Cummins 4BT From Onan Generator Engine Oil Capacity and Key Specs

[MikePhua]

Engine Overview

• The Cummins 4BT (also known as 3.9L 4-BT) is a robust, 4-cylinder inline diesel used in many industrial, generator, and retrofit applications.
  
• It belongs to the Cummins B-Series family, which spans light-to-medium duty diesel engines.
  

Oil Capacity

• For most 4BT variants, the oil capacity with filter change is about 10 quarts (≈ 9.5 liters).
  
• The oil pan (sump) will hold slightly less, depending on whether it's a standard, deep, or midsize pan version.
  
• The specification sheet for the Onan-DGCB generator set (with a 4BT engine) indicates 11.5 quarts, though it’s unclear whether that includes the filter and full system or just up to the oil pan.
  

Other Key Specifications

• Horsepower/Torque: Depending on the application, many 4BTs are rated around 105 hp @ ~2,300 RPM and produce roughly 265 lb-ft torque @ ~1,600 RPM.
  
• Oil Pan Types: There are variations (standard, deep, etc.) that affect oil volume.
  
• Recommended Oil Spec: Diesel engine oil with proper viscosity for operating temperature; filter should be replaced with oil change.
  

Practical Application / What to Do
If you have a 4BT from an Onan generator and are missing the dipstick or unsure of the exact oil pan, here’s a method to determine correct fill:

1. Use a dipstick (even non-original) and mark “Full” level roughly ½ inch below the oil pan-to-block joint, and “Low” about 1 inch below that. This gives a working range.
   
2. Slowly fill oil up to the “Full” mark and measure how much oil you added. That becomes your pan fill number. Then, deposit the filter and top off if needed.
   
3. Always run the engine a few minutes, shut down, recheck level once oil settles.
   

Conclusion
For most Cummins 4BT engines (including those in Onan generators), the proper oil fill with filter is ~10 quarts (≈9.5 liters). If your engine pan is deeper or different, it might need ~11.5 quarts by some specifications. Using a dipstick method for accuracy is wise when original markings are missing.