Product Description
Cummins 3978820 — Main Bearing (Standard), Lower Shell for B-Series 5.9L/6.7L
MPN: 3978820 | Also Known As: lower main bearing shell (STD) | Related: 3978818 (upper shell), 3978822 (thrust), 4955853 (STD bearing set)
Why This Part
- ✓ OE lower main shell for precise crank support and oil clearance.
- ✓ Direct-fit for 5.9L & 6.7L B-Series/ISB platforms (see fitment).
- ✓ Standard size shell for stock crank journals; pair with correct upper & thrust.
- ✓ Durable overlay construction to handle diesel loads and long service life.
Quick Specs
| Type | Main bearing, lower shell (STD) |
|---|---|
| OE Part # | 3978820 (lower). Match with 3978818 (upper) & 3978822 (thrust) as required. |
| Qty per Engine | Typically 7 lowers; sold individually. Verify by ESN/build. |
| Set Reference | For a complete STD set, see Cummins 4955853. |
Verified Fitment (Automotive)
| Year Range | Engine | Chassis |
|---|---|---|
| 1989–2002 | Ram 5.9L (12V & 24V VP44) | Ram 2500/3500 |
| 2003–2007 | Ram 5.9L ISB (24V Common-Rail) | Ram 2500/3500 |
| 2007.5–2024 | Ram 6.7L ISB (24V Common-Rail) | Ram 2500/3500 |
Notes: Bearings are selected by journal size and grade. Verify by ESN and crank specs, especially on 2019+ trucks and any machined cranks.
Often Paired With
- Upper Main Shells (STD): Cummins 3978818
- Thrust Bearing: Cummins 3978822
- Complete STD Set: Cummins 4955853 (includes uppers/lowers + thrust)
- Lower Engine Gasket Kit: match to model year when doing bottom-end work
What’s In The Box
- (1) Genuine Cummins Main Bearing Lower Shell (STD) — 3978820
- Other shells and thrust bearings sold separately (see “Often Paired With”).
Install Tips
- Measure crank journals; confirm standard size vs. undersize (.010, .020, etc.).
- Clean oil passages; use assembly lube and observe correct shell orientation.
- Torque main caps to spec; verify oil clearance and crank endplay.
FAQ
IS 3978820 THE UPPER OR LOWER MAIN BEARING?3978820 is the lower main bearing shell (STD). Pair with 3978818 (upper) and 3978822 (thrust) as required.
HOW MANY MAIN BEARINGS DOES MY ENGINE USE?Typical B-Series/ISB engines use multiple pairs plus a thrust location. A standard set (4955853) includes lowers, uppers, and thrust components; this listing is for one lower shell.
WILL THIS FIT BOTH 5.9L AND 6.7L?Yes—3978820 is used across 5.9L (1989–2007) and 6.7L (2007.5–2024) applications; always verify by ESN and journal size.
WHAT IF MY CRANK IS MACHINED UNDERSIZE?Select the correct undersize bearing set (e.g., .010, .020). 3978820 is the standard size lower shell.
CAN I JUST REPLACE ONE SHELL?Best practice is to inspect all journals and replace bearings as a matched set when wear is present. Mixing worn and new shells can shorten life.
