Replacing Seals on the Hitachi EX165 Dipper Cylinder Under Pressure

[MikePhua]

The EX165 and Hitachi’s Hydraulic Excavator Heritage
The Hitachi EX165 hydraulic excavator was introduced in the late 1990s as part of Hitachi’s EX-series, designed to deliver high breakout force, precision control, and long-term durability in mid-size earthmoving applications. With an operating weight around 38,000 lbs and powered by a six-cylinder Isuzu diesel engine, the EX165 was widely adopted in road building, quarrying, and utility trenching across Europe and Asia.
Hitachi Construction Machinery, founded in 1970, became a global leader in hydraulic excavator design by integrating advanced metallurgy, efficient hydraulic systems, and modular component architecture. The EX165 featured a robust boom-dipper-arm configuration, with double-acting hydraulic cylinders controlling each articulation point. The dipper cylinder, responsible for pulling the arm inward, is subjected to high loads and frequent cycling—making seal integrity critical to performance.
Understanding Cylinder Anatomy and Terminology
Before replacing seals, it’s essential to understand the internal structure of the dipper cylinder.
Terminology clarification:

• Rod seal: Prevents hydraulic fluid from leaking around the piston rod as it exits the cylinder.
  
• Wiper seal: Scrapes dirt and debris off the rod during retraction, protecting internal seals.
  
• Piston seal: Maintains pressure separation between the two sides of the piston inside the cylinder.
  
• Wear band: A non-metallic ring that prevents metal-to-metal contact between piston and cylinder wall.
  
• Gland nut: The threaded cap that retains the seal head and allows access to internal components.
  

In a 2020 rebuild in Malaysia, a contractor discovered that a worn wiper seal on the EX165 dipper cylinder had allowed grit to enter the gland, leading to rod scoring and internal leakage. Replacing the seals alone was insufficient—the rod had to be polished and the gland replaced to restore full pressure retention.
Preparation and Disassembly Strategy
Replacing seals on the dipper cylinder requires careful planning, especially when the machine is needed urgently.
Steps before disassembly:

• Park the machine on level ground and lower the boom fully
  
• Release hydraulic pressure by cycling controls with engine off
  
• Disconnect battery to prevent accidental activation
  
• Clean the cylinder exterior thoroughly to prevent contamination
  
• Mark hose positions and cap all hydraulic lines
  

Disassembly procedure:

• Remove the cylinder from the dipper arm using lifting straps and a crane
  
• Secure the cylinder horizontally on a bench or stand
  
• Unscrew the gland nut using a spanner wrench or hydraulic nut tool
  
• Extract the rod and piston assembly carefully to avoid scoring
  
• Remove old seals using plastic picks or seal removal tools
  
• Inspect rod, piston, and bore for wear, corrosion, or pitting
  

Recommended tools:

• Seal installation kit with picks, drivers, and lubricant
  
• Torque wrench calibrated for gland nut specs
  
• Hydraulic press for stubborn wear bands
  
• Micrometer and calipers for measuring seal grooves
  
• Clean rags and lint-free cloths for assembly
  

In a 2022 emergency repair in Alberta, a mining crew used a portable hydraulic nut tool to remove a seized gland nut on an EX165 dipper cylinder. The job was completed in under four hours, restoring the machine for overnight trenching.
Seal Selection and Installation Tips
Choosing the correct seal kit is essential. Hitachi cylinders often use metric seals with specific profiles.
Seal selection guidelines:

• Match seal dimensions to OEM specs using cylinder serial number
  
• Choose high-pressure rated materials such as polyurethane or Viton
  
• Verify temperature and fluid compatibility (ISO 46 hydraulic oil)
  
• Include backup rings and wear bands in the kit
  

Installation tips:

• Lubricate seals with hydraulic fluid or assembly grease
  
• Use seal stretchers or cone tools to avoid tearing
  
• Install wiper seal last to prevent contamination
  
• Align wear bands carefully to avoid binding
  
• Torque gland nut to manufacturer spec and mark for inspection
  

Preventive upgrades:

• Install dual wipers in dusty environments
  
• Use low-friction piston seals to reduce heat buildup
  
• Add rod protectors or bellows in abrasive applications
  
• Replace breather caps and tank filters to reduce contamination
  

In a 2023 municipal fleet in Wisconsin, switching to dual-lip rod seals on all EX-series dipper cylinders reduced seal failures by 60%, especially during winter operations with frozen soil.
Testing and Reinstallation
After reassembly, pressure testing ensures the cylinder is leak-free and ready for service.
Testing steps:

• Mount cylinder in test jig and pressurize to 2,500 psi
  
• Inspect for leaks at gland, rod, and hose ports
  
• Cycle rod fully and check for smooth movement
  
• Monitor for drift under load over 15 minutes
  
• Reinstall cylinder and torque mounting bolts to spec
  

Final checks:

• Bleed air from hydraulic lines during startup
  
• Monitor fluid level and top off as needed
  
• Recheck gland nut torque after 10 hours of operation
  
• Document seal kit used and service date for future reference
  

In a 2023 excavation job in Shanghai, a technician completed a dipper cylinder seal replacement on an EX165 in under six hours using preassembled kits and a mobile test bench. The machine returned to full duty with no drift or leakage.
Conclusion
Replacing seals on the Hitachi EX165 dipper cylinder is a high-stakes task that demands precision, cleanliness, and mechanical insight. With the right tools, seal selection, and testing protocol, technicians can restore full hydraulic integrity and avoid costly downtime. In excavators, the dipper cylinder is more than a hydraulic actuator—it’s the muscle behind every trench, lift, and reach. And keeping that muscle sealed means keeping the job on schedule, even under pressure.