Installing a Hydraulic Thumb on the Kobelco 70SR Excavator

[MikePhua]

The 70SR and Kobelco’s Compact Excavator Lineage
The Kobelco 70SR is a short-radius compact excavator designed for urban construction, utility trenching, and tight-access demolition. Introduced in the early 2000s, the 70SR was part of Kobelco’s push to deliver high-performance machines with minimal tail swing, allowing operators to work close to walls, traffic lanes, or other obstacles. With an operating weight of approximately 16,000 pounds and a digging depth of over 14 feet, the 70SR balances reach, breakout force, and maneuverability.
Kobelco, founded in 1905 as part of Kobe Steel Ltd., has built a reputation for hydraulic refinement and operator comfort. The 70SR features a load-sensing hydraulic system, pilot controls, and a well-insulated cab. However, like many compact excavators, it does not come standard with a hydraulic thumb—an attachment that dramatically expands its versatility.
What a Hydraulic Thumb Adds to the Machine
A hydraulic thumb is a pivoting clamp mounted opposite the bucket, allowing the operator to grasp, lift, and manipulate irregular objects such as logs, concrete debris, stumps, and scrap. Unlike fixed thumbs, hydraulic thumbs are powered by a cylinder and can be retracted when not in use.
Benefits include:

• Improved material handling without switching attachments
  
• Enhanced control during demolition and sorting
  
• Ability to grip and rotate objects for precise placement
  
• Reduced reliance on ground crew for manual positioning
  

One operator in Oregon described his thumb-equipped 70SR as “a mini material handler,” capable of loading brush, placing boulders, and even setting culverts without leaving the cab.
Choosing the Right Thumb Configuration
Thumbs come in several styles, and selecting the right one depends on bucket size, linkage geometry, and hydraulic capacity.
Options include:

• Progressive link thumbs: Offer extended rotation range, ideal for tight spaces
  
• Direct link thumbs: Simpler design, fewer moving parts, lower cost
  
• Full-width thumbs: Match the bucket width for maximum grip
  
• Narrow thumbs: Allow better visibility and reduce weight
  

For the 70SR, a progressive link thumb with a four-tine configuration is often preferred. It provides nearly 180 degrees of rotation and maintains parallel movement with the bucket, improving grip consistency.
Key parameters to match:

• Thumb length: Should cover 75–90% of bucket length
  
• Tine spacing: Should align with bucket teeth for secure grip
  
• Cylinder stroke: Must allow full retraction without interference
  
• Mounting bracket: Should match stick profile and weld spec
  

Hydraulic Plumbing and Valve Integration
Installing a hydraulic thumb requires tapping into the auxiliary hydraulic circuit. The 70SR typically includes a single auxiliary line for breaker or auger use, but may lack a second return line or control valve for thumb operation.
Installation steps:

• Add a diverter valve or proportional control valve to manage flow
  
• Install a thumb cylinder with appropriate bore and stroke
  
• Route hoses along the stick with clamps and guards
  
• Connect to pilot controls or add a foot pedal or joystick button
  
• Test flow rate and pressure (typically 2,500–3,000 psi)
  

Recommendations:

• Use steel-braided hoses for durability
  
• Install check valves to prevent drift
  
• Add quick couplers for easy removal
  
• Use a flow restrictor if thumb movement is too fast
  

One technician in British Columbia added a proportional thumb valve with joystick integration, allowing feathered control for delicate tasks like placing riprap or handling pipe.
Welding and Structural Considerations
Mounting the thumb bracket requires precision welding. The stick must be cleaned, beveled, and preheated to avoid cracking. Welds should be full-penetration and inspected for porosity or undercut.
Tips:

• Use low-hydrogen rods or MIG wire rated for structural steel
  
• Avoid welding near hydraulic lines or seals
  
• Reinforce the bracket with gussets if operating in rocky terrain
  
• Paint and seal welds to prevent corrosion
  

One operator in Texas reinforced his thumb bracket with a 3/8-inch gusset after noticing flex during log handling. The modification improved stability and reduced wear on the cylinder pin.
Operational Adjustments and Safety
Once installed, the thumb changes how the machine is operated. Operators must learn to coordinate bucket curl and thumb movement for secure gripping. Overloading or side-loading the thumb can damage the cylinder or bracket.
Best practices:

• Avoid using the thumb to pry or dig
  
• Retract the thumb fully when trenching or grading
  
• Inspect pins and bushings weekly for wear
  
• Grease all pivot points daily
  
• Train operators on thumb control and load limits
  

One contractor in Maine added a thumb to his 70SR and saw a 40% reduction in ground crew labor during demolition. However, he also had to replace a bent cylinder after using the thumb to pull rebar embedded in concrete.
Conclusion
Adding a hydraulic thumb to the Kobelco 70SR transforms it from a digging machine into a multi-purpose handler. With careful selection, precise installation, and disciplined operation, the thumb becomes an extension of the operator’s hand—gripping, lifting, and placing with finesse. Whether clearing brush, sorting debris, or setting stone, the right thumb turns a compact excavator into a jobsite problem solver. In the hands of a skilled operator, it’s not just an attachment—it’s a capability upgrade.