Valve Adjustment on the 855 Cummins Engine

[MikePhua]

The 855 Cummins engine is a well-known and widely used diesel engine, known for its durability and reliability in heavy-duty applications. Commonly found in industrial machinery, agricultural equipment, trucks, and generators, this engine has a reputation for providing a solid performance when properly maintained. One of the critical maintenance tasks that must be carried out periodically on the 855 Cummins is the valve adjustment. Proper valve adjustment ensures the engine runs smoothly, efficiently, and without unnecessary wear.
Why Valve Adjustment Is Necessary
Engines like the 855 Cummins are equipped with overhead valves that control the intake and exhaust processes. These valves open and close based on the movement of the camshaft, allowing air and fuel to enter the combustion chamber while expelling exhaust gases. Over time, these valve components—particularly the valve tappets and rocker arms—can experience wear, causing the clearances between the valve and rocker to become misaligned. This misalignment can lead to issues like poor engine performance, increased emissions, and even engine damage if not addressed.
Regular valve adjustments are essential to maintain proper valve clearance. If the clearance is too tight, it can cause the valves to remain slightly open, leading to a loss of compression and poor combustion. If it's too loose, excessive valve noise, misfires, or even valve damage can occur. A proper valve adjustment ensures that the engine's performance remains at its peak and extends the life of its components.
How Valve Adjustment Works on the 855 Cummins
Valve adjustment on the 855 Cummins involves setting the correct clearance between the valve tappets and rocker arms, ensuring proper valve timing. The adjustment procedure can be done using feeler gauges, a tool that measures the gap between two surfaces. Here’s an overview of the process:

1. Preparation
   Before starting, make sure the engine is cool and has not been running for some time. This ensures that metal parts have contracted to their normal operating size and will prevent burns from hot components. Additionally, ensure the engine is parked on a flat surface, and the wheels or vehicle is properly secured.
   
2. Remove the Valve Cover
   The first step in valve adjustment is to remove the valve cover to expose the rocker arms and valves. Depending on the setup, this may involve removing bolts and potentially disconnecting any hoses or components that block access to the engine’s top side.
   
3. Turn the Engine to TDC (Top Dead Center)
   Valve adjustments must be performed at the correct position in the engine’s cycle. For the 855 Cummins, you’ll need to rotate the engine to TDC on the compression stroke for the cylinder you wish to adjust. At this point, both the intake and exhaust valves on that cylinder should be closed, and the piston should be at its highest point in the cylinder.
   
4. Adjust the Valve Clearance
   Using the recommended valve clearance specifications for your particular engine model (typically specified in the Cummins service manual), insert the appropriate feeler gauge between the rocker arm and the valve tappet. Adjust the nut on the rocker arm to achieve the correct clearance. It’s important to set the clearance to the exact specification for the engine to run smoothly and efficiently.
   
5. Repeat for Other Cylinders
   Once the first cylinder is adjusted, turn the engine to TDC for the next cylinder in the firing order, and repeat the valve adjustment procedure. Be sure to check each cylinder individually, as each valve requires its own adjustment. In a typical 6-cylinder 855 Cummins engine, you would need to repeat this process six times.
   
6. Reassemble and Test
   After all cylinders have been adjusted, replace the valve cover and tighten all bolts properly. It is also a good idea to check that all other engine components are reconnected properly, such as hoses and electrical connections. Once everything is reassembled, start the engine and listen for any unusual sounds. A smooth, quiet engine suggests the adjustments have been successful.
   

Tools Needed for Valve Adjustment
To perform a valve adjustment on the 855 Cummins engine, you will need the following tools:

• Feeler Gauges: These are used to measure the clearance between the valve and the rocker arm. The feeler gauge should be calibrated to the manufacturer’s recommended thickness for the specific valve clearance.
  
• Wrench or Socket Set: For adjusting the rocker arm nuts and other engine fasteners.
  
• Torque Wrench: Used to ensure that the nuts and bolts are tightened to the manufacturer’s specifications.
  
• Screwdrivers: Depending on the engine configuration, you may need screwdrivers to remove components that obstruct access to the valve cover.
  
• Piston Positioning Tool (Optional): This tool helps ensure the piston is at TDC for each cylinder.
  

Common Issues Caused by Improper Valve Adjustment

1. Engine Misfire: If the valve clearance is set too tight, the valve may not fully close, causing compression loss and misfires. A misfire can lead to rough engine operation, higher emissions, and potential damage to the cylinder head or piston.
   
2. Excessive Engine Noise: If the valve clearance is set too loose, the engine may experience excessive tapping or knocking sounds from the rocker arms. Over time, this can cause undue wear on the engine components and reduce overall engine efficiency.
   
3. Poor Fuel Economy: Incorrect valve clearance affects the combustion process, leading to incomplete combustion, which in turn can result in reduced fuel efficiency. Proper valve adjustment ensures optimal engine performance and fuel usage.
   
4. Increased Exhaust Emissions: When the valves do not open or close properly, it can lead to incomplete combustion, which contributes to higher emissions. Proper valve adjustment ensures better combustion and cleaner exhaust gases.
   

Valve Adjustment Intervals for the 855 Cummins
The 855 Cummins engine, like many diesel engines, has specific maintenance intervals for valve adjustment. Depending on the engine's use, these intervals can vary. However, as a general guideline:

• For heavy-duty applications (e.g., industrial equipment or machinery working long hours), valve adjustments are often recommended every 1,000 to 1,500 hours of operation.
  
• For lighter-duty use, adjustments may be needed less frequently, typically around 2,000 to 3,000 hours of operation.
  

The engine’s performance and the environment in which it operates will determine how often valve adjustments should be performed. For example, engines that operate in dusty or harsh conditions may require more frequent adjustments to account for faster wear of the valve components.
Conclusion
Regular valve adjustments on the 855 Cummins engine are essential for ensuring the engine’s longevity, performance, and efficiency. A well-adjusted valve system leads to optimal combustion, reduced wear on the engine’s internal components, and smoother operation. While the valve adjustment process itself is not overly complicated, it does require attention to detail, precision, and the right tools. By following the proper procedure and adhering to manufacturer guidelines, owners and operators can keep their 855 Cummins engines running smoothly for many years to come.