Repairing Severely Worn Trunnion Assemblies on Caterpillar Dozers

[MikePhua]

The Role of Trunnions in Blade Control
Trunnions are spherical bearing assemblies that allow the dozer blade to pivot and articulate under load. On Caterpillar machines—especially larger models like the D7, D8, and D6 series—the trunnion ball sits between the push arms and the blade, absorbing torsional stress and enabling multi-directional movement. These components are critical for maintaining blade stability, especially on machines equipped with U-blades or 4-way configurations.
Over time, trunnion assemblies wear due to constant pressure, vibration, and exposure to abrasive materials. When neglected, the wear can become severe enough to cause excessive blade slop, misalignment, or even mechanical failure.
Terminology and Component Notes
- Trunnion Ball: A spherical steel bearing that allows articulation between the blade and push arms.
- Bearing Cap: A cast or machined cover that holds the trunnion ball in place within the push arm.
- Bearing Insert: A weld-in seat or socket that houses the ball and allows for rotational movement.
- Shim Pack: A set of thin metal spacers used to adjust preload and eliminate slack in the assembly.
- Push Arm: The structural linkage connecting the dozer blade to the mainframe.
Symptoms of Trunnion Wear and Field Observations
Operators often notice excessive blade movement, difficulty grading, or audible clunking when trunnions are worn. In extreme cases, the push arm may detach entirely, as reported by one operator running a D8. The blade dropped unexpectedly when the trunnion arm fell off, forcing an emergency field repair using a torch to reshape the cap and clamp it tighter.
Another technician shared that on a D6R, the wear was so advanced that the ball had flattened and the bearing seat was ovalized. Despite the damage, the machine continued working until the job was complete—highlighting the resilience of CAT’s structural design but also the risks of deferred maintenance.
Repair Options and Rebuild Strategies
There are several approaches to restoring worn trunnion assemblies:
- Weld Build-Up and Machining
Weld material is added to the worn bearing seat or ball, then ground to match a template. This method is labor-intensive, requiring 5–6 hours per side on larger machines, but can restore functionality without full replacement.
- Cap Replacement and Seat Rebuild
New bearing caps (typically four per assembly) can be welded onto the push arms after burning off the old seats. This avoids cutting into the arms and preserves structural integrity. The ball may still need to be replaced or built up.
- Oversize Ball Fabrication
In cases of extreme wear, custom oversized trunnion balls can be machined and welded onto a stub shaft. This requires precise measurement and balancing to avoid introducing stress points.
- Aftermarket Kits
Suppliers like H-E Parts and Regal offer weld-in bearing inserts, trunnion balls, and caps at significantly lower cost than OEM parts. For example, a full rebuild kit for a D6R may cost $1,200 aftermarket versus $3,000 from Caterpillar.
Recommended Parts and Specifications
For a typical D7 or D6 machine, the following parts are commonly used:

• Trunnion Ball: 4T-5405 or classic 5J-4321
  
• Bearing Cap: 6F-7523 or classic 325-6039
  
• Bearing Insert: 1J-6740
  
• Bolts: 7X-0434
  
• Nuts: 6F-7522
  
• Shims: 9W-7602
  

Always verify compatibility using the machine’s serial number and consult a parts catalog or online breakdown before ordering.
Preventative Measures and Maintenance Tips
To extend trunnion life and avoid costly repairs:

• Grease bearing caps regularly to prevent dry friction
  
• Inspect shim packs annually and adjust preload as needed
  
• Avoid aggressive blade movements in rocky terrain
  
• Replace worn caps and balls before they deform
  
• Monitor blade articulation for signs of uneven wear or binding
  

One operator noted that aftermarket undercarriage parts from H-E Parts outperformed expectations, with a D6R LGP running 1,200 hours on a full set without issue—despite skepticism from a CAT dealer.
Historical Context and Design Evolution
Caterpillar’s trunnion design has evolved over decades, with earlier models using simpler mechanical seats and later machines incorporating shim packs and sealed bearings. The modular nature of the push arm and blade assembly allows for field repairs, a feature that has kept older machines like the 79Z-series D7s in service long after their production ended.
In the 1980s, CAT introduced weld-on trunnion kits to simplify rebuilds. These kits allowed operators to restore blade articulation without replacing entire push arms, reducing downtime and cost.
Conclusion
Worn trunnion assemblies are a common issue in aging Caterpillar dozers, but they’re far from a death sentence. With the right tools, parts, and techniques, operators can rebuild these critical joints and restore blade control. Whether through welding, aftermarket kits, or precision machining, the solution lies in understanding the mechanics—and respecting the iron. In the world of heavy equipment, even the smallest ball can carry the weight of the job.