Restoring and Maintaining the 1990 Hitachi EX120 Excavator

[MikePhua]

The EX120 and Its Historical Significance
The Hitachi EX120 excavator was introduced in the late 1980s as part of Hitachi’s push to dominate the mid-size hydraulic excavator market. Built between 1988 and 1990, this model quickly earned a reputation for reliability, simplicity, and robust performance. With an operating weight of approximately 11.8 metric tons and a bucket capacity ranging from 0.5 to 0.6 cubic meters, the EX120 was designed for trenching, foundation work, and general excavation across varied terrain.
Hitachi Construction Machinery, founded in 1970, had already established itself as a global leader in hydraulic systems and machine control. The EX120 became one of their most widely sold models in the 12-ton class, with thousands deployed across Asia, Europe, and North America. Its mechanical simplicity and durable components made it a favorite among owner-operators and small contractors.
Terminology Note

• Hydraulic Pump: The heart of the excavator’s hydraulic system, converting mechanical energy into fluid pressure.
  
• Swing Motor: A hydraulic motor that rotates the upper structure of the excavator.
  
• Travel Motor: A motor that powers the tracks, enabling movement across terrain.
  
• Boom and Arm Cylinders: Hydraulic actuators that control the digging motion.
  

Core Specifications and Performance Metrics
Key specifications of the 1990 Hitachi EX120 include:

• Engine: Nissan PE6 diesel, 6-cylinder, producing approximately 90 hp
  
• Operating weight: 11,800 kg
  
• Maximum digging depth: 5.6 meters
  
• Maximum reach: 8.7 meters
  
• Bucket breakout force: ~9,000 kgf
  
• Track width: 600 mm
  
• Swing speed: 11 rpm
  

These specs positioned the EX120 as a versatile machine capable of handling both confined urban sites and open rural grading. Its mechanical fuel injection system and analog controls made it easy to repair and diagnose without specialized tools.
Common Issues and Field Diagnostics
Despite its reliability, the EX120 is not immune to age-related wear. Frequent issues include:

• Hydraulic system degradation: Symptoms include slow boom response, weak bucket curl, or erratic travel. Causes range from worn pump seals to clogged filters or internal leakage in control valves.
  
• Engine performance problems: Hard starts, excessive smoke, or reduced power often stem from clogged fuel filters, worn injectors, or air intake restrictions.
  
• Electrical faults: Inconsistent gauge readings or dead starter circuits may result from corroded connectors, brittle wiring, or failing relays.
  
• Undercarriage wear: Uneven track tension, excessive vibration, or difficulty turning can be traced to worn sprockets, idlers, or track chains.
  
• Bucket and linkage fatigue: Cracks, pin wear, and loose bushings reduce digging efficiency and increase stress on hydraulic cylinders.
  

A technician in British Columbia diagnosed a sluggish boom on a 1990 EX120 and traced it to a partially collapsed suction hose feeding the hydraulic pump. Replacing the hose and flushing the system restored full responsiveness.
Maintenance Strategies and Upgrade Options
To keep the EX120 running smoothly:

• Replace hydraulic filters every 500 hours and fluid every 1,000 hours
  
• Inspect and adjust track tension monthly
  
• Clean or replace air filters quarterly
  
• Use OEM or high-quality aftermarket seals in cylinder rebuilds
  
• Upgrade electrical connectors to weather-sealed types
  
• Retrofit LED work lights and auxiliary hydraulic lines for modern attachments
  

A contractor in Texas retrofitted his EX120 with a hydraulic thumb and quick coupler, enabling faster material handling and reducing downtime during attachment changes.
Parts Sourcing and Restoration Challenges
Finding parts for a 1990 EX120 can be challenging due to its age and discontinued status. Strategies include:

• Searching by engine model (Nissan PE6) for fuel system components
  
• Using casting numbers on hydraulic pumps and motors to match replacements
  
• Contacting salvage yards and rebuild shops specializing in Japanese equipment
  
• Consulting parts cross-reference databases for compatible seals and bearings
  
• Rebuilding components like swing motors and travel drives using aftermarket kits
  

A fleet manager in New Zealand sourced a replacement swing motor from a dismantled EX100 and adapted the mounting flange to fit the EX120. The repair cost less than half of a new OEM unit and restored full rotation.
Operator Stories and Practical Advice
A land-clearing operator in Maine shared that his EX120, purchased used in 2005, still runs daily with over 12,000 hours. He attributes its longevity to regular fluid changes, careful warm-up routines, and avoiding overloading the bucket.
Meanwhile, a technician in Alberta rebuilt the main control valve after noticing slow boom response and internal leakage. He replaced all O-rings, polished the spool surfaces, and reinstalled with fresh fluid. The machine returned to full performance and passed a pressure test with factory-spec readings.
Conclusion
The 1990 Hitachi EX120 remains a workhorse in the mid-size excavator class, combining mechanical simplicity with durable engineering. While parts sourcing and age-related wear present challenges, the machine’s straightforward design makes it ideal for restoration and long-term use. With proper maintenance, thoughtful upgrades, and field-tested repair strategies, the EX120 continues to deliver reliable performance decades after its debut. In the world of heavy equipment, some machines earn their legacy not through innovation—but through endurance.