In marine diesel engines, the connecting rod is one of the most highly stressed components. During every engine cycle, it transfers enormous combustion forces from the piston to the crankshaft. Over time, wear, overheating, improper lubrication, and uneven loading can distort the circular shape of the connecting rod bore. This distortion is known as ovality.
If the ovality exceeds the manufacturer’s limit, it can lead to bearing failure, abnormal vibration, overheating, crankshaft damage, and even catastrophic engine breakdown. That is why checking connecting rod ovality is an essential part of auxiliary engine and main engine maintenance onboard ships.
Ovality is the difference between the maximum and minimum diameter measured inside the connecting rod bore. Ideally, the bore should be perfectly circular. When it becomes slightly oval due to wear or stress, the bearing clearance becomes uneven.
In marine engines, ovality commonly occurs at the big-end bore of the connecting rod where the crank pin bearing is fitted.
Causes of Connecting Rod Ovality
Common causes include:
These factors gradually distort the bore from its original circular shape.
Instruments Used to Measure Ovality
The following precision instruments are commonly used:
Among these, the inside micrometer and dial bore gauge are most commonly used onboard ships for accurate measurement.
Thoroughly clean the connecting rod and bearing cap. Remove all oil, dirt, carbon deposits, and old bearing material.
Also inspect:
Minor fretting can be removed using an oilstone, but severe damage requires replacement of the connecting rod.
Fit the bearing cap onto the connecting rod without installing the bearing shells.
Ensure:
This step is critical because improper seating can produce incorrect readings.
Tighten the connecting rod bolts or nuts to the manufacturer’s specified torque or hydraulic tension.
Proper tightening simulates actual operating conditions and ensures accurate bore measurement.
Using an inside micrometer or dial bore gauge, measure the bore diameter at multiple locations.
Typical measurement positions include:
Usually, five readings are taken to determine the maximum and minimum diameters.
Measurement Pattern
Take readings:
This helps identify:
Use the formula:
Ovality=Maximum Diameter−Minimum Diameter
For example:
Then:
Ovality=210.05−210.00=0.05 mm
Compare the measured ovality with the engine maker’s allowable limit.
A common marine engineering guideline states:
Maximum Allowable Ovality≤1/4×Bearing Clearance
However, always follow the engine manufacturer’s manual because limits vary between engine models.
If ovality exceeds permissible limits:
In severe cases, the connecting rod must be replaced.
While checking ovality, also inspect for:
Cracks
Use:
Fretting
Minor fretting may be corrected with an oilstone.
Corrosion
Check serrated surfaces carefully.
Bolt Condition
Inspect connecting rod bolts for:
Checking the ovality of a connecting rod is a critical maintenance procedure in marine diesel engines. By accurately measuring the bore diameter using an inside micrometer or dial bore gauge, marine engineers can identify wear before serious damage occurs.
Regular ovality checks improve:
A properly calibrated and perfectly round connecting rod ensures smooth engine operation and prevents costly breakdowns at sea.
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