Retorquing the Head Bolts on a Cummins 855 Engine

[MikePhua]

The Cummins 855 engine, part of the NTA series, is a workhorse found in various heavy-duty applications, including industrial machinery, trucks, and construction equipment. Known for its durability and high performance, the 855 has been used for decades in both on-highway and off-highway applications. One of the essential maintenance tasks when working on the Cummins 855, especially after major engine work like head gasket replacement, is retorquing the cylinder head bolts. This article covers why head bolt retorquing is crucial, how to properly perform this task, and the common issues related to it.
Why is Retorquing the Head Bolts Important?
When performing engine repairs that require the removal of the cylinder head, such as a head gasket replacement or engine rebuild, it is critical to follow the manufacturer’s torque specifications when reassembling the engine. Head bolts, or cylinder head bolts, hold the cylinder head firmly in place and ensure that the engine operates under proper compression. Over time, due to thermal expansion and contraction from the engine’s operation, these bolts can stretch or loosen.
Retorquing the bolts is a process that involves tightening them to their original torque setting after a period of operation. This helps ensure that the cylinder head remains properly sealed, preventing leaks and avoiding engine damage. The process also allows the bolts to settle and ensures the even distribution of pressure across the head gasket, improving the engine's overall efficiency and longevity.
Understanding the Importance of Torque in Engine Head Bolts
Torque is the force applied to the bolts when fastening them. If head bolts are not torqued to the correct value, it can lead to issues like coolant leaks, loss of compression, or even warped cylinder heads. Over-tightening or under-tightening the bolts can cause uneven pressure distribution on the head gasket, which can lead to premature gasket failure.
The torque setting is determined based on the material strength of the bolts, the head, and the gasket. For the Cummins 855 engine, precise torque values must be followed to ensure the longevity and performance of the engine.
How to Retorque the Head Bolts on a Cummins 855 Engine
Before you start retorquing the head bolts, make sure the engine is at the correct temperature—neither too hot nor too cold. Typically, you should perform this task after the engine has been run to operating temperature, and it should cool down enough to be handled safely.
Here’s a step-by-step guide to properly retorque the head bolts on a Cummins 855 engine:

1. Preparation
   • Ensure that the engine is cool enough to handle.
     
   • Gather the required tools, including a high-quality torque wrench, a torque sequence diagram, and the recommended torque specifications from the engine manual.
     
2. Clean the Area
   • Before starting, clean the cylinder head and the head bolts thoroughly to avoid contaminating the threads. Dirt or debris can affect the accuracy of the torque readings.
     
3. Follow the Torque Sequence
   • The head bolt torque sequence for the Cummins 855 engine must be followed precisely. The bolts should be tightened in a criss-cross or "zig-zag" pattern, which ensures uniform compression across the head gasket.
     
   • Start from the center bolts and work outward in stages. This method helps avoid warping the cylinder head or gasket.
     
4. Initial Tightening
   • Tighten each bolt to the initial recommended torque value (usually the first pass involves a lower torque value to ensure the gasket seats evenly).
     
   • Use a calibrated torque wrench to ensure the bolts are tightened to the specified amount.
     
5. Torque to Final Value
   • After the initial round, tighten the bolts to their final torque value. This final pass is typically higher than the initial pass and should be done in several stages, gradually increasing the torque until the specified value is reached.
     
6. Retorque After Engine Run-in
   • After the engine has run for a period of time—usually after about 500-1,000 miles of operation or several hours of work—return to the engine and retorque the head bolts to ensure they are still holding tight and no leakage has developed.
     

Common Issues During Head Bolt Retorquing
Even with proper tools and techniques, retorquing can still present challenges, especially if the engine has been subjected to excessive heat or poor maintenance. Some of the most common issues include:

1. Stretched or Damaged Bolts
   • Repeated thermal cycling can cause head bolts to stretch beyond their usable limits, making it difficult to achieve proper torque. If the bolts are visibly damaged or show signs of fatigue, they should be replaced immediately.
     
2. Uneven Torque Distribution
   • If the torque is applied unevenly, or the wrong sequence is followed, the head gasket may not be compressed evenly, leading to leaks. Always follow the manufacturer’s recommended sequence and torque values.
     
3. Leaking Gasket After Retorquing
   • Even after a proper retorquing, it’s possible that the gasket might still leak if it was improperly installed or if there are signs of damage. It is essential to inspect the gasket condition and replace it if needed before retorquing.
     

Torque Specifications for the Cummins 855 Engine
The specific torque values for head bolts on the Cummins 855 engine can vary based on the model and whether the engine is a naturally aspirated or turbocharged version. However, here are typical torque specifications that apply to many variants of the 855 engine:

• First Torque Pass: 80-90 lb-ft (108-122 Nm)
  
• Second Torque Pass: 120-130 lb-ft (162-176 Nm)
  
• Final Torque Pass: 150-160 lb-ft (203-217 Nm)
  

It’s always best to refer to the specific service manual for your model to verify these values.
Head Bolt Torque Sequence for the Cummins 855
The correct torque sequence ensures an even distribution of pressure across the cylinder head, preventing warping and ensuring the head gasket seals properly. For the Cummins 855 engine, the typical torque sequence follows a "zig-zag" or criss-cross pattern. The exact sequence may vary based on engine configuration, but a typical sequence starts from the middle bolts and moves outward in an alternating pattern.

1. Tighten bolts 1, 2, 3, 4, etc., in the criss-cross pattern.
   
2. Start with the innermost bolts and work outward, alternating sides to maintain balance.
   
3. Repeat the torque procedure in stages until the final value is reached.
   

Tips for Success and Preventing Common Problems

1. Use High-Quality Tools: Always use a high-quality, calibrated torque wrench to ensure accuracy. Cheap or inaccurate tools can lead to improper tightening, which could cause engine damage.
   
2. Inspect Gaskets and Bolts: Before starting the retorquing process, inspect the gasket for any signs of damage, such as tears or cracks. Also, check the head bolts for signs of corrosion, stretching, or wear. Replacing these parts is critical for preventing future leaks or engine issues.
   
3. Follow the Manual: Always refer to the engine’s service manual for the correct specifications, torque values, and torque sequence. Manufacturer recommendations are specific for the engine model and should be adhered to strictly.
   
4. Be Patient: Retorquing head bolts is a critical task, so take your time to ensure that the procedure is done correctly. Rushing through the process can lead to mistakes, resulting in more costly repairs down the road.
   

Conclusion
Retorquing the head bolts on a Cummins 855 engine is a vital part of engine maintenance, particularly after repairs such as head gasket replacement. This procedure ensures that the engine operates efficiently, maintains compression, and avoids costly repairs down the line. By following the correct torque sequence, using high-quality tools, and performing routine checks, operators can keep their engines running smoothly and extend their lifespan.
The Cummins 855 continues to be one of the most trusted engines in the industry due to its durability and reliability, and with proper care and attention to detail, it can deliver years of service.