Rebuilding the Track Adjuster on a Caterpillar 931 Loader

[MikePhua]

The Legacy of the CAT 931 Track Loader
The Caterpillar 931 track loader was introduced in the 1970s as part of CAT’s push into compact crawler loaders for construction, forestry, and utility work. Positioned between the smaller 941 and larger 955 models, the 931 offered a balance of maneuverability and breakout force. Powered by a four-cylinder diesel engine and equipped with a hydrostatic transmission, the 931 became a staple in municipal fleets and small contractor yards. Caterpillar, founded in 1925, had already established dominance in earthmoving, and the 931 contributed to its reputation for rugged, repairable machinery. Thousands of units were sold globally, and many remain in service today.
Understanding the Track Adjuster Assembly
The track adjuster is a hydraulic tensioning mechanism that maintains proper track sag and alignment. It consists of:

• A grease-filled cylinder housing
  
• A spring-loaded piston or ram
  
• A front idler that slides forward under pressure
  
• Seals and O-rings to contain grease and prevent contamination
  

When grease is pumped into the adjuster, the piston pushes the idler forward, tightening the track. Over time, seals degrade, grease leaks out, and the adjuster collapses—leading to excessive track slack, derailment risk, and accelerated undercarriage wear.
Diagnosing a Collapsing Adjuster
A collapsing track adjuster typically presents with:

• Visible grease leakage at the barrel-to-ram interface
  
• Track sag beyond manufacturer specifications (often more than 2 inches of droop between carrier rollers)
  
• Difficulty maintaining tension after greasing
  
• Grease fitting or relief valve failure
  

In one case, the adjuster was leaking at both the large O-ring and the nylon backup ring where the barrel bolts to the front idler ram. This dual failure strongly suggests that the internal piston seals are also compromised. A full rebuild is recommended rather than partial repair.
Seal Kit Selection and Sourcing
Seal kits for the 931 adjuster are available from both OEM and aftermarket suppliers. While genuine Caterpillar kits offer guaranteed compatibility, aftermarket kits from reputable vendors like Offroad Equipment can be significantly more affordable—often under CAD $25 per side. It’s advisable to order kits for both sides simultaneously, as freight costs are similar and the opposite adjuster may fail soon after.
When ordering, provide:

• Machine model (931)
  
• Serial number (typically stamped on the loader frame)
  
• Adjuster type (some variants differ slightly in barrel length or seal stack)
  

Disassembly and Repair Procedure
The repair process involves:

• Opening the grease relief valve to depressurize the adjuster
  
• Removing the grease fitting to allow full drainage
  
• Unbolting the adjuster barrel from the idler frame (usually four 15/16" bolts)
  
• Extracting the ram and inspecting the sealing surfaces
  
• Replacing all O-rings, backup rings, and piston seals
  
• Reassembling with fresh grease and verifying tension
  

Technicians often use a 5-foot cheater bar to break the adjuster bolts loose. Mild heat may be applied to the barrel, but caution is required due to residual grease inside. The second side typically takes half the time once the procedure is familiar.
Common Pitfalls and Field Advice
Several issues can complicate the rebuild:

• Pitted or scored ram surfaces that damage new seals
  
• Misalignment during reassembly causing uneven tension
  
• Over-tightening bolts leading to barrel distortion
  
• Failure to fully relieve pressure before disassembly
  

One retired operator in Tennessee recalled using a loader for 20 years before the first adjuster failure. He emphasized the importance of inspecting the ram surface with a flashlight and mirror, as even minor imperfections can cause premature seal wear.
Another technician noted that improperly torqued relief valves can leak under vibration. He recommended tightening valves “firm but not gut-busting,” using thread sealant sparingly.
Preventative Maintenance and Long-Term Reliability
To extend adjuster life:

• Grease adjusters monthly during active use
  
• Inspect for leaks during undercarriage service
  
• Replace seals every 2,000–3,000 hours or when tension loss is observed
  
• Keep relief valves clean and protected from debris
  
• Use high-quality grease rated for hydraulic applications
  

Operators should also monitor track sag visually and measure it periodically. Excessive slack not only stresses the adjuster but also accelerates wear on sprockets, rollers, and track links.
Conclusion
Rebuilding the track adjuster on a Caterpillar 931 is a straightforward but essential task for maintaining undercarriage integrity. With the right seal kit, tools, and attention to detail, even aging machines can be restored to reliable service. In a world where new equipment costs continue to rise, keeping legacy loaders like the 931 running strong is both economical and rewarding. Whether in a rural yard or a city jobsite, a well-tensioned track is the foundation of productive earthmoving.