Rod bearings play a critical role in an engine’s operation, ensuring smooth movement between the connecting rod and crankshaft. Over time, wear and tear or engine rebuilds may require the use of undersized rod bearings to compensate for material loss. But when should undersized bearings be used?
Understanding the correct application of undersized rod bearings is essential to maintain engine performance, prevent excessive clearance, and ensure proper lubrication. This topic will discuss when to use undersized rod bearings, why they are necessary, and how to determine the correct size for your engine.
What Are Rod Bearings?
Rod bearings are semi-circular metal shells that support the movement of the connecting rod on the crankshaft journal. These bearings reduce friction, allowing the crankshaft to rotate smoothly while maintaining proper oil clearance.
Rod bearings are designed with specific clearances that control oil flow and prevent metal-to-metal contact. If the clearance becomes too large due to wear, it can lead to low oil pressure, engine knocking, and bearing failure.
What Are Undersized Rod Bearings?
Undersized rod bearings, also known as undersized shells, are bearings that have a slightly reduced inner diameter to compensate for crankshaft journal wear or machining. They help restore proper oil clearance when the crankshaft has been ground down or resurfaced.
Undersized bearings are labeled with sizes like 0.010", 0.020", or 0.030", indicating how much material has been removed from the crankshaft journal.
When to Use Undersized Rod Bearings
1. When the Crankshaft Has Been Reground
One of the most common reasons to use undersized rod bearings is when the crankshaft journals have been machined (reground) due to wear or damage.
Over time, high mileage, poor lubrication, or contaminants in the oil can cause crankshaft journals to develop scratches, grooves, or uneven surfaces. To restore a smooth surface, the crankshaft is ground down, making the journal smaller.
Since the original-sized bearings would now be too loose, undersized bearings are installed to compensate for the material removed.
2. To Restore Proper Oil Clearance
Oil clearance is the small gap between the rod bearing and the crankshaft journal. This gap allows engine oil to flow and create a lubricating film, preventing direct metal contact.
If the clearance is too large due to bearing wear or crankshaft refinishing, it can cause:
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Low oil pressure
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Excessive engine noise
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Bearing failure due to lack of lubrication
Using undersized rod bearings helps restore factory-specified oil clearance, ensuring proper lubrication and reducing excessive wear.
3. When Bearings Show Excessive Wear or Damage
Rod bearings naturally wear out over time, especially in high-performance or high-mileage engines. Signs of worn-out bearings include:
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Knocking noises (rod knock)
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Low oil pressure warnings
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Metal shavings in engine oil
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Loss of engine performance
If bearings are excessively worn but the crankshaft is still in good condition, a mechanic may micropolish the crankshaft and install slightly undersized bearings to maintain proper oil clearance.
4. During an Engine Rebuild
If you are rebuilding an engine, it is important to check the condition of the crankshaft journals. If they show signs of wear, scoring, or out-of-round shape, a machine shop may grind them to an undersized specification.
Once the crankshaft is machined, matching undersized rod bearings must be used to ensure proper fitment and engine longevity.
5. To Compensate for Crankshaft Ovality
Crankshaft journals may become out of round (ovality) due to prolonged use or improper lubrication. This can lead to uneven bearing wear and inconsistent oil pressure.
Grinding the crankshaft back to a uniform size and using undersized bearings ensures the journals are perfectly round, allowing even oil distribution and preventing premature bearing failure.
6. For High-Performance Applications
Performance engines operate under higher stress, RPMs, and loads, causing increased bearing wear. Some racing or high-performance engines may use custom-machined crankshafts that require undersized bearings to achieve precise tolerances for oil clearance and lubrication.
How to Determine the Right Undersized Bearing Size
Before installing undersized rod bearings, it is essential to determine the correct size needed for your engine. Here’s how:
1. Measure the Crankshaft Journal Diameter
Use a micrometer to measure the crankshaft journal diameter. Compare the measurement with the factory specifications to see how much material has been removed or worn away.
2. Check the Service Manual
Refer to the engine service manual to find the acceptable oil clearance range and recommended bearing sizes.
3. Use Plastigage to Verify Clearance
Plastigage is a simple tool used to measure bearing clearance. Place a strip of Plastigage between the crankshaft journal and rod bearing, then tighten the rod cap. The flattened Plastigage will indicate the oil clearance, helping you determine if undersized bearings are required.
4. Consult a Machine Shop
If your crankshaft requires machining, a professional machine shop will typically grind it to a standard undersized specification, such as 0.010", 0.020", or 0.030", and recommend the correct undersized bearings.
Potential Issues with Undersized Bearings
1. Improper Clearance
Using the wrong undersized bearing can lead to too much or too little oil clearance, causing lubrication failure or excessive friction.
2. Increased Friction and Heat
If oil clearance is too tight, friction increases, leading to overheating and potential bearing seizure.
3. Reduced Engine Life
Incorrectly installed bearings can result in premature wear, reducing the overall lifespan of the engine.
To avoid these issues, always ensure accurate measurements and use the correct undersized bearings for your specific engine.
Undersized rod bearings are essential when rebuilding or repairing an engine with a regrinded or worn crankshaft. They help restore proper oil clearance, ensuring smooth engine operation, adequate lubrication, and extended engine life.
You should consider using undersized bearings when:
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The crankshaft has been machined to remove wear or damage.
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The oil clearance is too large due to bearing wear.
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The engine is being rebuilt, and the crankshaft requires resizing.
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The crankshaft journals are out of round (oval) and need to be reground.
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The engine is used in high-performance applications where precise tolerances are required.
Before selecting an undersized bearing, always measure the crankshaft, check factory specifications, and consult a professional machine shop if needed. Proper installation of undersized bearings can significantly enhance engine performance and reliability.