LO 7-10 Outline techniques for connecting rod inspection and service

1. Connecting Rod Inspection and Service

Connecting Rod Inspection and Service

Connecting rods should be checked for twists, bends, wear, nicks, and cracks. The oil passageways drilled through the connecting rod should also be checked to ensure that they are clear of any obstructions Figure 77. If a connecting rod uses a lubrication hole between the connecting rod journal and the wrist pin, a quick visual check is made by looking through the hole. The hole should shine light through the rod with a clear, concentric appearance.

FIGURE 07-77: A piston pin lubrication hole drilled through some rods needs to be inspected and cleaned.

FIGURE 07-78: This connecting rod was bent due to a hydrostatic lock in the cylinder.

Twists and bends in connecting rods are usually due to hydrostatic locking. A hydrostatic lock occurs when liquids enter the combustion chamber, eliminating the little clearance volume remaining in a diesel engine Figure 78. When the piston compresses the liquid, it will stop moving while the crankshaft continues to move, causing the connecting rod to bend and twist. Sources of liquids entering the combustion chamber include head gasket leakage, block porosity (from cavitation erosion), leaking injectors or injector tubes, and water entering the exhaust system through an open exhaust stack. Water, snow, oil, or fuel may also enter through the intake system.

A hydrostatically locked engine will turn freely when rotated backward because fluids are pushed out of the intake valve. When the engine is rotated again in the correct direction, the engine will lock up once more. Never use a starter motor to break free a hydrostatically locked engine. The cylinder components, valves, and starter motor can be easily damaged by the starter motor torque.

Bent connecting rods in diesels can be checked for by seeing how far above the block deck a piston protrudes. Most pistons do rise into the area occupied by the cylinder head gasket. A comparison can be made between the manufacturer’s specifications and the actual observed measurement. All pistons should rise uniformly to the same dimension when measured using a dial indicator Figure 79. Connecting rods can also be checked by a machine shop specialist using a fixture to determine whether the rod is bent or twisted and whether the small and large bores are parallel. If a bent or twisted rod is found, the replacement rod should be matched to the same weight classification as the other rods in the engine. The weight classification is usually indicated by a paint mark on the rod. In LD and MD engines, a slightly bent rod is indicated by a speed-dependent loud engine noise in the cylinder that will often disappear when the cylinder is cut out. The noise is caused by contact between the piston skirt and the crankshaft counterweight during every engine revolution.

FIGURE 07-79: Measure piston protrusion above the block deck to check for a bent or twisted connecting rod. A bent rod will have less protrusion than other cylinders.

1.1. Crack Detection

Crack Detection

Connecting rods may be checked for cracks by using dye penetrant or magnetic particle detection. Stresses encountered during operation can lead to stress risers opening and a catastrophic engine failure. Machine shops can also check connecting rod bores for roundness and wear. Worn bushings in the small end can be replaced.

1.2. Assembling Split Fracture Rods

Assembling Split Fracture Rods

If the connecting rod cap of a split fracture rod is fitted incorrectly and tightened, then the connecting rod must be replaced. This is necessary because the unique profile of the mating surfaces will be damaged when the cap is tightened, causing distortion of the two mating surfaces. The distorted rod and cap parting halves will no longer locate correctly, even if they are returned to their original position. To minimize the possibility of this error, connecting rod bolts or cap screws are often located off-center of the connecting rod to ensure that the cap is fitted to the rod in the correct orientation.