Product Description
The Cummins 3978820 OEM Main Bearing (Standard) – Lower Shell is a genuine crankshaft main-bearing insert used across 5.9L and 6.7L Cummins B-Series engines from 1989–2024. Engineered for precise oil clearance, long-term durability, and optimal crankshaft support, this lower main bearing shell meets strict Cummins metallurgical and dimensional standards—ensuring reliability for stock, towing, fleet, and performance applications. Whether completing an in-frame overhaul or a full engine build, this genuine Cummins bearing provides the exact fit and lubrication profile required to protect your crankshaft and bottom-end components.
Fitment
Automotive Applications:
- 1989–1998 5.9L 12-Valve Cummins (6BT)
- 1998.5–2002 5.9L 24-Valve ISB Cummins
- 2003–2007 5.9L Cummins Common Rail
- 2007.5–2024 6.7L Cummins (ISB / ISBe / B6.7)
Industrial / Off-Highway B-Series Engines:
- 4BT / 4BTA 3.9L
- 6BT / 6BTA 5.9L
- QSB / ISB / ISBe / B6.7 platforms
Note: Always verify bearing size (standard vs oversize) and crankshaft journal condition before installation.
Cross-Reference (Cummins-Only)
Other genuine Cummins bottom-end components:
• Cummins 3978819 OEM Main Bearing – Upper Shell (Standard)
• Cummins 4891178 Connecting Rod Bearing (Standard)
• Cummins 3900267 Tappet Cover Bolt Grommet Seal
FAQ
Is Cummins 3978820 the lower or upper main bearing shell?
The 3978820 is the lower main bearing shell. Most Cummins B-Series engines use different part numbers for upper vs. lower bearings.
Does this bearing fit both 5.9L and 6.7L engines?
Yes — this lower main bearing is used across 1989–2024 B-Series platforms, including 5.9L and 6.7L ISB engines.
How do I confirm standard bearing size?
Measure crankshaft journals and review service history. Engines that have been machined previously may require oversize bearings.
Should I replace all main bearings at once?
Yes. Cummins recommends replacing the entire bearing set during overhaul to ensure even wear, lubrication, and crankshaft stability.
Specifications for cummins 3978820
| Specification | Value |
|---|---|
| Brand | Cummins Genuine Parts |
| Part Number | 3978820 |
| Bearing Type | Main Bearing – Lower Shell (Standard Size) |
| UPC | Not Published |
| GTIN | Not Published |
| Cross Reference | Related: 3978819 (Upper Standard Shell) |
| E-Codes | N/A |
| Shipping Weight | 0.20 lbs |
| Package Dimensions | 3 x 3 x 1 in |
| Prop 65 Warning | Not Applicable |
