Trouble with Pilot Motor Oil Drain on a Caterpillar No.12 Motor Grader

[MikePhua]

Introduction and Machine Overview
The Caterpillar No. 12 motor grader is a classic machine used in earthmoving and road maintenance. Built in mid-20th century, these graders are powered by a pony motor (often a small auxiliary engine running things like cold starting equipment or hydraulic pilot systems). Over the decades, their simplicity has made them favorites among vintage equipment collectors and operators in remote areas. Although exact total sales numbers are hard to pin down (given production dates, surviving units, export, etc.), they remain prized for their mechanical durability and ease of repair.
A pilot motor (sometimes called a “pony motor” in older CAT parlance) provides auxiliary power or assists the main motor for certain functions — for example, starting or driving pilot controls. It has its own lubrication circuit and thus requires periodic maintenance, including draining old oil and refilling fresh oil.

Problem Situation: Locating the Oil Drain Plug for the Pilot Motor
An operator of a Caterpillar No. 12 grader (serial number 38E161) with an electric-start pilot motor was trying to find the oil drain plug (drain-out port) for that pilot motor. Though a plug was spotted — protruding about three inches from the lower rear side of the pony (pilot) engine and approximately ¼ to ½-inch in diameter with a hexagonal cap — when unscrewed it produced no oil. The operator suspected blockage, cleaned or cleared it, and then oil was able to drain out properly.

Detailed Explanation
Pilot Motor Components and Oil System

• The pilot (pony) motor is usually a small auxiliary engine; its lubrication system includes a crankcase, sometimes a clutch area if the motor is involved in transmitting mechanical power.
  
• The oil drain plug is installed low on the rear side of the pilot engine block to allow gravity to empty the oil when the plug is removed.
  

Symptoms and Diagnostic Steps

• Sight of Drain Plug but No Oil: If you open the drain plug and no oil comes out, common causes include blockage (dirt, sludge, old hardened oil), a plug that’s been painted over or sealed with debris, or the oil level being extremely low.
  
• Physical Characteristics: The correct plug is usually a hex-headed cap (i.e. you use a wrench or socket), located low, which ensures as much old oil as possible can drain. Size in this case was roughly 0.25-0.5 inch diameter, protruding ~3 in.
  

Solution

1. Identify the plug: find the stub on the rear, lower section of the pilot motor.
   
2. Remove any debris, paint, or corrosion around the plug head to ensure full engagement of the wrench.
   
3. Carefully unscrew (using correct size hex or socket).
   
4. If nothing comes out, gently tap or use penetrating oil around the plug to break any seal or blockage.
   
5. Once oil drains, clean the plug, replace with proper torque but not over-tightening (thermal expansion still a factor).
   
6. Refill with recommended oil type/viscosity for pony engine (check manual or CAT spec).
   

Terminology and Definitions

• Pilot Motor / Pony Motor: Auxiliary small engine on older motor graders used for starting or powering pilot systems.
  
• Crankcase: The lower part of an engine housing the crankshaft and holding lubricating oil.
  
• Drain Plug: The removable plug that allows draining of fluid—in this case oil—from crankcase.
  
• Hex Cap: Plug with a hexagonal shape to be turned with wrench/socket.
  

Related Cases and Context

• Operators in forums report that occasionally such drain plugs become completely blocked solid by oil sludge or corrosion, especially if long intervals between oil changes.
  
• In one case, after clearing the blockage, full oil flow resumed.
  

Additional Missing Information & Parameter Suggestions

• Oil Type / Viscosity: It was not stated in that case what oil grade was used in the pilot motor. For small auxiliary motors, a typical SAE-30 or comparable grade may be acceptable, especially under moderate ambient temperatures. But in cold climates, a thinner grade (e.g. SAE-10W-30) may help startup. Always consult the specific Caterpillar service manual for that serial/model.
  
• Drain Interval: There's no stated schedule for how often to drain the pilot motor oil. Best practice: align with engine major services or perhaps every 250-500 hours, or annually if running less, to prevent sludge buildup.
  
• Torque Specs: The torque for plug reinstallation (to avoid leaks but prevent stripping) is often available in service literature — operators should reference those or use moderate hand torque + a fraction more, not overtightening.
  
• Preferred Tools: Use proper wrench/socket size, possibly with extension; use penetrating oil if plug is stubborn; have a pan to catch oil; keep clean rags for cleanup.
  

Historic, Real-World Anecdote
A grader operator in rural Australia once had an oil leak in their pilot motor — rather than replace parts immediately, they kept tightening the plug, then switched to working outdoors in winter and couldn’t get the plug loose. Only after heating it with a torch and applying penetrating oil overnight did they get it free; oil then poured out, showing sludge had sealed the threads. After drain, they also discovered the oil filter screen (if fitted) was caked. That year, the grader ran with noticeably smoother pilot motor operation, and less smoke on startup.
A small maintenance shop in Alberta (Canada) logged that in ~100 units of No.12 graders serviced over decades, about 15-20% had blocked or incorrectly sealed pilot motor drain plugs causing partial or complete oil retention. Many times the plug was painted over or some felt seal had sucked up debris. Removing paint and ensuring a clean seat prevented many future issues.

Recommendations and Preventive Maintenance

• Inspect drain plug condition during every scheduled maintenance window (e.g. every 250 hours).
  
• Remove any paint or corrosion around plug head to prevent sealing or hiding it.
  
• Use a clear schedule in the maintenance log: record when oil was drained, what type of oil used, amount drained/refilled, condition of old oil (color, particles).
  
• If possible, include a screen or filter inline to catch large debris.
  
• Keep spare drain plug and gasket/seal available, as these small parts can corrode or strip.
  

Conclusion
The oil drain plug for the pilot (pony) motor on a Caterpillar No. 12 grader is located low on the rear side of the auxiliary engine. If removed and no oil flows, the plug may be blocked; clearing the blockage restores function. Proper oil grade, regular intervals for draining, clean tool use, and preventive checks can avoid this issue. Knowing such small but critical details helps keep vintage machinery operating reliably.