Cummins 250 Truck Motor Overhead Adjustment Procedure

[MikePhua]

Cummins engines are renowned for their durability and performance, making them a popular choice in heavy-duty truck applications. However, like any engine, they require regular maintenance to ensure optimal performance. One essential maintenance task for maintaining the engine’s efficiency is adjusting the overhead, or valve clearance, on the Cummins 250 truck motor.
The overhead adjustment procedure is critical because it ensures the valves are operating within the correct tolerances. Incorrect valve adjustment can lead to performance issues, including rough idling, decreased engine power, and, in extreme cases, engine damage. In this article, we will explore the overhead adjustment procedure for the Cummins 250 truck motor, common issues that can arise during the process, and solutions to address these problems.
Overview of the Cummins 250 Truck Motor
The Cummins 250 truck motor is part of the Cummins NTA885 engine family. This particular engine is known for its power and efficiency in various heavy-duty applications, including trucks, buses, and construction machinery. The Cummins 250 delivers 250 horsepower, with a torque output designed to handle demanding tasks.
One of the standout features of Cummins engines like the 250 is their mechanical simplicity. While the engine is packed with advanced technology, the mechanical components like the valve train still require periodic adjustment to ensure they function properly. This is where the overhead adjustment procedure comes into play.
Why Overhead Adjustment is Important
Overhead adjustment refers to the fine-tuning of the engine’s valve clearance. This is essential because the clearance between the valve and the camshaft needs to be within a specific range to ensure that the engine operates efficiently.

• If the clearance is too tight, the valves may not fully close, leading to a loss of compression and, over time, valve damage.
  
• If the clearance is too loose, the valves may not open fully, reducing engine performance and potentially leading to more significant engine issues such as overheating or misfiring.
  

Proper valve adjustment ensures that the engine runs smoothly, the combustion process is efficient, and that the engine components don’t experience unnecessary wear.
The Overhead Adjustment Procedure
Adjusting the overhead on a Cummins 250 truck motor requires precision and the right tools. The process typically involves adjusting the rocker arms, which control the opening and closing of the engine’s valves.
Tools Required
Before starting the adjustment process, ensure you have the following tools:

• Torque wrench
  
• Feeler gauges
  
• Socket set
  
• Screwdriver or wrench for valve cover removal
  
• Engine manual (to verify specifications)
  

Steps for Overhead Adjustment

1. Preparation: Begin by ensuring that the engine is cool. The overhead adjustment procedure should always be performed when the engine is not running, as the valve temperatures can affect the clearance readings.
   
2. Remove the Valve Cover: Use your wrench or socket set to remove the valve cover. Be cautious of any debris or old gaskets that might be present.
   
3. Rotate the Engine to Top Dead Center (TDC):
   • Locate the flywheel and rotate it until the number one cylinder is at top dead center. This is essential for accurately adjusting the valves.
     
   • You’ll know the engine is at TDC when the intake valve for cylinder one just begins to close, and the exhaust valve opens.
     
4. Adjust the Valves:
   • Using a feeler gauge, measure the clearance between the rocker arm and valve stem. The specific clearance will be detailed in your engine’s manual, but for a typical Cummins 250 engine, the intake valve clearance is around 0.010 inches, and the exhaust valve clearance is 0.020 inches.
     
   • Adjust the rocker arms using the adjusting screw to achieve the correct clearance. Tighten or loosen the nut as necessary to adjust the clearance.
     
5. Torque the Nuts: Once the desired clearance is achieved, use the torque wrench to ensure the adjusting nuts are tightened to the manufacturer’s specified torque.
   
6. Repeat for Other Cylinders: Rotate the engine to the next cylinder and repeat the adjustment procedure for each valve. For a six-cylinder engine, ensure each cylinder’s intake and exhaust valves are adjusted properly.
   
7. Reassemble: After all adjustments have been made, replace the valve cover, ensuring that any gaskets are properly seated and that no debris is left behind.
   
8. Test the Engine: After completing the overhead adjustment, start the engine to check for any unusual sounds. The engine should run smoothly with no ticking or clattering noises from the valve train.
   

Common Issues During Overhead Adjustment
While performing overhead adjustments on the Cummins 250 truck motor, several issues may arise. Let’s explore some of the most common problems and their potential solutions.
1. Incorrect Valve Clearance Readings
Sometimes, even after making adjustments, the engine still seems to exhibit performance issues. This can occur if the clearance measurements were incorrect.

• Solution: Double-check the feeler gauge measurements. Make sure the correct gauge thickness is used for both intake and exhaust valves. Refer to the service manual for the specific clearance values.
  

2. Difficulty Adjusting the Rocker Arms
If the rocker arms seem difficult to adjust or won't hold the proper clearance, the rocker arm assembly may be worn out, or the adjusting screws may be damaged.

• Solution: Inspect the rocker arms for wear. If they appear worn, it may be necessary to replace them. Also, check the adjusting screws to ensure they aren’t stripped or damaged. Replacing worn components will help maintain proper valve adjustment.
  

3. Over-tightened Valve Nuts
Over-tightening the valve nuts can lead to valve damage or excessive stress on the valve springs, which can affect engine performance.

• Solution: Always use the correct torque specifications when tightening the nuts. Over-tightening can lead to additional wear, while under-tightening may cause the valves to lose their adjustment.
  

4. Inconsistent Valve Operation After Adjustment
In some cases, the engine may still run poorly after the overhead adjustment, with the valves not operating smoothly or efficiently.

• Solution: After adjusting, check the entire valve train for lubrication. Ensure the rocker arms and push rods are properly lubricated. Insufficient lubrication can lead to poor valve operation and further engine problems.
  

Maintenance Tips for Longer Engine Life
Regular valve adjustments are just one aspect of maintaining the Cummins 250 engine. Here are a few additional tips to keep the engine running smoothly:

• Check for Leaks: After performing maintenance, always check for oil leaks, especially around the valve cover and gaskets.
  
• Use Quality Oil: Always use the manufacturer-recommended oil type and change it at regular intervals.
  
• Monitor Engine Temperatures: Overheating can lead to valve and gasket damage. Keep an eye on engine temperatures during operation to avoid costly repairs.
  

Conclusion
Overhead adjustment is a crucial part of maintaining the performance and longevity of the Cummins 250 truck motor. By ensuring the correct valve clearance, operators can prevent many common engine issues, such as rough idling, poor performance, or even catastrophic engine failure. Regular maintenance, correct adjustments, and the use of quality parts will keep the Cummins 250 running at its best for years to come. Whether you’re an owner-operator or a fleet manager, understanding the overhead adjustment procedure will help keep your engine in top shape, minimizing downtime and costly repairs.