Auxiliary Engine Big-End Bearing

A thorough inspection of all parts is essential before reassembly to ensure reliability and prevent premature failure.

1. Bearing Shells (Thin-walled):

• Visual Inspection: Examine the white metal surface for signs of wiping, pitting, or scuffing. Also check for fatigue cracks or cavitation damage, which indicate deterioration of the bearing surface.
• Back of the Shell: Inspect for fretting marks (black spots). These indicate that the shell has been moving inside the housing, usually due to loss of proper “crush.”
• Thickness Measurement: Measure the shell thickness at several locations using a ball-ended micrometer. Compare the readings with manufacturer limits to assess wear.

2. Crank Pin:

• Ovality and Taper: Measure the diameter at multiple positions (both vertical and horizontal planes) using an outside micrometer. Compare readings to detect ovality or taper.
• Surface Condition: Check for scoring, ridges, or heat discoloration (blue spots), which suggest overheating or lubrication failure.
• Oil Hole Condition: Ensure the oil hole is clean and free from obstruction. The edges should be smooth and properly radiused to avoid stress concentration.

3. Serrations (Joining Faces):

• Condition of Mating Surfaces: Inspect the serrated faces between the connecting rod and bearing cap for fretting or crushing.
• Cleanliness: These surfaces must be perfectly clean. Even small particles can cause misalignment, leading to uneven loading and bearing failure.

4. Connecting Rod Bolts:

• Crack Detection: Inspect bolts carefully, especially at thread roots and the transition between shank and head, using methods such as Liquid Penetrant Testing (LPT).
• Elongation Check: Measure the bolt length and compare it with the original unstretched length. If permanent elongation exceeds permissible limits, the bolt must be replaced.

After satisfactory inspection, the bearing is assembled as follows:

1. Preparation: Clean the bearing housing and the back of the shells thoroughly. Wipe the crank pin with a lint-free cloth and apply a light coat of clean lubricating oil.
2. Fitting of Shells: Place the bearing shells correctly into the connecting rod and cap, ensuring the locating lugs fit properly into their recesses.
3. Mounting on Crank Pin: Position the connecting rod onto the crank pin (or bring the pin into position). Fit the bearing cap, ensuring that the match marks on the rod and cap align correctly.
4. Tightening of Bolts:
   • Clean and lightly lubricate the bolt threads and seating surfaces.
   • Tighten the bolts in stages (for example, 30% → 60% → 100%) using a hydraulic jack or torque wrench as specified.
   • If the angle-of-turn method is used, ensure reference marks are correctly aligned during tightening.

Once assembly is complete, several checks must be carried out before the engine is put into operation:

1. Bearing Clearance (Oil Clearance): Measure the clearance using long feeler gauges inserted between the crank pin and bearing shell at the point of maximum gap (top or bottom). Ensure it is within specified limits (typically around 0.15–0.25 mm).
2. Side Clearance (Axial Float): Check the clearance between the connecting rod and crank web. This ensures there is sufficient space for free movement and thermal expansion without binding.
3. Swing Test: If the piston is disconnected, the connecting rod should swing freely under its own weight (in smaller engines) or move easily by hand, confirming correct alignment and clearance.
4. Turning Gear Test: Engage the turning gear and rotate the engine through at least two full revolutions. Observe for smooth movement, absence of hard spots, and listen for any abnormal metallic sounds.
5. Lubrication Check: Start the pre-lubrication pump and inspect the bearing area. Oil should be seen flowing or weeping from the edges of the bearing, confirming proper lubrication supply.
6. Securing Arrangements: Ensure all locking devices such as tab washers, locking wires, or pins are properly fitted to prevent loosening of bolts during operation.