Cold Injector and Valve Settings for the Cummins 855 Big Cam Engine

[MikePhua]

The Cummins 855 Big Cam and Its Legacy
The Cummins 855 Big Cam series was introduced in the mid-1970s as an evolution of the original NH series, bringing electronic fuel control and improved combustion efficiency to heavy-duty diesel applications. With a displacement of 14 liters and horsepower ratings ranging from 250 to over 400 depending on configuration, the 855 Big Cam became a staple in mining, trucking, and construction. Its robust inline-six design, mechanical simplicity, and long service intervals earned it a reputation for reliability across North America and beyond.
Cummins Inc., founded in 1919 in Columbus, Indiana, has produced millions of engines globally. The 855 Big Cam was one of its most successful platforms, with multiple CPL (Control Parts List) variants tailored to specific applications. Though production ceased decades ago, the engine remains in service in legacy fleets, rebuilt units, and restoration projects.
Terminology Annotation
- Injector Setting: The calibrated adjustment of the fuel injector plunger or rack to ensure proper fuel delivery timing and volume.
- Valve Clearance: The gap between the valve stem and rocker arm, measured cold to ensure correct valve timing and prevent premature wear.
- CPL Number: A unique identifier used by Cummins to specify the exact configuration of an engine, including camshaft profile, injectors, and timing.
- Top Stop Injector: A type of injector used in later Big Cam engines, designed to simplify adjustment by eliminating the need for rack settings.
Why Cold Settings Matter
Setting injectors and valves cold—typically at ambient temperature before the engine has run—ensures accurate measurements unaffected by thermal expansion. This is especially critical in older engines like the 855 Big Cam, where mechanical tolerances are tight and wear can introduce variability.
Incorrect settings can lead to:

• Hard starting or misfiring
  
• Excessive smoke or poor fuel economy
  
• Valve damage due to insufficient clearance
  
• Injector overfueling or underfueling
  

A Story from the Field
In Lancashire, England, a restorer working on a Cummins 855 Big Cam in a vintage haul truck needed cold settings for injectors and valves. Without the CPL number, he faced uncertainty, as different variants used different injectors and cam profiles. A retired Cummins technician advised checking the injector type—Top Stop or non-Top Stop—and using a feeler gauge to set intake valves to 0.014" and exhaust valves to 0.027" cold, a common baseline for many Big Cam configurations. The injector setting, if Top Stop, was adjusted using the zero-lash method with the cam follower on base circle. The truck started smoothly and ran clean, confirming the settings were appropriate.
Recommended Cold Settings (Typical Values)
While exact settings depend on CPL and injector type, common cold values for the Cummins 855 Big Cam are:

• Intake Valve Clearance: 0.014 inches
  
• Exhaust Valve Clearance: 0.027 inches
  
• Injector Setting (Top Stop): Zero lash at base circle, then torque to spec
  
• Injector Setting (Non-Top Stop): Requires rack gauge and dial indicator, varies by CPL
  

Always verify with the engine’s CPL number and consult the Cummins service manual for precise specifications.
Adjustment Procedure Overview
To set valves and injectors cold:

• Rotate engine to Top Dead Center (TDC) for cylinder 1
  
• Use valve adjustment chart to determine firing order and adjustment sequence
  
• Insert feeler gauge between rocker arm and valve stem
  
• Adjust lock nut until slight drag is felt on gauge
  
• Torque lock nut to manufacturer spec
  
• For injectors, ensure cam follower is on base circle and follow Top Stop or rack procedure
  

Use a barring tool or flywheel wrench to rotate the engine manually. Always mark the crankshaft position to avoid misalignment during multi-cylinder adjustment.
Preventative Maintenance and Best Practices
To maintain optimal performance:

• Check valve and injector settings every 10,000 to 15,000 miles or annually
  
• Use OEM gaskets and torque specs during adjustment
  
• Inspect rocker arms and pushrods for wear or bending
  
• Replace injector O-rings and seals during adjustment if signs of leakage are present
  
• Keep a record of CPL number and previous settings for future reference
  

For engines operating in extreme climates, consider adjusting clearances slightly to account for thermal expansion and contraction.
Industry Trends and Legacy Support
As of 2025, Cummins continues to support legacy engines through its QuickServe platform and dealer network. Rebuilt 855 Big Cam engines are still in demand for off-road applications, and aftermarket suppliers offer complete overhaul kits, injectors, and camshafts.
Some enthusiasts have begun retrofitting electronic monitoring systems to track valve lash and injector timing in real time, though most restorers prefer the tactile precision of manual adjustment.
Conclusion
Setting cold injector and valve clearances on the Cummins 855 Big Cam is a foundational task for maintaining engine health and performance. With attention to CPL details, injector type, and proper procedure, operators can ensure smooth operation and extend the life of this legendary powerplant. Whether in a haul truck, scraper, or generator, the 855 Big Cam continues to earn its place in diesel history—one adjustment at a time.