Komatsu D21-6 Steering System and Troubleshooting Insights

[MikePhua]

The D21 Series and Komatsu’s Compact Dozer Legacy
Komatsu, founded in 1921 in Japan, has long been a global leader in construction and mining equipment. The D21 series, including the D21-6 variant, was introduced as a compact crawler dozer designed for light grading, land clearing, and agricultural use. With an operating weight of approximately 8,000 pounds and a footprint narrow enough for orchard rows or tight construction sites, the D21 became a favorite among small contractors and landowners.
The D21-6 model features a direct mechanical drivetrain, simple hydraulic systems, and manual steering clutches—making it highly serviceable and durable in remote or rugged environments. Thousands of units were sold across Asia, North America, and Australia, with many still in operation decades later.
Steering System Configuration and Function
The Komatsu D21-6 uses a dual steering clutch and brake system to control direction. Unlike hydrostatic or power-assisted steering found in larger dozers, the D21 relies on mechanical linkages and friction clutches to engage or disengage drive to each track independently.
Key components include:

• Steering clutch packs (left and right)
  
• Brake bands and drums
  
• Control levers connected via rods and bellcranks
  
• Return springs and adjustment bolts
  
• Final drive housings with gear reduction
  

When the operator pulls a steering lever, the clutch disengages drive to that side, and the brake applies friction to slow or stop the track. This allows the opposite track to continue driving, causing the machine to pivot or turn.
Common Steering Issues and Their Causes
Over time, the steering system can develop problems due to wear, contamination, or misadjustment. Frequent symptoms include:

• Difficulty turning in one direction
  
• Steering lever feels loose or lacks resistance
  
• Machine turns sluggishly or not at all
  
• Grinding or squealing noises during turns
  
• Brake band overheating or smoking
  

Typical causes:

• Worn clutch discs or glazed friction surfaces
  
• Oil contamination in clutch housing
  
• Broken or stretched return springs
  
• Misaligned linkage or seized bellcrank pivots
  
• Brake band wear or drum scoring
  

In one case from rural Queensland, a D21-6 began pulling to one side during grading. The operator discovered that the left clutch pack was saturated with hydraulic oil due to a leaking seal. After replacing the seal and cleaning the clutch discs with solvent, steering performance was restored.
Inspection and Adjustment Procedures
Routine inspection and adjustment are essential to maintain steering responsiveness. Recommended steps include:

• Remove side covers to access clutch and brake housings
  
• Check clutch disc thickness and surface condition
  
• Inspect brake bands for wear and drum scoring
  
• Verify linkage alignment and lever travel
  
• Adjust clutch engagement bolts to factory spec
  
• Replace return springs if stretched or broken
  

Adjustment tips:

• Clutch free play should be ~1 inch at the lever
  
• Brake band tension should allow full engagement without dragging
  
• Linkage rods must be straight and pivot freely
  
• Use high-temperature grease on bellcrank bushings
  

In a rebuild project in Oregon, a D21-6 was restored after years of orchard work. The steering system was rebuilt with new clutch discs, brake bands, and linkage bushings. The operator noted that the machine turned smoothly even on soft soil and tight slopes.
Preventive Maintenance and Fluid Management
To extend the life of the steering system:

• Change transmission and final drive oil every 250 hours
  
• Use SAE 30 or 40 non-detergent oil in clutch housings
  
• Inspect seals and gaskets for leaks quarterly
  
• Clean clutch housings annually to remove dust and oil residue
  
• Grease all linkage points monthly
  

Avoid using hydraulic or multi-grade oils in clutch housings, as they can reduce friction and cause slippage. In cold climates, pre-warming the machine helps prevent condensation and clutch drag.
Parts Availability and Restoration Tips
Though the D21-6 is no longer in production, parts remain available through Komatsu dealers, aftermarket suppliers, and salvage yards. Common replacement items include:

• Clutch discs and pressure plates
  
• Brake bands and drums
  
• Linkage rods and bellcranks
  
• Return springs and adjustment bolts
  
• Final drive seals and bearings
  

In one restoration in British Columbia, a mechanic fabricated new bellcrank bushings from bronze stock and installed aftermarket clutch discs. The machine was later used for trail building and performed reliably under load.
Operator Stories and Field Wisdom
In Texas, a rancher used a D21-6 to clear mesquite and grade access roads. After noticing steering lag, he adjusted the clutch bolts and replaced a broken return spring. The dozer regained full turning ability and was used daily for fence line maintenance.
In New Zealand, a vineyard owner relied on a D21-6 for slope grading. After rebuilding the brake bands and installing a new seat, the machine operated comfortably and safely on steep terrain.
Conclusion
The Komatsu D21-6 steering system is a mechanically elegant solution for compact dozer control. With proper maintenance, adjustment, and occasional rebuilding, it offers precise maneuverability and long-term reliability. Whether clearing brush or shaping orchard rows, the D21-6 continues to prove that simplicity and durability are a winning combination in small-scale earthmoving.