ul><li>Discuss connecting rod reconditioning procedures. </li></ul></ul><ul><ul><li>Explain how piston rings operate and how to install them on a piston. </li></ul></ul>
  4. 4. PISTONS
  5. 5. Pistons <ul><li>Purpose and Function </li></ul><ul><ul><li>Three purposes </li></ul></ul><ul><ul><ul><li>Transfers force </li></ul></ul></ul><ul><ul><ul><li>Seals the combustion chamber </li></ul></ul></ul><ul><ul><ul><li>Conducts heat </li></ul></ul></ul>
  6. 6. Pistons <ul><li>Parts Involved </li></ul><ul><ul><li>Piston </li></ul></ul><ul><ul><li>Wrist pin </li></ul></ul><ul><ul><li>Crank throw/crankpin/connecting rod bearing journal </li></ul></ul><ul><ul><li>Piston rings </li></ul></ul>
  7. 7. Figure 33-1 The piston seals the bottom of the combustion chamber and is attached to a connecting rod.
  8. 8. Pistons <ul><li>Piston Operation </li></ul><ul><ul><li>Starts, accelerates, and stops twice in each crankshaft revolution </li></ul></ul><ul><ul><li>Piston starts at the top of the cylinder and ends at the top of the stroke </li></ul></ul>
  9. 9. Pistons <ul><li>Piston Operation </li></ul><ul><ul><li>Piston head exposed to the hot combustion gases </li></ul></ul><ul><ul><li>Skirt contacts the relatively cool cylinder wall </li></ul></ul><ul><ul><ul><li>Results in a temperature difference of about 275°F (147°C) between the top and bottom of the piston </li></ul></ul></ul>
  10. 10. Pistons <ul><li>Piston Operation </li></ul><ul><ul><li>NOTE: A typical piston in an engine operating at 4000 RPM accelerates from 0 to 60 mph (97 km/h) in about 0.004 second (4 ms) as it travels about halfway down the cylinder. </li></ul></ul>
  12. 12. Piston Construction <ul><li>Piston Ring Grooves </li></ul><ul><ul><li>Located between the piston head and skirt </li></ul></ul><ul><ul><li>Factors that determine minimum piston height: </li></ul></ul><ul><ul><ul><li>Width of grooves </li></ul></ul></ul>
  13. 13. Piston Construction <ul><li>Piston Ring Grooves </li></ul><ul><ul><li>Factors that determine minimum piston height: </li></ul></ul><ul><ul><ul><li>Width of lands between ring grooves </li></ul></ul></ul><ul><ul><ul><li>Number of rings </li></ul></ul></ul>
  14. 14. Piston Construction <ul><li>Piston Ring Grooves </li></ul><ul><ul><li>Outside diameter lands is about 0.02 to 0.04 in. (0.5 to 1 mm) smaller than the skirt diameter </li></ul></ul>
  15. 15. Figure 33-2 All pistons share the same parts in common.
  16. 16. Figure 33-3 A piston diameter is measured across the thrust surfaces.
  17. 17. Piston Construction <ul><li>Cast Pistons </li></ul><ul><ul><li>Usually made using gravity die casting </li></ul></ul><ul><ul><li>Molten aluminum alloy and about 10% silicon are poured into a mold </li></ul></ul>
  18. 18. Piston Construction <ul><li>Cast Pistons </li></ul><ul><ul><li>Silicon increases the strength and helps control the expansion of the piston when it gets hot </li></ul></ul>
  19. 19. Piston Construction <ul><li>Cast Pistons </li></ul><ul><ul><li>Metals used in the aluminum alloy include copper, nickel, manganese, and magnesium </li></ul></ul>
  20. 20. Figure 33-4 A cast piston showing the sprues which were used to fill the mold with molten aluminum alloy.
  21. 21. Piston Construction <ul><li>Hypereutectic Pistons </li></ul><ul><ul><li>Eutectic pistons contain about 9% to 12% silicon </li></ul></ul><ul><ul><li>Hypereutectic pistons are stronger due to a 16% silicon content </li></ul></ul>
  22. 22. Piston Construction <ul><li>Hypereutectic Pistons </li></ul><ul><ul><li>Advantages: </li></ul></ul><ul><ul><ul><li>Strength </li></ul></ul></ul><ul><ul><ul><li>25% weight reduction </li></ul></ul></ul><ul><ul><ul><li>Lower expansion rate </li></ul></ul></ul>
  23. 23. Piston Construction <ul><li>Hypereutectic Pistons </li></ul><ul><ul><li>Disadvantages: </li></ul></ul><ul><ul><ul><li>Higher cost </li></ul></ul></ul><ul><ul><ul><li>More difficult to cast and machine </li></ul></ul></ul>
  24. 24. Piston Construction <ul><li>Forged Pistons </li></ul><ul><ul><li>Dense grain structure </li></ul></ul><ul><ul><li>Very strong </li></ul></ul><ul><ul><li>Often used in turbocharged or supercharged engines </li></ul></ul>
  25. 25. Piston Construction <ul><li>Forged Pistons </li></ul><ul><ul><li>Less porous than cast pistons </li></ul></ul><ul><ul><li>Conduct heat more quickly than cast pistons </li></ul></ul><ul><ul><li>Run about 20% cooler than cast pistons </li></ul></ul>
  26. 26. Figure 33-5 The top of the piston temperature can be 100°F (38°C) lower on a forged piston compared to a cast piston.
  27. 27. Piston Construction <ul><li>Piston Head Designs </li></ul><ul><ul><li>Shape is vital to the combustion process </li></ul></ul>
  28. 28. Piston Construction <ul><li>Piston Head Designs </li></ul><ul><ul><li>Flat-top pistons </li></ul></ul><ul><ul><ul><li>Close to the cylinder head </li></ul></ul></ul><ul><ul><ul><li>Recesses are cut in the piston top for valve clearance </li></ul></ul></ul><ul><ul><ul><ul><li>Commonly called: eyebrows, valve reliefs, valve pockets </li></ul></ul></ul></ul>
  29. 29. Piston Construction <ul><li>Piston Head Designs </li></ul><ul><ul><li>Pistons in high-powered engines may have raised domes (pop-ups) on the heads </li></ul></ul>
  30. 30. Piston Construction <ul><li>Piston Head Designs </li></ul><ul><ul><li>Pistons in other engines may be provided with a depression (dish) </li></ul></ul><ul><ul><li>Varying dish depths provide different compression ratios </li></ul></ul>
  31. 31. Piston Construction <ul><li>Piston Head Designs </li></ul><ul><ul><li>NOTE: Newer engines do not use valve reliefs because this requires that the thickness of the top of the piston be increased to provide the necessary strength. The thicker the top of the piston, the farther down from the top the top piston ring sits. </li></ul></ul>
  32. 32. Piston Construction <ul><li>Piston Head Designs </li></ul><ul><ul><li>NOTE: To reduce unburned hydrocarbon (HC) exhaust emissions, engineers attempt to place the top piston ring as close to the top of the piston as possible to prevent the unburned fuel from being trapped (and not burned) between the top of the piston and the top of the top piston ring. </li></ul></ul>
  33. 33. Figure 33-6 Valve reliefs are used to provide valve clearance.
  34. 34. Piston Construction <ul><li>Slipper Skirt Pistons </li></ul><ul><ul><li>Shorter on the two sides that are not thrust surfaces </li></ul></ul><ul><ul><li>Advantages include: </li></ul></ul><ul><ul><ul><li>Lighter weight </li></ul></ul></ul>
  35. 35. Piston Construction <ul><li>Slipper Skirt Pistons </li></ul><ul><ul><li>Advantages include: </li></ul></ul><ul><ul><ul><li>Allows for shorter overall engine height </li></ul></ul></ul><ul><ul><li>Most engines today use a slipper skirt piston </li></ul></ul>
  36. 36. Piston Construction <ul><li>Cam Ground Pistons </li></ul><ul><ul><li>Provides method of expansion control </li></ul></ul><ul><ul><li>Piston thrust surfaces closely fit the cylinder </li></ul></ul><ul><ul><li>Piston pin boss diameter is fitted loosely </li></ul></ul>
  37. 37. Piston Construction <ul><li>Cam Ground Pistons </li></ul><ul><ul><li>Expands along the piston when heated </li></ul></ul><ul><ul><li>Nearly round at its normal operating temperatures </li></ul></ul>
  38. 38. Figure 33-7 Piston cam shape. The largest diameter is across the thrust surfaces and perpendicular to the piston pin (labeled A).
  39. 39. Piston Construction <ul><li>Piston Finish </li></ul><ul><ul><li>Varies with manufacturer </li></ul></ul><ul><ul><li>All finishes reduce scuffing (condition where the metal of the piston actually contacts the cylinder wall) </li></ul></ul>
  40. 40. Piston Construction <ul><li>Piston Finish </li></ul><ul><ul><li>Reduce scuffing by coating piston skirts with tin 0.0005 in. (0.0125 mm) thick or a moly graphite coating </li></ul></ul>
  41. 41. Figure 33-8 A moly graphite coating on this piston from a General Motors 3800 V-6 engine helps to prevent piston scuffing.
  42. 42. Piston Construction <ul><li>Piston Head Size </li></ul><ul><ul><li>Smaller in diameter than the rest of piston </li></ul></ul><ul><ul><li>Horizontal separation slots act as heat dams </li></ul></ul>
  43. 43. Piston Construction <ul><li>Piston Head Size </li></ul><ul><ul><li>Slots reduce heat transfer from the piston head to the lower skirt </li></ul></ul><ul><ul><ul><li>Keeps the skirt temperature lower to reduce skirt expansion </li></ul></ul></ul><ul><ul><ul><li>Can be used for oil drainback and expansion control </li></ul></ul></ul>
  44. 44. Figure 33-9 The head of the piston is smaller in diameter than the skirt of the piston to allow it to expand when the engine is running.
  45. 45. Piston Construction <ul><li>Piston Strut Inserts </li></ul><ul><ul><li>Add strength to the piston in the piston pin area </li></ul></ul><ul><ul><li>Help control thermal expansion </li></ul></ul>
  46. 46. Piston Construction <ul><li>Piston Strut Inserts </li></ul><ul><ul><li>Good piston-to-cylinder wall clearance at normal temperatures </li></ul></ul><ul><ul><li>Cold operating clearance as small as 0.0005 in. (0.0127 mm) </li></ul></ul><ul><ul><ul><li>Will prevent cold piston slap and noise </li></ul></ul></ul>
  47. 47. Figure 33-10 Steel struts cast inside the piston help control expansion and add strength to the piston pin area.
  48. 48. PISTON PINS
  49. 49. Piston Pins <ul><li>Terminology </li></ul><ul><ul><li>Attaches the piston to the connecting rod </li></ul></ul><ul><ul><li>Also known as wrist pins or gudgeon pins </li></ul></ul>
  50. 50. Piston Pins <ul><li>Terminology </li></ul><ul><ul><li>Transfers the force produced by combustion chamber pressures and piston inertia to the connecting rod </li></ul></ul>
  51. 51. Piston Pins <ul><li>Terminology </li></ul><ul><ul><li>Made from high-quality steel in the shape of a tube </li></ul></ul><ul><ul><li>Interior hole is sometimes tapered </li></ul></ul>
  52. 52. Figure 33-11 Most piston pins are hollow to reduce weight and have a straight bore. Some pins have a tapered bore to reinforce the pin.
  53. 53. Piston Pins <ul><li>Piston Pin Offset </li></ul><ul><ul><li>Some piston pin holes are not centered in the piston </li></ul></ul><ul><ul><li>Located toward the major thrust surface, approximately 0.062 in. (1.57 mm) from the piston centerline </li></ul></ul>
  54. 54. Piston Pins <ul><li>Piston Pin Offset </li></ul><ul><ul><li>Designed to reduce piston slap and the noise </li></ul></ul><ul><ul><ul><li>Minor thrust </li></ul></ul></ul><ul><ul><ul><li>Major thrust </li></ul></ul></ul>
  55. 55. Piston Pins <ul><li>Piston Pin Offset </li></ul><ul><ul><li>NOTE: Not all piston pins are offset. In fact, many engines operate without the offset to help reduce friction and improve power and fuel economy. </li></ul></ul>?
  56. 56. Figure 33-12 Piston pin offset toward the major thrust surface.
  57. 57. Figure 33-13 Engine rotation and rod angle during the power stroke cause the piston to press harder against one side of the cylinder, called the major thrust surface.
  58. 58. Piston Pins <ul><li>Piston Pin Fit </li></ul><ul><ul><li>Size is held to tens of thousandths of an inch </li></ul></ul><ul><ul><li>Loose pins will make a sound while engine is running (double knock) </li></ul></ul>
  59. 59. Piston Pins <ul><li>Piston Pin Fit </li></ul><ul><ul><li>Tight pins will restrict piston expansion along the pin diameter and lead to piston scuffing </li></ul></ul><ul><ul><li>Normal clearances range from 0.0005 to 0.0007 in. (0.0126 to 0.018 mm) </li></ul></ul>
  61. 61. Piston Pin Retaining Methods <ul><li>Full Floating </li></ul><ul><ul><li>Free to “float” in the connecting rod and the piston </li></ul></ul><ul><ul><li>Retaining device keeps piston from scraping against the cylinder wall </li></ul></ul>
  62. 62. Piston Pin Retaining Methods <ul><li>Full Floating </li></ul><ul><ul><li>Most often used in high-performance modified engines </li></ul></ul><ul><ul><li>Some type of lock ring used to retain the piston pin </li></ul></ul>
  63. 63. Piston Pin Retaining Methods <ul><li>Full Floating </li></ul><ul><ul><li>Two common types of lock rings: </li></ul></ul><ul><ul><ul><li>Internal snap ring </li></ul></ul></ul><ul><ul><ul><li>Spiral lock ring </li></ul></ul></ul>
  64. 64. Figure 33-14 Circlips hold full-floating piston pins in place.
  65. 65. Piston Pin Retaining Methods <ul><li>Interference Fit </li></ul><ul><ul><li>Connecting rod hole slightly smaller than the piston pin </li></ul></ul><ul><ul><li>Pin is installed by heating the rod or by pressing it into the rod </li></ul></ul>
  66. 66. Piston Pin Retaining Methods <ul><li>Interference Fit </li></ul><ul><ul><li>Be careful to have the correct hole sizes and center the pin </li></ul></ul><ul><ul><li>Least expensive method to use </li></ul></ul>
  67. 67. Figure 33-15 A typical interference fit piston pin.
  68. 68. PISTON RINGS
  69. 69. Piston Rings <ul><li>Purpose and Function </li></ul><ul><ul><li>Form a sliding combustion chamber seal to prevent the high-pressure combustion gases from leaking past the piston </li></ul></ul><ul><ul><li>Keep engine oil from entering the combustion chamber </li></ul></ul>
  70. 70. Piston Rings <ul><li>Purpose and Function </li></ul><ul><ul><li>Transfer some of the piston heat to the cylinder wall, where it is removed from the engine through the cooling system </li></ul></ul>
  71. 71. Figure 33-16 The rings conduct heat from the piston to the cylinder wall.
  72. 72. Piston Rings <ul><li>Classifications </li></ul><ul><ul><li>Classified in two types: </li></ul></ul><ul><ul><ul><li>Two compression rings </li></ul></ul></ul><ul><ul><ul><li>One oil control ring </li></ul></ul></ul>
  73. 73. Piston Rings <ul><li>Classifications </li></ul><ul><ul><li>NOTE: Some engines, such as Honda high-fuel economy engines, use pistons with only two rings: one compression ring and one oil control ring. </li></ul></ul>
  74. 74. Piston Rings <ul><li>Compression Rings </li></ul><ul><ul><li>Forms seal between the moving piston and the cylinder wall to get maximum power from the combustion pressure </li></ul></ul>
  75. 75. Piston Rings <ul><li>Compression Rings </li></ul><ul><ul><li>Must keep friction at a minimum </li></ul></ul><ul><ul><li>Space in the ring groove above the ring is called side clearance </li></ul></ul><ul><ul><li>Space behind the ring is called the back clearance </li></ul></ul>
  76. 76. Figure 33-17 Combustion chamber pressure forces the ring against the cylinder wall and the bottom of the ring groove to effectively seal the cylinder.
  77. 77. Figure 33-18 The side and back clearances must be correct for the compression rings to seal properly.
  78. 78. Piston Rings <ul><li>Oil Control Rings </li></ul><ul><ul><li>Scraping action allows oil to return through the expander and openings in the piston </li></ul></ul><ul><ul><li>Spacer expander lies between the top and bottom rails </li></ul></ul>
  79. 79. Piston Rings <ul><li>Oil Control Rings </li></ul><ul><ul><li>Spacer expander keeps the rails separated and pushes them out against the cylinder wall </li></ul></ul>
  80. 80. Figure 33-19 This typical three-piece oil control ring uses a hump-type stainless steel spacer-expander. The expander separates the two steel rails and presses them against the cylinder wall.
  81. 81. Piston Rings <ul><li>Ring Gap </li></ul><ul><ul><li>Allow some leakage past the top compression ring </li></ul></ul><ul><ul><ul><li>Leakage provides pressure on the second ring to create seal </li></ul></ul></ul>
  82. 82. Piston Rings <ul><li>Ring Gap </li></ul><ul><ul><li>Amount of gap is critical </li></ul></ul><ul><ul><ul><li>A gap that is too great will allow excessive blowby (leakage of combustion gases past the rings) </li></ul></ul></ul>
  83. 83. Piston Rings <ul><li>Ring Gap </li></ul><ul><ul><li>Amount of gap is critical </li></ul></ul><ul><ul><ul><li>A gap that is too little will allow the piston ring ends to touch together when the engine is hot, causing excessive wear and possible engine failure </li></ul></ul></ul>
  84. 84. Piston Rings <ul><li>Ring Gap </li></ul><ul><ul><li>Gaps reduce losses of high-pressure combustion gases </li></ul></ul>
  85. 85. Figure 33-20 Typical piston ring gaps.
  86. 86. Piston Rings <ul><li>Piston Ring Shapes </li></ul><ul><ul><li>Taper face ring </li></ul></ul><ul><ul><li>Positive twist ring </li></ul></ul><ul><ul><li>Reverse twist ring </li></ul></ul>
  87. 87. Piston Rings <ul><li>Piston Ring Shapes </li></ul><ul><ul><li>Scraper ring </li></ul></ul><ul><ul><li>Barrel face ring </li></ul></ul>
  88. 88. Figure 33-21 The taper face ring provides oil control by scraping the cylinder wall. This style of ring must be installed right side up or the ring will not seal and oil will be drawn into the combustion chamber.
  89. 89. Figure 33-22 Torsional twist rings provide better compression sealing and oil control than regular taper rings.
  90. 90. Figure 33-23 Scraper-type rings provide improved oil control.
  91. 91. Figure 33-24 The upper barrel face ring has a line showing contact with the cylinder wall. The second taper face ring shows contact along the lower edge of the ring.
  93. 93. Piston Ring Construction <ul><li>Piston Ring Materials </li></ul><ul><ul><li>Plain cast iron </li></ul></ul><ul><ul><li>Pearlitic cast iron </li></ul></ul><ul><ul><li>Nodular cast iron </li></ul></ul>
  94. 94. Piston Ring Construction <ul><li>Piston Ring Materials </li></ul><ul><ul><li>Steel </li></ul></ul><ul><ul><li>Ductile iron </li></ul></ul>
  95. 95. Piston Ring Construction <ul><li>Chromium Piston Rings </li></ul><ul><ul><li>Greatly increases piston ring life </li></ul></ul><ul><ul><li>Slightly chamfered at the outer corners </li></ul></ul><ul><ul><li>Rings are prelapped or honed before packaging </li></ul></ul>
  96. 96. Figure 33-25 The chrome facing on this compression ring is about 0.004 in. (0.10 mm) thick.
  97. 97. Piston Ring Construction <ul><li>Molybdenum Piston Rings </li></ul><ul><ul><li>Introduced in the early 1960s </li></ul></ul><ul><ul><li>Plasma method is a spray method used to deposit molybdenum on cast iron to produce a long-wearing and low-friction piston ring </li></ul></ul>
  98. 98. Piston Ring Construction <ul><li>Molybdenum Piston Rings </li></ul><ul><ul><li>Most have a 0.004 to 0.008 in. (0.1 to 0.2 mm) groove filled with molybdenum cut into its face </li></ul></ul>
  99. 99. Piston Ring Construction <ul><li>Molybdenum Piston Rings </li></ul><ul><ul><li>Will survive under high-temperature and scuffing conditions better than chromium face rings </li></ul></ul>
  100. 100. Figure 33-26 The moly facing on this compression ring is about 0.005 in. (0.13 mm) thick.
  101. 101. Piston Ring Construction <ul><li>Moly-Chrome-Carbide Rings </li></ul><ul><ul><li>Used in some original equipment (OE) and replacement applications </li></ul></ul>
  102. 102. Piston Ring Construction <ul><li>Moly-Chrome-Carbide Rings </li></ul><ul><ul><li>Coating properties hardness of chrome and carbide combined with heat resistance of molybdenum </li></ul></ul>
  103. 103. Piston Ring Construction <ul><li>Ceramic-Coated Rings </li></ul><ul><ul><li>Ceramic coating applied through a process called physical vapor deposition (PVD) </li></ul></ul><ul><ul><li>Used when additional heat resistance is needed </li></ul></ul>
  105. 105. Connecting Rods <ul><li>Purpose and Function </li></ul><ul><ul><li>Transfers the force and reciprocating motion of the piston to the crankshaft </li></ul></ul>
  106. 106. Connecting Rods <ul><li>Purpose and Function </li></ul><ul><ul><li>Small end reciprocates with the piston </li></ul></ul><ul><ul><li>Large end rotates with the crankpin </li></ul></ul>
  107. 107. Connecting Rods <ul><li>Purpose and Function </li></ul><ul><ul><li>Connecting rods are manufactured by casting, forging, and powdered (sintered) metal processes </li></ul></ul>
  108. 108. Figure 33-27 The connecting rod is the most highly stressed part of any engine because combustion pressure tries to compress it and piston inertia tries to pull it apart.
  109. 109. Figure 33-28 The I-beam shape (top rod) is the most common, but the H-beam shape is common in high-performance and racing engine applications.
  110. 110. Connecting Rods <ul><li>Connecting Rod Design </li></ul><ul><ul><li>Big end must be a perfect circle </li></ul></ul><ul><ul><li>Once a rod and cap are initially machined, they must remain a “matched set” </li></ul></ul>
  111. 111. Connecting Rods <ul><li>Connecting Rod Design </li></ul><ul><ul><li>Assembly bolt holes are closely reamed in both the cap and connecting rod to ensure alignment </li></ul></ul>
  112. 112. Connecting Rods <ul><li>Connecting Rod Design </li></ul><ul><ul><li>Bolts have piloting surfaces that closely fit the reamed holes </li></ul></ul><ul><ul><li>Made with balancing bosses (pads) so weight can be adjusted to specifications </li></ul></ul>
  113. 113. Connecting Rods <ul><li>Connecting Rod Design </li></ul><ul><ul><li>Some have a spit hole that bleeds some of the oil from the connecting rod journal </li></ul></ul>
  114. 114. Figure 33-29 Rod bolts are quickly removed using a press.
  115. 115. Figure 33-30 Some rods have balancing pads on each end of the connecting rod.
  116. 116. Figure 33-31 Some connecting rods have spit holes to help lubricate the cylinder wall or piston pin.
  117. 117. Figure 33-32 Some engines, such as this Ford diesel, are equipped with oil squirters that spray or stream oil toward the underneath side of the piston head to cool the piston.
  118. 118. Connecting Rods <ul><li>Cast Connecting Rods </li></ul><ul><ul><li>Can be identified by their narrow parting line </li></ul></ul>
  119. 119. Connecting Rods <ul><li>Forged Connecting Rods </li></ul><ul><ul><li>Generally used in heavy-duty and high-performance engines </li></ul></ul><ul><ul><li>Lighter weight and stronger but more expensive </li></ul></ul>
  120. 120. Connecting Rods <ul><li>Forged Connecting Rods </li></ul><ul><ul><li>Can be identified by their wide parting line </li></ul></ul><ul><ul><li>Many high-performance rods use a bronze bushing in the small end </li></ul></ul>
  121. 121. Figure 33-33 A cast connecting rod is found on many stock engines and can be identified by the thin parting line.
  122. 122. Figure 33-34 This high-performance connecting rod uses a bronze bushing in the small end of the rod and oil hole to allow oil to reach the full-floating piston pin.
  123. 123. Connecting Rods <ul><li>Powdered Metal Connecting Rods </li></ul><ul><ul><li>Used by most new production engines </li></ul></ul><ul><ul><li>Advantages over convention cast (forged) rods including precise weight control </li></ul></ul>
  124. 124. Connecting Rods <ul><li>Powdered Metal Connecting Rods </li></ul><ul><ul><li>Created with measured amount of material so rod and engine are balanced without extra weighting and machining operations </li></ul></ul>
  125. 125. Connecting Rods <ul><li>Powdered Metal Connecting Rods </li></ul><ul><ul><li>Start as powdered metal (iron, copper, carbon, other alloying agents) </li></ul></ul><ul><ul><li>Powder placed in a die and compacted (forged) </li></ul></ul>
  126. 126. Connecting Rods <ul><li>Powdered Metal Connecting Rods </li></ul><ul><ul><li>Part is heated, without melting, to about 2,000°F. </li></ul></ul><ul><ul><li>Ingredients are transformed into metallurgical bonds </li></ul></ul>
  127. 127. Connecting Rods <ul><li>Powdered Metal Connecting Rods </li></ul><ul><ul><li>Machining includes boring and drilling </li></ul></ul><ul><ul><li>Big end is fractured to help ensure a perfect match when assembled </li></ul></ul>
  128. 128. Figure 33-35 Powdered metal connecting rods feature a fractured parting line at the big end of the rod.
  130. 130. Connecting Rod Service <ul><li>Removing Pistons From Rods </li></ul><ul><ul><li>Removed using a special fixture </li></ul></ul>
  131. 131. Connecting Rod Service <ul><li>Inspection </li></ul><ul><ul><li>Check rod for a twist before reconditioning </li></ul></ul><ul><ul><li>Hole at the small end and the hole at the big end should be parallel </li></ul></ul>
  132. 132. Connecting Rod Service <ul><li>Inspection </li></ul><ul><ul><li>Twists greater than 0.002 in. (0.05 mm) are not acceptable </li></ul></ul><ul><ul><li>Some specialty shops can remove twist by bending the rod cold </li></ul></ul>
  133. 133. Connecting Rod Service <ul><li>Inspection </li></ul><ul><ul><li>Both cast and forged rods can be straightened </li></ul></ul><ul><ul><li>Many engine builders replace the connecting rod if it is twisted </li></ul></ul>
  134. 134. Figure 33-36 A press used to remove the connecting rod from the piston.
  135. 135. Figure 33-37 If the rod is twisted, it will cause diagonal-type wear on the piston skirt.
  136. 136. Figure 33-38 A rod alignment fixture is used to check a connecting rod for bends or twists.
  137. 137. Connecting Rod Service <ul><li>Reconditioning Procedure </li></ul><ul><ul><li>STEP 1: Parting surfaces of the rod and cap are smoothed. A couple thousandths of an inch of metal is removed from the rod cap parting surface. The amount removed from the rod and cap only reduces the bore size 0.003 to 0.006 in. (0.08 to 0.15 mm). </li></ul></ul>
  138. 138. Connecting Rod Service <ul><li>Reconditioning Procedure </li></ul><ul><ul><li>STEP 2: The cap is installed on the rod, and the nuts or cap screws are properly torqued. The hole is then bored or honed to be perfectly round and of the size and finish required to give the correct connecting rod bearing crush. </li></ul></ul>
  139. 139. Connecting Rod Service <ul><li>Reconditioning Procedure </li></ul><ul><ul><li>NOTE: Powdered metal connecting rods cannot be reconditioned using this method. Most manufacturers recommend replacing worn powdered metal connecting rods. </li></ul></ul>
  140. 140. Figure 33-39 Rod bearing bores normally stretch from top to bottom, with most wear concentrated on the rod cap.
  141. 141. Figure 33-40 To help ensure that the big ends are honed straight, many experts recommend placing two rods together when performing the honing operation.
  143. 143. Piston and Rod Assembly <ul><li>Interference Fit Rods </li></ul><ul><ul><li>Pin is put in one side of the piston </li></ul></ul><ul><ul><li>Check small end of connecting rod for proper size </li></ul></ul>
  144. 144. Piston and Rod Assembly <ul><li>Interference Fit Rods </li></ul><ul><ul><li>Small eye on connecting rod is heated before pin is installed causing rod eye to expand </li></ul></ul><ul><ul><li>Pin must be rapidly pushed into the correct center position </li></ul></ul>
  145. 145. Figure 33-41 The small end of the rod is being heated in an electric heater and the piston is positioned properly so the piston pin can be installed as soon as the rod is removed from the heater.
  146. 146. Piston and Rod Assembly <ul><li>Full-Floating Rods </li></ul><ul><ul><li>Full-floating piston pins operate in a bushing in the small eye of the connecting rod </li></ul></ul>
  147. 147. Piston and Rod Assembly <ul><li>Full-Floating Rods </li></ul><ul><ul><li>Bushings and pistons are honed to the same diameter to allow the pin to slide freely through both </li></ul></ul>
  148. 148. Piston and Rod Assembly <ul><li>Full-Floating Rods </li></ul><ul><ul><li>Pin is held in place with a lock ring at each end </li></ul></ul><ul><ul><li>Lock rings expand into a small groove in the pin hole </li></ul></ul><ul><li>NOTE: The original lock rings should always be replaced with new rings. </li></ul>
  150. 150. Piston Ring Service <ul><li>Steps </li></ul><ul><ul><li>Check side clearance </li></ul></ul><ul><ul><li>Check ring gap </li></ul></ul><ul><ul><li>Install oil control ring </li></ul></ul>
  151. 151. Piston Ring Service <ul><li>Steps </li></ul><ul><ul><li>Install compression rings </li></ul></ul><ul><ul><li>Double-check everything </li></ul></ul>
  152. 152. Figure 33-42 The side clearance of the piston ring is checked with a feeler gauge.
  153. 153. Figure 33-43 The ring gap is measured using a feeler gauge.
  154. 154. Figure 33-44 A hand-operated piston ring end gap grinder being used to increase the end gap of a piston ring so that it is within factory specifications.
  155. 155. Figure 33-45 A typical ring expander being used to install a piston ring on a piston.
  156. 156. Figure 33-46 Identification marks used to indicate the side of the piston ring to be placed toward the head of the piston.
  157. 157. TECH TIP <ul><li>Piston Weight Is Important! </li></ul><ul><ul><li>All pistons in an engine should weigh the same to help ensure a balanced engine. Piston weight becomes a factor when changing pistons. Most aluminum pistons range in weight from 10 to 30 ounces (280 to 850 grams) (1 oz = 28.35g). </li></ul></ul>BACK TO PRESENTATION A typical paper clip weighs 1 g. If the cylinder has been bored, larger replacement pistons are obviously required. If the replacement pistons weigh more, this puts additional inertia loads on the rod bearings. Therefore, to help prevent rod bearing failure on an overhauled engine, the replacement pistons should not weigh more than the original pistons CAUTION: Some less expensive replacement cast pistons or high-performance forged pistons are much heavier than the stock pistons, even in the stock bore size. This means that the crankshaft may need heavy metal added to the counterweights of the crankshaft for the engine to be balanced. For the same reason, if one piston is being replaced, all pistons should be replaced or at least checked and corrected to ensure the same weight.
  158. 158. FREQUENTLY ASKED QUESTION <ul><li>Which Side Is the Major Thrust Side? </li></ul><ul><ul><li>The thrust side is the side the rod points to when the piston is on the power stroke. Any V-block engine (V-6 or V-8) that rotates clockwise is viewed from the front of the engine. The left bank piston thrust side faces the inside (center) of the engine. The right bank piston thrust side faces the outside of the block. </li></ul></ul>? BACK TO PRESENTATION <ul><li>This rule, called the lefthand rule, states the following: </li></ul><ul><ul><li>Stand at the rear of the engine and point toward the front of the engine. </li></ul></ul><ul><ul><li>Raise your thumb straight up, indicating the top of the engine. </li></ul></ul><ul><ul><li>Point your other fingers toward the right. This represents the major thrust side of the piston. </li></ul></ul><ul><li>This rule, called the lefthand rule, states the following: </li></ul><ul><ul><ul><li>Always assemble the connecting rods onto the rods so that the notch or “F” on the piston is pointing toward the front of the engine and the oil squirt hole on the connecting rod is pointing toward the major thrust side with your left hand. </li></ul></ul></ul>
  159. 159. REAL WORLD FIX <ul><li>Big Problem, No Noise </li></ul><ul><ul><li>Sometimes the piston pin can “walk” off the center of the piston and score the cylinder wall. This scoring is often not noticed because this type of wear does not create noise. Because the piston pin is below the piston rings, little combustion pressure is lost past the rings until the groove worn by the piston pin has worn the piston rings. </li></ul></ul>BACK TO PRESENTATION Troubleshooting the exact cause of the increased oil consumption is difficult because the damage done to the oil control rings by the groove usually affects only one cylinder. Often, compression tests indicate good compression because of the cylinder seals, especially at the top. More than one technician has been surprised to see the cylinder gouged by a piston pin when the cylinder head has been removed for service. In such a case, the cost of the engine repair immediately increases far beyond that of normal cylinder head service.