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The Big Cam III and Its Legacy
The Cummins Big Cam III engine, part of the iconic 855 cubic inch series, was introduced in the early 1980s as a refinement of the Big Cam II. Cummins, founded in 1919, had already established dominance in the heavy-duty diesel market, and the Big Cam III continued that legacy with improved fuel control, dual-volute turbocharging, and enhanced emissions compliance. These engines powered thousands of Class 8 trucks across North America, and many remain in service today due to their rebuildability and parts availability.
Why Add Jake Brakes
Jake brakes, or engine compression release brakes, are essential for controlling speed on grades without relying solely on service brakes. On the Big Cam III, installing a Jake brake system improves downhill safety, reduces brake wear, and enhances driver control. The 401B model Jake brake is compatible with the Big Cam III, provided the engine has a dual-entry turbo and meets other configuration requirements.
Terminology Notes
Proper valve and injector adjustment is critical after installing Jake brakes. The Big Cam III follows a specific sequence:
Field Insights and Final Outcome
After sourcing the correct parts and performing a full overhead adjustment, the engine ran smoothly and the Jake brakes functioned as intended. However, a dead #2 cylinder was later diagnosed, traced to an injector rocker that couldn’t take up slack. This was resolved by inspecting the push tubes and confirming proper seating.
Recommendations for Future Installations
Installing Jake brakes on a Cummins Big Cam III requires careful attention to part compatibility, adjustment procedures, and mechanical fitment. While the process can be labor-intensive, the result is a safer, more controllable truck with extended brake life. With the right tools and knowledge, even older engines like the Big Cam III can be upgraded to meet modern performance expectations.
The Cummins Big Cam III engine, part of the iconic 855 cubic inch series, was introduced in the early 1980s as a refinement of the Big Cam II. Cummins, founded in 1919, had already established dominance in the heavy-duty diesel market, and the Big Cam III continued that legacy with improved fuel control, dual-volute turbocharging, and enhanced emissions compliance. These engines powered thousands of Class 8 trucks across North America, and many remain in service today due to their rebuildability and parts availability.
Why Add Jake Brakes
Jake brakes, or engine compression release brakes, are essential for controlling speed on grades without relying solely on service brakes. On the Big Cam III, installing a Jake brake system improves downhill safety, reduces brake wear, and enhances driver control. The 401B model Jake brake is compatible with the Big Cam III, provided the engine has a dual-entry turbo and meets other configuration requirements.
Terminology Notes
- Jake Brake: A device that opens exhaust valves during the compression stroke, converting engine power into braking force.
- Crosshead: A bridge component between the rocker arm and valve stem, often modified to accommodate Jake brake actuation.
- Slave Piston: A hydraulic actuator inside the Jake housing that pushes on the crosshead to open the exhaust valve.
- Valve Lash: The clearance between the rocker arm and valve stem, critical for proper operation.
- Adjustment Screw Length: The original injector rocker adjustment screws were too short for the Jake brake setup. A longer screw was sourced from a salvage yard, resolving the initial issue.
- Exhaust Valve Clearance Screws: The screws provided with the Jake brake kit were also too short. Investigation revealed that the correct configuration requires a hollow adjustment screw with an internal pin that transmits force from the slave piston to the exhaust valve. The solid screws from non-Jake crossheads were incompatible.
- Crosshead Compatibility: The kit included wider crossheads, but installation was blocked by interference with the rocker boxes. The solution involved removing the rocker boxes entirely, replacing the crossheads, and re-running the overhead valve adjustment.
- Part Number Discovery: The correct adjustment screw and pin assembly was identified as part number 013555, which allowed the Jake brake to function properly.
Proper valve and injector adjustment is critical after installing Jake brakes. The Big Cam III follows a specific sequence:
- Valve Lash Settings:
- Intake: 0.011 inches
- Exhaust: 0.023 inches
- Intake: 0.011 inches
- Injector Torque: 6 inch-pounds
- Adjustment Sequence:
- A valves #5, injector #3
- B valves #3, injector #6
- C valves #6, injector #2
- A valves #2, injector #4
- B valves #4, injector #1
- C valves #1, injector #5
Field Insights and Final Outcome
After sourcing the correct parts and performing a full overhead adjustment, the engine ran smoothly and the Jake brakes functioned as intended. However, a dead #2 cylinder was later diagnosed, traced to an injector rocker that couldn’t take up slack. This was resolved by inspecting the push tubes and confirming proper seating.
Recommendations for Future Installations
- Verify Compatibility Using Cummins/Jacobs Charts: Ensure the Jake brake model matches the engine CPL and turbo configuration.
- Inspect Salvage Parts Carefully: Not all components from used kits are correct or complete.
- Use OEM or Verified Rebuild Sources: The 401 series Jake brakes are increasingly difficult to source, and parts availability is limited.
- Run the Overhead Twice: After any major top-end work, recheck valve and injector settings to catch misalignment or seating issues.
Installing Jake brakes on a Cummins Big Cam III requires careful attention to part compatibility, adjustment procedures, and mechanical fitment. While the process can be labor-intensive, the result is a safer, more controllable truck with extended brake life. With the right tools and knowledge, even older engines like the Big Cam III can be upgraded to meet modern performance expectations.
