Grease Fittings and Relief Valves on the Caterpillar 955K Track Loader

[MikePhua]

The 955K and Its Mechanical Heritage
The Caterpillar 955K crawler loader was part of Caterpillar’s mid-20th century lineup of track-type loaders, built for rugged earthmoving and material handling. Produced during the 1960s and 1970s, the 955K featured a direct-injection diesel engine, torque converter drive, and a robust undercarriage designed for heavy-duty applications. With a bucket capacity of approximately 1.5 cubic yards and an operating weight near 30,000 pounds, the 955K was widely used in construction, mining, and land clearing.
Caterpillar’s K-series loaders were known for their mechanical simplicity and field serviceability. Unlike modern machines with electronic diagnostics, the 955K relied on manual adjustments, mechanical linkages, and grease-based tensioning systems to maintain track alignment and undercarriage integrity.
Terminology Annotation
- Grease zerk fitting: A small, threaded nozzle used to inject grease into a mechanical component under pressure.
- Track adjuster: A hydraulic or spring-loaded mechanism that maintains proper track tension.
- Relief valve: A pressure-release component that allows grease or hydraulic fluid to escape when servicing or adjusting a system.
- Release valve: A manually operated plug or bolt that vents pressure from a sealed chamber, often used in track adjusters.
- STOR fitting: A type of hydraulic fitting with an integrated O-ring groove, used for sealing under pressure.
Track Tensioning and the Role of Grease Fittings
The 955K uses a grease-charged track adjuster system to maintain proper tension between the idler and the track chain. Grease is injected through a zerk fitting into the adjuster cylinder, pushing the idler forward and tightening the track. Over time, grease may leak or lose pressure, requiring recharging. If the track becomes too tight or needs removal, the relief valve is loosened to vent grease and retract the idler.
Operators often confuse the grease fitting with the valve itself. In most cases, the zerk is a replaceable component threaded into the top of the valve body. If damaged or missing, it can be sourced from standard auto parts suppliers, as it follows common thread and pressure specifications.
Part Identification and Cross-Reference
For machines with the 85J serial prefix, Caterpillar lists the fill valve as part number 2S-5925 and the relief valve as 2S-5926. These components are relatively inexpensive, typically priced under $10 each. However, confirming compatibility requires the machine’s serial number, as variations exist across production years and regional builds.
Recommendations:

• Always verify part numbers using the serial number and a parts manual
  
• Inspect valve threads and sealing surfaces before installation
  
• Use high-pressure grease rated for track adjuster systems
  
• Replace both fill and relief valves during undercarriage service to prevent future leaks
  

Aftermarket Sources and Cost-Saving Options
While Caterpillar dealers offer OEM parts, aftermarket suppliers like Undercarriage Specialist Inc. and H-E Parts provide competitive pricing and support. These vendors can cross-reference part numbers and offer bulk discounts for fleet maintenance. Some operators report sourcing compatible grease fittings from Amazon or local hydraulic shops, especially when only the zerk is needed.
Tips for sourcing:

• Provide serial number and machine model when contacting suppliers
  
• Request photos or dimensional specs to confirm compatibility
  
• Keep a spare set of valves and fittings in the service truck for field repairs
  
• Use thread sealant rated for hydraulic systems to prevent leaks
  

Field Anecdotes and Practical Advice
One technician recalled servicing a 955K that had lost its grease fitting during a brush clearing job. The operator had attempted to inject grease using a makeshift adapter, resulting in a clogged valve and uneven track tension. After replacing the valve and properly bleeding the adjuster, the machine returned to normal operation.
Another mechanic noted that older machines often have non-standard fittings due to previous repairs or regional modifications. In such cases, custom adapters or rethreading may be necessary. He recommended carrying a thread pitch gauge and a small assortment of metric and SAE fittings when working on legacy equipment.
Conclusion
Maintaining the track adjuster system on a Caterpillar 955K requires attention to detail and familiarity with grease fittings and relief valves. While the components are simple, their role in undercarriage performance is critical. By sourcing the correct parts, verifying fitment, and using proper service techniques, operators can ensure smooth operation and extend the life of their machine. In the world of vintage iron, even a $10 valve can make the difference between downtime and productivity.