Repairing Worn Equalizer Bar Bushing Bores on the Caterpillar D8N

[MikePhua]

The Caterpillar D8N and Its Legacy
The Caterpillar D8N crawler dozer was introduced in the late 1980s as part of Caterpillar’s ongoing evolution of the D8 series, which dates back to the 1930s. Caterpillar Inc., founded in 1925, has long been a global leader in heavy equipment manufacturing, and the D8 line has been one of its most iconic offerings. The D8N featured a 3406 diesel engine producing around 300 horsepower, a torque converter drive train, and a suspended undercarriage system that improved traction and reduced shock loads. With thousands of units sold worldwide, the D8N became a staple in mining, forestry, and large-scale earthmoving operations. Its robust frame and modular design made it a favorite among mechanics and operators alike.
Understanding the Equalizer Bar System
The equalizer bar is a critical component in the undercarriage of large track-type tractors like the D8N. It connects the left and right track roller frames to the machine’s main frame, allowing vertical movement and distributing ground forces evenly. This system enhances stability and ride comfort, especially on uneven terrain.
Key components include:

• Equalizer bar
  
• Bushing bores (also called saddle bores)
  
• Pivot shaft
  
• Retainer caps
  
• Grease fittings and seals
  

The bar pivots within bushings housed in the saddle bores on each side of the main frame. These bores are machined to precise tolerances and are designed to hold bronze or steel bushings that support the rotating shaft.
Symptoms of Wear and Damage
Over time, the saddle bores can become worn due to:

• Lack of lubrication
  
• Contaminant ingress (mud, sand, water)
  
• Excessive side loading during operation
  
• Misalignment of the equalizer bar or pivot shaft
  
• Bushing rotation within the bore instead of on the shaft
  

Common symptoms include:

• Excessive movement or “clunking” during turns
  
• Uneven track wear
  
• Grease leakage around the bore caps
  
• Difficulty maintaining alignment during bushing replacement
  
• Visible ovaling or scoring inside the bore
  

In one case from Nevada, a D8N used in a copper mine developed severe saddle bore wear after 18,000 hours of operation. The bushings had spun inside the bores, enlarging them beyond spec and causing the pivot shaft to shift under load.
Inspection and Measurement Techniques
Before any repair, thorough inspection is essential. Recommended steps include:

• Remove equalizer bar and pivot shaft
  
• Clean bore surfaces with solvent and wire brush
  
• Use inside micrometer or bore gauge to measure diameter at multiple points
  
• Compare readings to OEM specifications (typically within ±0.005 inches)
  
• Check for taper, ovality, and surface pitting
  

If bore wear exceeds tolerance, standard bushing replacement will not restore proper fit. In such cases, bore repair is necessary.
Repair Options for Worn Bores
There are several approaches to restoring worn saddle bores:

1. Line boring and oversize bushings
   • Machine the bores to a larger diameter
     
   • Install custom or oversize bushings
     
   • Ensure concentricity between left and right bores
     
   • Requires portable line boring equipment and skilled technician
     
2. Sleeving the bore
   • Install a hardened steel sleeve into the worn bore
     
   • Machine sleeve ID to accept standard bushing
     
   • Retains original bushing size and simplifies future maintenance
     
   • Must ensure sleeve is press-fit and secured with Loctite or welds
     
3. Welding and remachining
   • Build up bore surface with MIG or TIG weld
     
   • Machine back to original diameter
     
   • Risk of heat distortion and cracking if not done properly
     
   • Best suited for minor wear or non-critical applications
     
4. Epoxy-based bore restoration
   

• Use industrial-grade metal-filled epoxy to rebuild bore
  
• Machine epoxy after curing
  
• Not recommended for high-load applications like dozers
  

Terminology Clarification

• Line boring: A precision machining process that restores concentric bores using a rotating cutting bar
  
• Ovality: Deviation from a perfect circle, often caused by uneven wear
  
• Sleeving: Inserting a cylindrical liner to restore bore dimensions
  
• Pivot shaft: The rotating shaft that passes through the equalizer bar and bushings
  
• Saddle bore: The machined cavity in the frame that houses the bushing
  

Field Story and Lessons Learned
In 2021, a contractor in Queensland, Australia, faced downtime on a D8N due to severe equalizer bar movement. After discovering bore wear beyond 0.030 inches, they opted for line boring and oversize bushings. The repair took two days and cost approximately $6,500 AUD, but restored full functionality. The operator noted improved track alignment and reduced vibration immediately after the fix.
Preventive Measures and Maintenance Tips
To extend the life of equalizer bar components:

• Grease pivot points every 50–100 hours
  
• Use high-pressure moly-based grease for better film strength
  
• Inspect seals and caps during routine service
  
• Avoid aggressive side loading during ripping or pushing
  
• Monitor track alignment and roller frame movement
  

Industry Trends and Support Solutions
With the rise of telematics, some fleets now monitor equalizer bar movement using frame-mounted sensors. These systems detect abnormal oscillation and alert operators before mechanical failure. In 2023, Caterpillar introduced a retrofit sensor kit for legacy D-series dozers, allowing predictive maintenance on undercarriage components.
Final Recommendations

• Measure bore wear before ordering bushings
  
• Choose repair method based on severity, budget, and technician skill
  
• Document all dimensions and part numbers for future reference
  
• Consider sleeving for long-term durability and ease of future service
  
• Train technicians on proper alignment and torque specs during reassembly
  

The equalizer bar system on the Caterpillar D8N is a vital part of its suspension and stability. While bore wear is inevitable over time, proper inspection, repair, and preventive care can restore performance and extend machine life. With the right tools and knowledge, even severe wear can be corrected without compromising structural integrity.