Cummins NT855 Engine Overhead Adjustment Guide

[MikePhua]

The Cummins NT855 is a diesel engine widely used in heavy equipment. Regular inspection and adjustment of the engine's overhead are crucial for ensuring the engine runs smoothly and lasts longer. Overhead adjustments primarily involve checking and adjusting the valve clearances, which helps maintain engine performance and extend its service life.
Why is Overhead Adjustment Necessary?
During engine operation, the valves can gradually change due to heat and wear, causing the valve clearances to increase or decrease. If the clearance is too large, it can lead to engine noise, reduced power, or increased fuel consumption. If the clearance is too small, the valve may not close properly, causing loss of compression and potentially severe mechanical failures. Therefore, regularly adjusting the valve clearance is essential.
Steps for Overhead Adjustment

1. Preparation
   Before beginning the adjustment, ensure the engine has fully cooled down. Metal parts expand when the engine operates at high temperatures, so adjustments must be made when the engine is cool. You will also need the necessary tools, such as a torque wrench and adjustment tools.
   
2. Locate the Valves
   The valve clearance adjustment on the Cummins NT855 requires accurately locating the piston at the top of the compression stroke. First, rotate the flywheel to find the top dead center (TDC). When the piston is at the top, the marks will align with the scale.
   
3. Check Valve Clearance
   Use a feeler gauge to check the clearance of each valve. Refer to the engine's specific model and operation manual to set the correct valve clearance. Typically, the clearance for intake and exhaust valves will differ, so make sure to follow the manufacturer's guidelines closely.
   
4. Adjust Valve Clearance
   If the clearance is not within the required range, use the adjustment tool to make fine adjustments. Be careful during the process to ensure the valve clearance is set to the manufacturer's recommended specifications. After adjustment, recheck the valve clearance to confirm there are no deviations.
   
5. Lock the Adjustment Nuts
   After the adjustment is complete, lock the adjustment nuts in place to ensure the valve clearance does not change due to vibration or prolonged use. Use a torque wrench to ensure the adjustment nuts are properly tightened.
   

Common Issues and Solutions

1. Excessive Valve Clearance
   If the valve clearance is too large, it can cause engine instability and a decrease in power. The issue can typically be resolved by making the proper adjustments.
   
2. Insufficient Valve Clearance
   Insufficient valve clearance can result in loss of compression, leading to severe engine damage. Adjust the clearance to the proper specification based on the manufacturer's recommendations.
   
3. Difficulty in Adjustment
   In some cases, the adjustment process may be difficult due to long-term use or worn-out components. If this happens, it's advisable to replace any damaged parts or consult a professional technician for assistance.
   

Conclusion
Overhead adjustment on the Cummins NT855 engine is a critical procedure for ensuring proper engine operation and extending its lifespan. By regularly checking and adjusting the valve clearances, you can prevent power loss, increased fuel consumption, and mechanical failures caused by improper clearances. To ensure accuracy and effectiveness, it's recommended to follow the manufacturer's manual and seek guidance from professional technicians when necessary.