Fine-Tuning the CAT 426B Extendahoe: A Guide to Shim Adjustment and Backhoe Stability

[MikePhua]

Introduction to the CAT 426B Extendahoe
The Caterpillar 426B Extendahoe is a versatile backhoe-loader known for its robust build and extendable dipper stick, commonly referred to as the Extendahoe. Designed for trenching, lifting, and material handling, this machine offers increased reach and digging depth compared to standard backhoes. However, as with any mechanical system under constant use, wear in pivot points and the extendable dipper can develop over time, often requiring adjustments via shimming to restore original performance.
Understanding Shims and Their Purpose
Shims are thin, precisely machined metal spacers used to adjust the spacing or preload between mechanical components. In heavy equipment, especially on the extendable dipper section of backhoes like the 426B, shims help:

• Eliminate excessive play in sliding components
  
• Restore correct alignment of the inner and outer dipper sections
  
• Reduce vibration, noise, and wear
  
• Maintain operator control and safety during operation
  

Where Shims Are Applied on the Extendahoe
On the CAT 426B Extendahoe, shims are typically installed in the following locations:

• Between the inner and outer dipper stick: These guide surfaces are prone to wear, especially on machines that have been used heavily in trenching or loading gravel.
  
• Slide block areas: To keep the inner extension section tracking properly inside the housing.
  
• End stops or retainers: Shims here help ensure that the extendable section doesn’t rattle or “clunk” when fully extended or retracted.
  

Common Symptoms That Call for Reshim

• Excessive lateral or vertical movement of the extendable arm
  
• Clunking or banging noise when the dipper is moved or hits limits
  
• Noticeable delay or slop between joystick input and mechanical response
  
• Uneven wear on the sliding pads or guides
  
• Hydraulic cylinder binding due to misalignment of the dipper sections
  

Shim Thickness Recommendations and Adjustments
There’s no one-size-fits-all shim thickness. Shims are typically stacked in combinations to achieve the desired fit. Common shim thicknesses include:

• 0.010 inch (0.25 mm)
  
• 0.020 inch (0.50 mm)
  
• 0.030 inch (0.75 mm)
  
• 0.060 inch (1.5 mm)
  

The goal is to adjust the fit until there is minimal play without creating excessive binding. You should be able to slide the extendahoe smoothly without resistance or rattling. Over-shimming can cause the inner dipper to stick or gall, while under-shimming allows for too much motion and premature wear.
Measuring for Shim Requirements

• Extend the dipper fully and inspect clearance at all contact points.
  
• Use feeler gauges to measure the gap between wear pads or contact faces.
  
• Compare measured clearance to manufacturer specifications or baseline from a similar machine.
  

Real-World Case: Rural Contractor’s Fix
A contractor working on septic systems in rural Kentucky noticed his CAT 426B’s extendahoe had significant side play and a loud bang when retracted. After removing the retaining bolts, he discovered the factory-installed shims had worn thin and were unevenly compressed. Instead of replacing the entire slide block assembly, he sourced a shim kit with varying thicknesses and rebuilt the system over a weekend.
His method:

• Cleaned the contact surfaces thoroughly
  
• Checked for galling or grooves and dressed them with a flat file
  
• Installed a stack of 0.020" and 0.010" shims until the arm moved snugly
  
• Applied anti-seize on bolt threads and torqued to spec
  

The result was a dramatic reduction in movement and smoother operation under load.
Technical Considerations During Shim Replacement

• Torque Specs Matter: Always torque the bolts holding shim stacks evenly and to factory specifications to prevent warping the components.
  
• Replace Any Distorted Shims: Bent or crushed shims will not restore correct alignment and may worsen the problem.
  
• Use Hardened Shims: Do not use makeshift materials like cut tin or soft washers. Only precision-ground, heat-treated steel shims should be used in structural applications.
  
• Inspect Slide Pads: If the UHMW or bronze slide pads are worn, shimming alone may not resolve the issue. Replacement is advised.
  

When to Consider Professional Help
While reshim procedures are within reach for mechanically inclined owners, certain issues should prompt consultation with a dealer or heavy equipment mechanic:

• Cracked dipper housings
  
• Severe misalignment of hydraulic cylinder pins
  
• Worn or damaged extendahoe cylinder bushings
  
• Unexpected movement under hydraulic load despite tight shims
  

Preventive Maintenance for Extendahoe Longevity

• Grease the slide areas frequently, especially before use in dusty or wet conditions.
  
• Inspect shims during every 500-hour maintenance cycle. Even if tight, they can compress over time.
  
• Clean the inner dipper section regularly. Dirt or sand trapped inside the housing accelerates wear.
  
• Avoid sudden slams or extensions. Operating the extendahoe with finesse can extend shim and component life.
  

Final Thoughts
The CAT 426B Extendahoe remains a trusted machine for contractors and municipalities alike. But like any hard-working tool, it requires attention to detail—especially in its extendable arm mechanics. Shimming may seem minor, but when done correctly, it restores confidence in the machine’s reach and power. Whether you’re trenching for water lines or lifting concrete barriers, a properly shimmed Extendahoe is not just more efficient—it’s safer and smoother to operate.