Restoring the Michigan 75GM IIIA Wheel Loader

[MikePhua]

Clark Michigan’s Industrial Legacy
The Michigan 75GM IIIA wheel loader was produced by Clark Equipment Company, a manufacturer with deep roots in American industrial machinery. Founded in 1903, Clark became a major player in the post-war construction boom, with its Michigan line of loaders gaining popularity for their rugged design and reliable performance. The 75GM IIIA, built in the late 1960s to early 1970s, was a mid-sized loader powered by Detroit Diesel two-stroke engines, often the 4-71 series. These machines were widely used in logging, aggregate handling, and municipal work, with thousands sold across North America.
The “GM” designation refers to the use of General Motors Detroit Diesel engines, while “IIIA” indicates a specific configuration and update package. The model 409-B-306-CAC, for example, reflects a later serial prefix that includes axle upgrades and transmission refinements.
Terminology Note

• Detroit Diesel 4-71: A two-stroke inline four-cylinder diesel engine known for its distinctive sound and high-revving characteristics.
  
• Power Shift Transmission: A hydraulic transmission allowing gear changes without clutching.
  
• Work Range/Travel Range Lever: A mechanical selector that shifts the transmission between low-speed torque mode and high-speed travel mode.
  
• Crankcase Breather: A ventilation system that releases pressure from the engine’s crankcase and prevents oil contamination.
  

Engine Identification and Oil Specifications
The engine serial number prefix “4A” confirms the presence of a Detroit Diesel 4-71. Based on the serial number 173476, this particular unit was manufactured in 1971. For optimal performance, the engine requires SAE 40 monograde oil with a sulfated ash content below 1.0%. Multi-grade oils like 15W-40 are not recommended due to incompatibility with the two-stroke design and potential for carbon buildup.
Preferred oils include:

• Chevron Delo 100 SAE 40
  
• Mobil Delvac 1240
  
• Shell Rotella SAE 40 (non-multigrade variant)
  

In one restoration case, a tree service operator in Washington discovered water contamination in the crankcase due to a missing dipstick bell. After draining the gray oil and flushing with diesel overnight, nearly a quart of water was removed. Switching to proper SAE 40 oil restored engine behavior and reduced white smoke from the breather.
Transmission Fluid and Range Selector Behavior
The transmission in the 75GM IIIA was originally filled with ATF due to Canadian climate considerations. However, in warmer regions, operators often use SAE 20 or SAE 30 engine oil. The fluid should be checked in neutral with the engine at low idle and warm. A lever near the operator’s left heel toggles between work range and travel range. This mechanical shift must be performed with the machine stopped and in neutral to avoid gear damage.
Symptoms of incorrect fluid or shifting technique include:

• Delayed engagement
  
• Sluggish response in forward or reverse
  
• Difficulty climbing grades in travel mode
  

A technician in Georgia noted that using multi-grade oil in the transmission caused erratic shifting and overheating. Replacing it with SAE 30 restored smooth operation.
Crankcase Breather and Valve Cover Modifications
The crankcase breather should be a sealed housing mounted atop the valve cover, not an open hole. Missing components can allow rainwater and debris into the engine, especially in regions with sideways rain like the Pacific Northwest. A proper breather assembly includes:

• Breather cap with mesh filter
  
• Piping to route vapors away from the engine
  
• Splash guard to prevent oil mist escape
  

In a rebuild project, a missing breather led to excessive white smoke and oil vapor discharge. Installing a factory-style breather reduced emissions and protected the engine from environmental contamination.
Oil Filter Conversion and Maintenance Tips
The original cartridge-style oil filter on the 4-71 can be difficult to service due to its location and alignment challenges. Spin-on conversion kits were once available and may still be sourced through aftermarket suppliers like Wix or Baldwin.
Benefits of spin-on conversion:

• Faster filter changes
  
• Reduced risk of gasket misalignment
  
• Improved sealing under vibration
  

One operator used a tarp strap to hold the cartridge in place while threading the bolt—a workaround that highlights the need for a more ergonomic solution.
Boom Hinge Wear and Bushing Repair Strategies
The lower boom hinge on the 75GM IIIA often develops slop due to worn bushings and pins. If the top hinge is inaccessible, operators can tie a rope to the boom lever and observe movement from the ground. A visible gap between the bushing OD and the pivot steel may require more than standard replacement.
Repair options include:

• Installing oversized bushings and machining the bore
  
• Pouring babbitt or bronze to fill the gap
  
• Using industrial epoxy for temporary fixes
  

A restoration in Oregon involved machining custom bushings and installing them one at a time to avoid full disassembly. The result was a tighter boom with improved control during tree removal.
Center Pivot Bearing and Structural Concerns
The center pivot bearing, located between the front and rear halves of the loader, is housed in a bolted casing. It may contain roller bearings or bushings depending on the model year. Wear in this area can cause frame misalignment and steering issues.
Inspection steps:

• Remove top plate and check for play
  
• Rotate the loader and observe bearing movement
  
• Look for signs of grease leakage or metal shavings
  

If the bearing is worn, replacement may require lifting the rear frame and supporting the front half. A shop job could exceed the value of the machine, so many operators opt for in-field repairs using shims or poured metal.
Conclusion
Restoring a Michigan 75GM IIIA wheel loader is a rewarding challenge that blends mechanical intuition with historical appreciation. From engine oil selection to hinge bushing repair, each subsystem demands attention to detail and respect for vintage engineering. With proper fluids, sealed breathers, and thoughtful upgrades, these machines can return to productive service—even decades after their prime. The legacy of Clark Michigan lives on in every rebuilt loader that rolls back into the field.