When it comes to the heart of an engine, the crankshaft plays a vital role — it’s the component that converts the linear motion of pistons into rotational power. But without the crankshaft bearings, this process wouldn’t be smooth at all.
These small yet powerful parts reduce friction, handle immense loads, and keep your engine running like a well-oiled machine. Whether you’re a mechanic, an engineering student, or just someone who loves learning how engines work, understanding crankshaft bearing sizes is essential.
In this post, we will break down what crankshaft bearings are, how they work, their types, and most importantly, provide you with a crankshaft bearing size chart for easy reference.
What Are Crankshaft Bearings?
Crankshaft bearings are precision components that support the crankshaft inside the engine block. Their main job is to:
Reduce friction between moving parts
Support the crankshaft under high pressure
Maintain proper oil clearance
Prevent metal-to-metal contact
Simply put, they ensure the crankshaft rotates freely while carrying the power from the pistons to the transmission. Without them, your engine would seize in seconds.
Types of Crankshaft Bearings
There are mainly two types of bearings used in most internal combustion engines:
1. Main Bearings
Main bearings hold the crankshaft in the engine block and allow it to spin freely. They are positioned in the main journals of the crankshaft and help distribute the engine load evenly.
2. Rod Bearings (Connecting Rod Bearings)
Rod bearings are placed between the connecting rods and the crankshaft journals. They handle the up-and-down motion from the pistons and convert it into the rotational motion of the crankshaft.
In high-performance engines, you might also hear about thrust bearings, which control the crankshaft’s forward and backward movement, preventing it from shifting under load.
Why Bearing Size Matters
Choosing the correct bearing size is crucial for:
Maintaining proper oil clearance
Preventing excessive wear
Ensuring balanced crankshaft rotation
Reducing vibration and friction
Even a 0.001-inch difference can change how your engine behaves. Too tight — and the crankshaft won’t spin freely. Too loose — and you’ll have oil pressure issues or even bearing failure.
Crankshaft Bearing Size Chart (Standard and Undersize Bearings)
Here’s a general crankshaft bearing size chart you can refer to. Keep in mind that exact dimensions may vary depending on the manufacturer and engine model. But these are the typical standard and undersized bearing dimensions used in automotive engines.
| Bearing Type | Standard Journal Diameter (inches) | Undersize Bearing (inches) | Common Clearance Range (inches) |
|---|---|---|---|
| Main Bearing | 2.000 | 1.998 (-0.002) | 0.0015 – 0.0025 |
| Connecting Rod Bearing | 1.850 | 1.848 (-0.002) | 0.0015 – 0.0025 |
| Thrust Bearing | 2.500 | 2.498 (-0.002) | 0.002 – 0.004 |
| Heavy-Duty Diesel Main Bearing | 3.000 | 2.998 (-0.002) | 0.002 – 0.003 |
| Performance Engine Main Bearing | 2.200 | 2.198 (-0.002) | 0.0018 – 0.0022 |
Note:
“Undersize” bearings are used when the crankshaft journal is ground down during reconditioning.
For example, if the journal is worn and needs machining, a 0.25 mm (0.010 inch) undersize bearing is used to maintain the correct clearance.
Crankshaft Bearing Size Chart PDF:
The Crankshaft Bearing Size Chart PDF provides a clear and detailed reference for standard and undersized bearing dimensions. It helps mechanics, students, and engine rebuilders choose the correct bearings for optimal crankshaft performance. This chart simplifies understanding of clearances and fits, ensuring smooth rotation, reduced friction, and longer engine life in both standard and performance engines.
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Understanding Bearing Codes and Sizes
Every bearing has a size code — for example, STD, 0.25, 0.50, or 1.00 — which shows how much material has been removed from the crankshaft during grinding.
| Bearing Code | Meaning |
|---|---|
| STD | Standard size |
| 0.25 | 0.25 mm undersize (0.010 inch) |
| 0.50 | 0.50 mm undersize (0.020 inch) |
| 0.75 | 0.75 mm undersize (0.030 inch) |
| 1.00 | 1.00 mm undersize (0.040 inch) |
So, if your engine’s crankshaft journals have been ground by 0.25 mm, you will need a 0.25 undersize bearing set to fit perfectly.
How to Measure Crankshaft Bearing Clearance
If you’ve ever rebuilt an engine, you know how important bearing clearance is. Too tight, and it overheats. Too loose, and you lose oil pressure.
Here’s how you can measure it:
Clean the crankshaft and bearing surfaces.
Place a strip of Plastigage (a soft measuring plastic) across the journal.
Install and torque the bearing cap to spec.
Remove it and compare the flattened Plastigage to the scale on the packet.
That width tells you your oil clearance — usually between 0.0015″ to 0.0025″ for most engines.
This small step helps ensure your new bearings won’t fail prematurely.
Signs of Worn or Damaged Crankshaft Bearings
Even the best bearings wear out over time. Here are some common symptoms:
Knocking noise from the engine bottom end
Low oil pressure on the gauge
Metal flakes in oil
Vibration or loss of smooth engine operation
Crankshaft play or excessive movement
If you notice any of these, it’s time to inspect your crankshaft and bearings before serious engine damage occurs.
How to Choose the Right Crankshaft Bearings
Here are a few quick tips when selecting bearings:
Check your crankshaft journals: Measure them carefully using a micrometer before ordering bearings.
Stick with OEM or trusted brands.: Brands like Clevite, King Bearings, and ACL are known for precision and durability.
Match the bearing type to your engine use: Daily driving → Standard bearings, Racing or high-performance → Tri-metal or high-load bearings
Don’t forget the oil clearance: Always verify with Plastigage before final assembly.
Crankshaft Bearing Material Types
Different materials perform differently under load and temperature:
| Bearing Material | Description |
|---|---|
| Bimetal Bearings | Aluminum alloy on a steel backing; common in modern cars. |
| Trimetal Bearings | Copper-lead alloy with overlay; best for performance and high-load engines. |
| Lead-Free Bearings | Eco-friendly versions using tin or aluminum alloys; found in newer vehicles. |
Each material offers unique strength and resistance to wear, depending on your engine type and power level.
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Maintenance Tips for Long Bearing Life
If you want your crankshaft bearings to last, follow these easy but effective maintenance steps:
Always use clean engine oil and change it regularly.
Warm up your engine before revving hard.
Check for oil pressure issues — never ignore the warning light.
Keep the oil passages clean and unclogged.
Use the right engine oil viscosity as recommended by your vehicle manufacturer.
These small habits can prevent costly engine rebuilds in the long run.
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Example: Bearing Sizes for a Common Engine
Let’s take a popular example — the Toyota 1NZ-FE engine (used in Corolla, Vitz, Yaris).
| Part | Standard Journal (mm) | Undersize (mm) |
|---|---|---|
| Main Bearing | 47.992 – 48.000 | 47.742 – 47.750 |
| Rod Bearing | 43.992 – 44.000 | 43.742 – 43.750 |
| Thrust Bearing | 48.000 | 47.750 |
This helps you understand how manufacturers specify both standard and undersized ranges for each bearing.
Final Thoughts
Crankshaft bearings may look small, but they carry the entire power of your engine. Understanding their sizes, materials, and clearances is the difference between a smooth-running engine and one that fails prematurely.
Whenever you rebuild or tune an engine, take your time to measure correctly, use quality parts, and double-check your oil clearances. It’s a little effort that saves you a big headache later.
The next time you hear someone talking about “bearing sizes,” you’ll know exactly what they mean — and maybe even help them pick the right one.
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FAQs:
What is the main purpose of crankshaft bearings?
Crankshaft bearings reduce friction between the crankshaft and engine block while supporting the rotating motion under heavy load. They ensure smooth movement of the crankshaft, maintain proper oil clearance, and prevent metal-to-metal contact. Without these bearings, the engine would overheat, seize, or fail due to excessive friction and wear.
How do I know if my crankshaft bearings are worn out?
You can detect worn bearings by listening for knocking or rumbling noises from the engine’s bottom end. Low oil pressure, metal shavings in the oil, and vibration are also signs. If ignored, worn bearings can lead to serious crankshaft damage, requiring a full rebuild or engine replacement.
What does an undersize bearing mean?
An undersize bearing is used when the crankshaft journal has been ground down to remove wear or damage. These bearings are slightly thicker to maintain the correct oil clearance. For example, if a journal is reduced by 0.25 mm, a 0.25 mm undersize bearing compensates for the difference to restore proper fit.
Can I reuse old crankshaft bearings?
It’s not recommended to reuse crankshaft bearings. Once installed, bearings conform to specific journal surfaces and lose their original structure when removed. Reusing them increases the risk of oil clearance issues, metal fatigue, and eventual engine failure. Always replace bearings during rebuilds for consistent performance and long-term reliability.
How do I measure crankshaft bearing clearance?
Use a tool called Plastigage to measure clearance. Place a small strip on the crankshaft journal, install and torque the bearing cap, then remove it to compare the flattened strip’s width with the scale. The reading tells you if your clearance falls within the safe range (typically 0.0015”–0.0025”).
What happens if bearing clearance is too tight or too loose?
If clearance is too tight, the crankshaft may seize or overheat due to lack of oil flow. If too loose, oil pressure drops, and bearings can knock or wear quickly. Both conditions lead to serious engine problems. Proper measurement and selection ensure ideal performance and engine longevity.