Restoring Blade Cylinders on the 1958 Caterpillar D7 17A Dozer

[MikePhua]

The D7 17A and Its Historical Significance
The Caterpillar D7 17A series was introduced in the mid-1950s as part of Caterpillar’s post-war expansion into heavier-duty track-type tractors. Built for logging, road construction, and military use, the 17A variant featured a D7 diesel engine, cable or hydraulic blade options, and a rugged undercarriage designed for long-term field service. Caterpillar Inc., founded in 1925, had by then become a global leader in earthmoving equipment, and the D7 was one of its most widely deployed mid-size dozers.
Sales of the 17A series extended through the late 1950s, with thousands of units shipped across North America, Europe, and Asia. Many remain in operation today, especially in restoration circles and remote logging camps, where mechanical simplicity and parts interchangeability are prized.
Blade Cylinder Wear and Common Failures
After decades of use, the blade lift cylinders on the D7 17A often show signs of fatigue. Typical issues include:

• Grooved cylinder shafts from abrasive grit exposure
  
• Cracked seal retainers due to bolt failure or corrosion
  
• Leaking seals and degraded packing
  
• Bent rods from overloading or side stress
  

Terminology annotation:

• Seal retainer: A metal plate that holds the hydraulic seal in place at the cylinder head.
  
• Cylinder shaft: The polished rod that extends and retracts to move the blade.
  
• Packing gland: The assembly that houses seals and prevents hydraulic fluid leakage.
  

In one case, the right-side seal retainer fractured after a bolt sheared, causing a corner of the retainer to break off. Both cylinder shafts were visibly grooved, indicating years of operation in dusty or sandy conditions without adequate wiper seal protection.
Identifying Parts Without a Serial Number
Many older machines have lost their serial number plates, complicating parts sourcing. Fortunately, Caterpillar’s hydraulic systems from that era often carried separate serial numbers and part numbers cast directly into the cylinder head or barrel.
To identify replacement parts:

• Inspect the cylinder head for cast-in part numbers
  
• Compare dimensions with known D7 17A cylinder specs
  
• Reference parts catalogs from the 1950s–1960s
  
• Consult with vintage parts dealers or Caterpillar legacy support
  

In one restoration case, a mechanic in Oregon identified his cylinder using a casting number on the head, bypassing the need for a chassis serial number entirely.
Repair vs. Replacement Strategy
Owners facing cylinder damage must decide between repair and replacement. Repair options include:

• Welding and machining the seal retainer
  
• Rechroming and grinding the cylinder shaft
  
• Installing new seals and bushings
  
• Fabricating a new retainer plate from aluminum or steel
  

Replacement options include:

• Sourcing original cylinders from salvage yards
  
• Purchasing aftermarket units compatible with D7 17A geometry
  
• Retrofitting cylinders from similar-era machines (e.g., D6 or D8)
  

In Quebec, a collector found two aluminum seal retainers (part number 1J344) and matching shims from a retired logging outfit. These parts allowed him to rebuild his blade cylinders without fabricating new components.
Hydraulic System Compatibility and Upgrades
The D7 17A used a basic hydraulic system with open-center valves and low-pressure lines. When replacing or rebuilding cylinders, ensure compatibility with:

• Hydraulic line fittings (typically flare or pipe thread)
  
• Mounting pin diameter and spacing
  
• Stroke length and bore size
  
• Operating pressure (usually under 2,000 psi)
  

Some restorers upgrade to modern seals or synthetic wipers to improve longevity. Others retrofit newer cylinders with adapter bushings to match the original mounts.
In British Columbia, a forestry operator replaced his D7 cylinders with units from a 1970s D6C, using custom bushings and line adapters. The result was a smoother lift and better seal retention.
Locating Engine Numbers and Casting Marks
Even if the machine’s serial number is missing, engine identification is still possible. On Caterpillar D7s, the engine number is typically cast into the block below the magneto, near the governor linkage.
To locate it:

• Clean the flat casting area beneath the magneto
  
• Look for stamped or cast numbers (e.g., 7M, D7E, etc.)
  
• Cross-reference with engine manuals or parts guides
  

This number can help verify engine type, production year, and compatibility with hydraulic components.
Preventative Maintenance and Preservation Tips
To extend the life of blade cylinders:

• Install wiper seals to prevent grit intrusion
  
• Grease pivot points regularly
  
• Flush hydraulic fluid annually
  
• Inspect bolts and retainers for fatigue
  
• Store the machine under cover when idle
  

In one Montana case, a rancher used canvas wraps over his cylinder rods during winter to prevent ice scoring. After 15 years, his cylinders showed minimal wear compared to neighboring machines.
Conclusion
Restoring the blade cylinders on a 1958 Caterpillar D7 17A is a blend of detective work, mechanical skill, and historical appreciation. Whether sourcing vintage parts, fabricating replacements, or adapting newer components, the goal is to preserve the machine’s functionality without compromising its legacy.
With careful inspection, creative sourcing, and attention to hydraulic detail, even a 70-year-old dozer can return to service—pushing dirt, clearing land, and reminding us why these machines were built to last.