Final Drive Motor Replacement for Mitsubishi MX35

[MikePhua]

The MX35 and Its Compact Excavator Heritage
The Mitsubishi MX35 mini excavator was part of Mitsubishi’s push into compact construction equipment during the 1990s. Designed for tight urban job sites and utility trenching, the MX35 featured a zero-tail swing design, a hydraulic quick coupler, and a robust undercarriage. With an operating weight around 3.5 metric tons and a dig depth exceeding 10 feet, it competed with models from Kubota, Yanmar, and Komatsu. Mitsubishi Heavy Industries, founded in 1884, had long been a leader in industrial machinery, and the MX series reflected their engineering pedigree.
Though production of the MX35 has ceased, many units remain in service globally, especially in Asia and the Pacific. Their longevity is owed to simple hydraulics, durable steel construction, and widespread parts interchangeability. However, one of the most critical components—the final drive motor—can pose challenges when replacement is needed.
Final Drive Motor Function and Anatomy
The final drive motor is a hydraulic component that powers the tracks of the excavator. It converts pressurized fluid into rotational motion, driving the sprockets that move the tracks. In the MX35, the final drive motor is a two-speed axial piston motor integrated with a planetary gear reduction system.
Terminology annotation:
- Final drive motor: A hydraulic motor that powers the track system of an excavator, converting fluid pressure into torque.
- Planetary gear: A gear system that multiplies torque through a central sun gear surrounded by planet gears and a ring gear.
- Axial piston motor: A hydraulic motor that uses pistons arranged parallel to the drive shaft to generate rotational force.
- Two-speed travel: A feature allowing the operator to switch between high-speed travel and low-speed torque modes.
In the MX35, the final drive motor is typically a Nachi PH300 unit. This motor includes internal seals, bearings, and a brake assembly. Over time, seals degrade, bearings wear, and internal leakage can reduce torque or cause complete failure.
Replacement Challenges and Obsolescence
One of the main issues with the MX35 is that the original Nachi PH300 motor is no longer supported by the manufacturer. Nachi has phased out many older models, making parts difficult to source. Even well-stocked hydraulic shops struggle to find seal kits or bearing replacements for this unit.
Options for replacement include:

• Purchasing a new aftermarket final drive motor compatible with the MX35
  
• Retrofitting a similar motor from another brand with matching flange and port dimensions
  
• Rebuilding the existing motor if planetary gears and housing are intact
  
• Sourcing used motors from salvage yards or decommissioned machines
  

Aftermarket suppliers like Precision Final Drives offer bolt-on replacements that match the MX35’s hydraulic specifications and mounting pattern. These units often include hydraulic fittings and arrive preassembled, ready for installation.
Installation Tips and Compatibility Checks
Before purchasing a replacement motor, verify the following:

• Spline count and shaft diameter match the sprocket hub
  
• Bolt hole pattern aligns with the track frame
  
• Hydraulic port size and thread type match existing hoses
  
• Flow rate and pressure ratings are compatible with the MX35’s hydraulic system
  

Installation steps:

• Safely lift and support the machine with cribbing or jack stands
  
• Remove track and sprocket to access the motor flange
  
• Disconnect hydraulic lines and cap them to prevent contamination
  
• Unbolt the motor and inspect the mounting surface for wear or debris
  
• Install the new motor, torque bolts to spec, and reconnect hoses
  
• Bleed air from the system by cycling the travel function slowly
  

Field Anecdotes and Practical Advice
One operator in Oregon reported that his MX35’s final drive motor failed after years of forestry work. While the planetary gears remained intact, the motor leaked internally and lost torque. Unable to source Nachi parts, he opted for a new aftermarket unit and completed the swap in under three hours. He noted that the replacement motor offered smoother travel and quieter operation.
Another technician in Queensland rebuilt his MX35 motor using a hybrid of seals from other models. While successful, he warned that mismatched seals can lead to premature failure and advised using OEM-grade kits when possible.
Preventative Maintenance and Longevity Tips
To extend final drive motor life:

• Change hydraulic fluid every 500 hours and inspect for contamination
  
• Use magnetic drain plugs to monitor for metal debris
  
• Avoid high-speed travel on rocky terrain to reduce shock loads
  
• Grease sprocket hubs and inspect track tension weekly
  
• Monitor for signs of leakage, noise, or reduced travel speed
  

For machines stored long-term, rotate the tracks monthly to prevent seal hardening and shaft corrosion.
Conclusion
Replacing the final drive motor on a Mitsubishi MX35 requires careful sourcing, compatibility checks, and mechanical diligence. With the original Nachi PH300 unit no longer supported, aftermarket solutions offer a practical path forward. Whether rebuilding or replacing, understanding the motor’s function and installation nuances ensures that this compact excavator continues to deliver reliable performance in the field. With proper care and informed decisions, even legacy machines like the MX35 can keep moving forward.