Comprehensive Guide to Bucket Bushing Replacement on the Kobelco SK35-2

[MikePhua]

Understanding the Role of Bushings in Excavator Arms
Bushings are critical wear components in excavator attachments such as buckets, booms, and dipper arms. They serve as replaceable sleeves fitted into larger bore holes where pins rotate or oscillate. Their function is to absorb wear and allow for smoother, more consistent pin movement. In compact excavators like the Kobelco SK35-2, bushings play a key role in ensuring tight pivoting, minimizing slack, and maintaining hydraulic precision.
Over time, due to friction, high load cycles, poor lubrication, or contaminants like dirt and grit, bushings can wear out or seize. When that happens, the result is usually pin slop, abnormal noises, or even uneven digging performance. In extreme cases, failure to replace bushings in time can damage the bore itself, leading to costly weld-and-bore repairs.
Symptoms of Worn Bucket Bushings

• Excessive play at the bucket pivot points
  
• Misalignment of linkage arms
  
• Irregular bucket movement or lag during operation
  
• Audible clunking noises when changing bucket direction
  
• Visible wear marks or scoring on the pins or inside the bushings
  

A well-maintained SK35-2 should not exhibit more than a few millimeters of play at the bucket connection. Anything beyond that indicates accelerated wear.
Diagnosis: Checking Pin and Bushing Wear
Routine inspection of the bushings involves checking for side-to-side and vertical movement at pin joints. A pry bar can be used to apply force on the bucket to test for movement. Measuring the pin and the bore diameter with calipers provides precise insights:

• If the pin is worn more than 0.5 mm from its original diameter, it should be replaced.
  
• If the bushing's internal diameter exceeds the spec by more than 0.25 mm, the bushing is likely worn out.
  

In the Kobelco SK35-2, the most common wear points are at the bucket linkage and the quick-attach coupler if equipped.
Removing the Worn Bushings: Tools and Cautions
Removing old bushings can be simple or highly involved, depending on corrosion levels and design tolerances.
Required tools:

• Hydraulic press or manual bushing driver
  
• Slide hammer or air hammer (in stubborn cases)
  
• Freezer and torch (for thermal contraction/expansion techniques)
  
• Penetrating oil and wire brush for bore cleaning
  

Removal steps:

1. Clean the area thoroughly to avoid contaminating the bore.
   
2. Apply penetrating oil and allow it to soak for several hours.
   
3. Use a driver or press to push out the old bushing.
   
4. If the bushing is frozen in place, carefully heat the housing and attempt removal with a slide hammer.
   

Pro tip: Some technicians place dry ice or freeze the new bushing to slightly shrink it before insertion. This helps reduce press-fit resistance.
Installing New Bushings: Precision Is Key
Reinstalling bushings is more than hammering them in. Misalignment can lead to premature failure. A hydraulic press ensures even force distribution during installation.
Installation checklist:

• Clean the bore completely, removing rust, debris, and leftover oil
  
• Use anti-seize compound or high-pressure grease if specified
  
• Align the bushing to match grease port holes if applicable
  
• Press in slowly and evenly using a bushing driver or properly sized socket
  
• Check final alignment—any tilt can cause binding
  

It's often advisable to install the pin after pressing the bushing to verify smooth rotation and fitment.
Sourcing the Correct Parts
Parts availability for the SK35-2 can vary depending on region. OEM bushings are ideal, but several aftermarket suppliers offer equivalent or improved wear-resistant options.
Things to consider when sourcing bushings:

• Material hardness: hardened steel bushings offer superior wear resistance
  
• Greasability: some aftermarket bushings come with spiral grooves to enhance grease flow
  
• Compatibility: even slight differences in outer diameter can affect press-fit integrity
  

One user story involved ordering a full set of aftermarket hardened bushings and pins, only to find out that the pin diameter was off by 0.2 mm—tight enough to seize after heating up in operation.
Avoiding Common Mistakes

• Never reuse visibly worn or scored pins with new bushings
  
• Do not attempt to hammer in bushings with improper alignment—use a press
  
• Always clean bores meticulously before reinstallation
  
• Do not mix metric and imperial bushings—SK35-2 uses metric sizing
  
• Grease immediately after installation, even if the machine hasn't been used yet
  

Case Study: Kobelco SK35-2 with a Wobbly Bucket
A small contractor in Ohio noticed that his SK35-2’s bucket started swinging excessively when digging tight trenches. Upon inspection, he found 4 mm of play at the bucket pin, with the bushing visibly elongated on one side. The pin itself was egg-shaped. After pressing in a new bushing and replacing the pin, the tightness returned and the machine regained precision control.
The key takeaway? Don't delay replacement. Worn bushings are not just a nuisance—they can become a costly repair if the bore is damaged.
Bushing Maintenance Best Practices

• Grease daily or at least every 8 hours of operation
  
• Avoid high-impact motions when bushings are dry or dirty
  
• Clean the joint areas after working in muddy or sandy conditions
  
• Inspect pins and bushings during every 100-hour service interval
  
• Replace in pairs (pin and bushing) to maximize service life
  

Final Thoughts: A Small Part With Big Impact
Though bushings are inexpensive compared to hydraulic cylinders or undercarriage components, their failure can cascade into more serious structural damage. Especially on mini-excavators like the Kobelco SK35-2, where compactness demands tighter tolerances, maintaining bushing integrity is essential for precision digging, grading, and control.
A well-documented example occurred on a construction site in Japan, where repeated under-lubrication of a SK35-2 bucket joint led to seized bushings mid-operation, resulting in a costly boom crack due to stress concentration. From that point forward, the contractor adopted a strict morning grease-check routine—saving thousands in potential repair bills.
In the end, keeping bucket bushings in top condition is not just a maintenance task—it's a form of insurance for your entire machine.