3978818 3978820 Main Bearing for Cummins 4B3.9/6A3.4/6B5.9

The Cummins 3978818 and 3978820 are main bearing sets designed for several Cummins engine series, including the 4B3.9, 6A3.4, and 6B5.9. Below are the detailed specifications and compatibility notes:

Main Bearing Specifications

Part Numbers:

• 3978818: Lower shell (thrust bearing)

• 3978820: Upper shell

Key Features:

• Material: Tri-metal (steel-backed with copper-lead intermediate layer and babbitt overlay)

• Width: Standard for 3.9L/5.9L crankshaft journals

• Thickness: Varies by engine application (measured in “thou” for proper clearance)

• Bearing Clearance: 0.05–0.10 mm (check with plastigauge)

Compatible Engines:

✔ 4B3.9 (Industrial/Agricultural)
✔ 6A3.4 (Older industrial models)
✔ 6B5.9 (Includes Dodge Ram 12V/24V engines up to 1998)

⚠ Not for:

• ISB 5.9L (1998.5+) – Uses different bearing designs (e.g., 4085643).

Installation Notes

1. Torque Sequence:

   • Main cap bolts: 160–180 Nm (varies by engine) + 90° turn (stretch bolts).

2. Critical Checks:

   • Measure crankshaft journal wear (max 0.025 mm taper/out-of-round).

   • Verify thrust bearing endplay (0.10–0.30 mm).

3. Lubrication:

   • Coat bearings with assembly lube during installation.

Cross-Reference

Warning

• Always replace bearings in matched sets (upper/lower).

• Do not mix old/new bearings – uneven wear risks crankshaft damage.