Hydraulic Cap Identification and Maintenance on the Hitachi EX35

[MikePhua]

The Hitachi EX35 and Its Compact Excavator Lineage
The Hitachi EX35 is part of the EX series of compact excavators developed in the 1990s to meet growing demand for maneuverable, high-performance machines in urban and utility environments. With an operating weight of approximately 3.5 metric tons and powered by a reliable three-cylinder diesel engine, the EX35 was designed for trenching, grading, and light demolition work. Its compact footprint and responsive hydraulics made it a popular choice across Asia, North America, and Europe.
Hitachi Construction Machinery, founded in 1970, became a global leader in hydraulic excavator technology by the early 2000s. The EX series helped solidify its reputation for durability and serviceability, especially in the compact equipment segment.
Hydraulic Cap Functions and Placement
Hydraulic caps on the EX35 serve multiple purposes:

• Reservoir access: The main hydraulic tank cap allows for fluid inspection, filling, and venting.
  
• System isolation: Smaller caps or plugs seal off unused ports or service access points.
  
• Contamination prevention: Caps protect internal components from dust, moisture, and debris.
  
• Pressure regulation: Some caps include breather valves to equalize pressure during operation.
  

Typical cap locations include:

• Hydraulic reservoir top
  
• Control valve block
  
• Pilot line junctions
  
• Return line manifolds
  
• Auxiliary circuit ports
  

In Ohio, a contractor discovered a missing breather cap on an EX35 hydraulic tank after noticing fluid seepage and erratic boom movement. Replacing the cap restored system pressure balance and eliminated contamination risk.
Identifying Cap Types and Thread Specifications
To replace or service hydraulic caps, accurate identification is essential. Key parameters include:

• Thread type: Metric (e.g., M22x1.5) or BSP (British Standard Pipe)
  
• Seal type: O-ring, flat washer, or tapered thread
  
• Material: Steel, aluminum, or polymer depending on location
  
• Vent type: Solid, vented, or pressure-relief integrated
  
• Torque rating: Typically 15–40 Nm for small caps, higher for reservoir lids
  

Use calipers to measure thread diameter and pitch. If the cap is damaged or missing, compare with similar Hitachi models or consult hydraulic schematics. Some operators use thread gauges or dye to confirm compatibility before ordering replacements.
Common Issues and Field Solutions
Hydraulic cap problems on the EX35 may include:

• Cracked or missing caps due to vibration or impact
  
• Cross-threading during installation
  
• Seal degradation leading to leaks
  
• Blocked breather vents causing pressure buildup
  
• Incorrect cap type used during repair
  

Solutions include:

• Replacing caps with OEM or high-quality aftermarket equivalents
  
• Using thread sealant or anaerobic compound for non-vented caps
  
• Installing breather filters to reduce dust ingress
  
• Adding tethered caps to prevent loss during service
  
• Inspecting cap threads and mating surfaces for wear
  

In Shanghai, a fleet manager retrofitted color-coded caps on all compact excavators to simplify maintenance and reduce mix-ups during fluid checks.
Preventive Maintenance and Inspection Tips
To maintain hydraulic cap integrity:

• Inspect all caps weekly for cracks, leaks, or looseness
  
• Clean breather vents monthly to ensure airflow
  
• Replace seals annually or during major service
  
• Torque caps to spec using calibrated tools
  
• Keep spare caps and seals in the service kit
  

Some operators install magnetic drain plugs with integrated caps to monitor metal wear in the hydraulic system. Others use UV-reactive dye in fluid to detect leaks around cap threads.
Field Anecdote and Practical Insight
In Alberta, a utility crew operating an EX35 noticed sluggish arm movement during cold mornings. Inspection revealed a blocked breather cap on the hydraulic tank, causing vacuum formation and restricted flow. After replacing the cap with a heated breather unit, performance normalized even in sub-zero conditions.
Conclusion
Hydraulic caps on the Hitachi EX35 may seem minor, but they play a critical role in protecting and regulating the machine’s fluid systems. Accurate identification, proper installation, and routine inspection ensure consistent performance and prevent costly failures. Whether sealing a pilot line or venting a reservoir, each cap contributes to the overall health of the excavator’s hydraulic architecture. With attention to detail and proactive care, operators can keep their EX35 running smoothly across seasons and job sites.