Can a PC60 or PC78US Boom Fit a Komatsu PC75UU-1 Excavator

[MikePhua]

The Komatsu PC75UU Series and Its Unique Boom Design
Komatsu’s PC75UU-1 excavator, part of the early 1990s compact utility lineup, was engineered for urban and roadside work where space constraints demanded minimal tail swing and maximum reach. The “UU” designation refers to its ultra-urban configuration, which includes a Z-shaped offset boom allowing the machine to dig parallel to walls or curbs. While this design offers unmatched maneuverability, it complicates boom replacement and customization.
Komatsu, founded in 1921 in Japan, has produced millions of excavators globally. The PC75UU series was particularly popular in Japan and later entered North America through grey market imports. These units often lack English documentation, making parts interchangeability a challenge for owners seeking to retrofit or repair.
Terminology Annotation

• Offset Boom: A boom design that allows lateral movement independent of the swing frame, ideal for tight spaces.
  
• Straight Boom: A conventional boom with a single pivot, offering simplicity and better lifting geometry.
  
• Boom Foot Width: The horizontal width of the boom base where it mounts to the upper frame.
  
• Pin Diameter: The thickness of the steel pin used to secure the boom to the frame or hydraulic cylinders.
  

Evaluating Boom Compatibility Across Models
Owners of the PC75UU-1 often seek to replace the offset boom with a straight boom from other Komatsu models like the PC60 or PC78US. The goal is to simplify maintenance and improve lifting performance. However, compatibility hinges on two critical dimensions:

• Boom foot width
  
• Pin diameter and spacing
  

Measurements from field conversions reveal:

• PC75UU-1 boom foot width: approximately 17.7 inches (450 mm)
  
• Pin diameter: 60 mm (2.36 inches)
  

In contrast:

• PC78US-6 boom foot width: approximately 15.75 inches (400 mm)
  
• PC60 boom foot width: similar or narrower than PC78US
  

This means both the PC60 and PC78US booms are too narrow to mount directly onto a PC75UU-1 without modification. While the pin diameters may match, the base width discrepancy requires either custom spacers or frame alterations.
Successful Conversions and Lessons Learned
One operator successfully retrofitted a PC90 boom onto a PC75UU-2. The boom foot width and pin diameter matched perfectly, though the hydraulic cylinder pin required bushing due to a mismatch in hole size. The stick-to-boom pin and width were also compatible, simplifying assembly.
However, the stick cylinder posed a challenge. The geometry of the new boom altered the cylinder’s angle, requiring fabrication of a new mounting bracket. With access to a machine shop and skilled welding, the issue was resolved without compromising structural integrity.
Sourcing Components and Cost Considerations
Retrofitting a boom is not just about fit—it’s about cost and availability. A typical donor package includes:

• Boom: $1,500
  
• Stick: $1,250
  
• Stick cylinder: $850
  
• Bucket cylinder: $650
  
• Inner bucket links: $125 each
  
• Outer bucket links: $75 each
  

Total cost without bucket: approximately $4,650
These prices reflect salvage yard rates and do not include labor, shipping, or inspection. Some components may require re-bushing or machining to eliminate slop and wear. In one Massachusetts case, a machine with a Ford extendahoe stick was retrofitted with a Komatsu boom and linkage, improving performance but requiring extensive pin and bushing work.
Recommendations for Retrofit Projects

• Measure boom foot width and pin diameter precisely using calipers and steel tape
  
• Avoid booms narrower than the frame unless willing to fabricate spacers
  
• Source complete assemblies to avoid mismatched linkage geometry
  
• Inspect donor parts for fire damage, corrosion, or structural fatigue
  
• Budget for machine shop time and welding if adapting cylinder mounts
  

If a PC90 or PC80 boom is available, it offers the best chance of direct fitment. Otherwise, expect to modify components or fabricate adapters.
Final Thoughts
Swapping a boom on a Komatsu PC75UU-1 is possible but requires careful measurement, mechanical skill, and realistic budgeting. While the PC60 and PC78US booms are tempting due to availability, their narrower base makes them poor candidates for direct replacement. With the right donor—ideally a PC90—and a willingness to adapt, the transformation can breathe new life into a well-used excavator.
In the end, retrofitting is part engineering, part detective work. And for those who enjoy rescuing machines as much as pets, it’s a challenge worth taking.