09-11-2025, 10:10 AM
Engine Overview
Oil Capacity
Other Key Specifications
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:
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.
- 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:
- 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.
- 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.
- 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.
