Product Description
Cummins 3978820 OEM Main Bearing (Standard) Lower Shell – 5.9L & 6.7L Cummins (1989–2024)
The Cummins 3978820 OEM Main Bearing (Standard) Lower Shell is a genuine Cummins replacement bearing designed for 5.9L and 6.7L Cummins diesel engines. Manufactured to meet original equipment specifications, this standard-size lower main bearing shell provides proper crankshaft support, oil clearance, and long-term durability for stock rebuilds, performance engine builds, and heavy-duty repair applications.
Whether you are rebuilding a 12-valve, 24-valve, Common Rail, or late-model 6.7L Cummins engine, the Cummins 3978820 main bearing helps maintain proper crankshaft alignment and oil film integrity under demanding operating conditions. Genuine Cummins bearings are engineered to deliver the fit, finish, and reliability required for long engine life.
Features
- Genuine Cummins OEM main bearing
- Standard-size lower shell bearing
- Precision manufactured to OEM specifications
- Designed for proper crankshaft support and alignment
- Helps maintain correct oil clearance
- Provides long service life and durability
- Ideal for stock rebuilds and performance engine builds
- Direct replacement for worn or damaged main bearings
- Built to Cummins factory quality standards
- Suitable for street, towing, fleet, and heavy-duty applications
Fitment
- 1989–1998 Cummins 5.9L 12-Valve Engines
- 1998.5–2002 Cummins 5.9L 24-Valve Engines
- 2003–2007 Cummins 5.9L Common Rail Engines
- 2007.5–2024 Cummins 6.7L Common Rail Engines
- Dodge Ram 2500 Applications
- Dodge Ram 3500 Applications
- RAM 2500 Applications
- RAM 3500 Applications
- Industrial and Commercial Cummins B-Series Engines
Note: This listing is for one standard-size lower main bearing shell. Verify bearing size requirements and engine specifications before ordering. Complete engine rebuilds typically require multiple bearing shells.
What's Included
- One Genuine Cummins 3978820 Standard Lower Main Bearing Shell
Specifications
- Manufacturer: Cummins
- Part Number: 3978820
- Product Type: Main Bearing
- Bearing Position: Lower Shell
- Bearing Size: Standard
- Material: OEM Bearing Alloy Construction
- Engine Family: Cummins B-Series
- Engine Compatibility: 5.9L and 6.7L Cummins
- Application: Engine Rebuild and Repair
- OEM Replacement: Yes
- Fitment Range: 1989–2024 Cummins Applications
- Condition: New Genuine Cummins Component
Cross Reference Numbers
3978820, Cummins 3978820, OEM Cummins Main Bearing, Standard Lower Main Bearing Shell, Cummins B-Series Main Bearing
FAQ
What does the Cummins 3978820 main bearing fit?
The Cummins 3978820 standard lower main bearing fits many 5.9L and 6.7L Cummins B-Series diesel engines used in Dodge Ram and RAM truck applications from 1989 through current production years.
Is this a standard-size bearing?
Yes. The 3978820 is a standard-size lower main bearing shell designed for crankshafts that remain within factory specifications.
Is this an OEM Cummins part?
Yes. This is a genuine Cummins replacement component manufactured to original equipment specifications.
How many bearings are required for a complete engine rebuild?
The number of bearings required depends on the engine configuration and rebuild requirements. Always consult the factory service manual and parts catalog for your specific engine.
Can this bearing be used in performance builds?
Yes. Genuine Cummins main bearings are commonly used in stock, towing, fleet, and performance engine builds where OEM fitment and reliability are desired.
Why is proper bearing clearance important?
Proper bearing clearance helps maintain oil film thickness, reduces friction, supports crankshaft alignment, and contributes to long engine life and reliability.
