The connecting rod ovality of the bottom end bearing is caused by cyclic loading and the angular motion of the connecting rod. Importance of connecting rod ovality:

• Ovality can disrupt the uniform oil film that facilitates hydrodynamic lubrication. A failure in this lubrication can result in direct metal-to-metal contact, overheating, and bearing failure.
• Ovality causes an uneven distribution of forces on the bearing, crankpin, and gudgeon pin, leading to localized wear, pitting, and damage.
• If the connecting rod’s deformation affects the piston’s motion, it may cause poor sealing of the piston rings, leading to blow-by gases, power loss, and increased emissions.
• Unchecked ovality can propagate further damage to critical engine components, such as the crankshaft, connecting rod, and bearings, resulting in catastrophic failure.

Remove the connecting rod bottom end bearing shell and inspect for any signs of wear or damage. Refit the bearing cover and tighten the bolts to the torque specified by the manufacturer.

• Use an inside micrometer or bore gauge to measure the internal diameter of the bearing housing at three positions:
  • Position a: Along the vertical (load) axis.
  • Position b and Position c: At two points along the horizontal axis (90° apart from position a).
  • Record the measurements for all three positions.
• Use the formula: Ovality = a - (b + c)/2, where:
  • a = vertical measurement.
  • b and c = horizontal measurements.

  Compare the calculated ovality to the manufacturer's specified maximum allowable limit. If it exceeds 25% of the bearing clearance, the connecting rod requires repair or replacement.

  Part (c)

  Impact of Ovality Beyond the Maximum Allowable Limit:

  • Loss of oil film integrity can lead to scoring, overheating, and eventual bearing failure.
  • Causes concentrated stress on certain areas of the bearing and crankpin, leading to pitting, wear, and cracks.
  • The uneven loading and lack of lubrication can result in surface damage, fatigue, and deformation of the crankpin and gudgeon pin.
  • Misalignment due to ovality affects the piston motion, leading to improper combustion, increased friction, and reduced engine output.
  Part (d)

  Method of Ascertaining Elongation of Connecting Rod Bolts

  • New bolts should be pre-tensioned outside the engine.
    • Tighten the bolts to the specified torque as per the manufacturer’s recommendations.
    • Repeat this process 3-4 times to ensure proper matching with the internal threads and uniform pre-tensioning.
  • Measure the length of the bolts after tightening using a micrometer.
  • Compare the measured length with the manufacturer’s specified limit.
  • If the elongation exceeds the permissible limit, discard and replace the bolts. Elongated bolts lose their ability to maintain proper tension, increasing the risk of failure under load.