Swivel Joint Leaks After Rebuild on the Bobcat 337

[MikePhua]

The Bobcat 337 and Its Hydraulic Architecture
The Bobcat 337 is a mid-sized compact excavator introduced in the early 2000s by Bobcat Company, a division of Doosan Group. Designed for utility work, trenching, and light demolition, the 337 features a zero-tail swing design, pilot-operated controls, and a robust hydraulic system capable of powering attachments and travel functions simultaneously. With an operating weight of approximately 7,500 lbs and a 50 hp diesel engine, the 337 became a popular choice among contractors seeking reliability in confined job sites.
Central to its hydraulic system is the swivel joint—also known as the center joint or rotary manifold—which allows hydraulic fluid to pass between the upper structure and the undercarriage while enabling 360-degree rotation. This component is critical for travel, blade, and auxiliary functions routed through the lower frame.
Understanding the Swivel Joint’s Role
The swivel joint consists of stacked rotary seals, internal fluid channels, and a rotating shaft housed in a fixed body. It connects multiple hydraulic lines—typically six to eight circuits—between the upper and lower halves of the machine. These include:

• Left and right travel motors
  
• Blade lift circuit
  
• Auxiliary hydraulic return and supply
  
• Case drain lines
  
• Swing motor return
  

Terminology notes:

• Swivel joint: A hydraulic rotary manifold that maintains fluid continuity during rotation.
  
• Case drain: A low-pressure return line that relieves excess fluid from hydraulic motors.
  
• O-ring stack: A series of elastomeric seals that prevent cross-leakage between circuits.
  

Why Leaks Occur After Rebuild
Swivel joint leaks shortly after rebuild are often caused by one or more of the following:

• Improper seal installation or orientation
  
• Use of incorrect or aftermarket seal kits
  
• Scoring or pitting on the shaft or bore surfaces
  
• Over-torquing of retaining bolts causing seal distortion
  
• Contamination during assembly leading to premature wear
  
• Misalignment during reinstallation causing uneven pressure
  

In one case from a contractor in Minnesota, a Bobcat 337 began leaking hydraulic fluid from the undercarriage within 20 hours of a swivel joint rebuild. Investigation revealed that the seal kit used was not OEM and lacked the correct durometer rating for the high-pressure circuits. Replacing the seals with factory-grade components resolved the issue permanently.
Inspection and Rebuild Best Practices
To ensure a successful rebuild:

• Use genuine Bobcat seal kits or verified equivalents with correct material specs
  
• Clean all components thoroughly with lint-free cloths and solvent
  
• Inspect the shaft and bore for wear, corrosion, or scoring
  
• Replace any damaged bearings or retaining rings
  
• Lubricate seals lightly with hydraulic fluid before installation
  
• Align the joint precisely during reassembly to avoid twisting or pinching
  
• Torque bolts to manufacturer specifications using a calibrated wrench
  

A technician in British Columbia developed a habit of photographing each seal layer during disassembly to ensure correct reinstallation. This practice reduced rebuild errors and improved first-pass success rates.
Testing and Post-Rebuild Monitoring
After reassembly:

• Pressurize the system gradually and observe for leaks
  
• Check each circuit independently using flow meters or pressure gauges
  
• Monitor case drain flow to detect internal leakage
  
• Inspect the joint after 10, 50, and 100 hours of operation
  
• Document fluid loss and pressure anomalies for trend analysis
  

If leaks persist, consider dye testing with UV-reactive hydraulic fluid to pinpoint the source. In some cases, micro-leaks may only appear under full load or thermal expansion.
Field Story from a Utility Crew
In 2019, a municipal crew in Ohio rebuilt the swivel joint on their Bobcat 337 after noticing fluid pooling near the blade. Despite careful installation, the leak returned within a week. A senior mechanic discovered that the retaining ring had been installed upside down, allowing the seal stack to shift under pressure. After correcting the orientation and re-torquing the assembly, the machine ran leak-free for the remainder of the season.
Preventive Measures and Recommendations
To avoid future failures:

• Replace seals proactively every 2,000–2,500 hours
  
• Use hydraulic fluid with anti-wear additives and proper viscosity
  
• Flush the system after major component replacement
  
• Train technicians in seal identification and installation techniques
  
• Keep a rebuild log with part numbers, torque specs, and service intervals
  

Conclusion
Swivel joint leaks on the Bobcat 337 after rebuild are often preventable with proper technique, quality components, and attention to detail. As a critical hydraulic pathway, the swivel joint demands precision during service. With the right approach, operators can restore full functionality and extend the life of their machine—keeping fluid where it belongs and productivity on track.