Is It Just Paint or a Structural Crack on the Boom

[MikePhua]

Visual Clues Can Be Misleading
When inspecting a repainted excavator boom, spotting a linear mark or surface blemish often raises alarms. In one case involving a 2000 Caterpillar 330BL with over 10,000 hours, a suspicious line appeared near the main boom pivot. The machine had been idle for months in a coastal port, and the mark emerged after purchase. The question was simple: is it a paint crack or a structural failure? But the answer required more than a glance.
Common Boom Crack Locations and Causes
Excavator booms are subject to intense cyclic stress, especially near weld joints, gussets, and pin bosses. Cracks often initiate:

• Behind boom cylinder mounts
  
• Below stick-to-boom connection pins
  
• Along welds where plate thickness transitions
  
• Near internal gusset terminations
  

These areas concentrate stress, especially if welding was done without proper preheating or cooling control. A poorly executed weld can develop microfractures that propagate under load, eventually surfacing as visible cracks.
Paint, Filler, or Metal Fatigue
A straight, narrow line under thick paint may be:

• A crack in body filler (commonly Bondo)
  
• A surface weld defect
  
• A true structural crack
  

Paint rarely cracks in a straight line unless applied over a flawed substrate. If the mark runs nearly the full thickness of the boom plate, it warrants deeper investigation. Some machines sold through auctions or after cosmetic repairs may conceal prior damage with filler and paint.
Diagnostic Techniques for Confirmation
Before assuming the worst, several field methods can help:

• Grinding Test: Use an angle grinder to remove paint and filler. If sparks fly and the metal is clean, it’s likely cosmetic. If the wheel digs into a void or reveals a fracture, it’s structural.
  
• Magnetic Particle Inspection: Apply iron filings and use a magnet. Cracks will align filings in arc-shaped patterns.
  
• Ultrasonic Testing: A certified technician can scan welds and plate interiors to detect subsurface cracks.
  
• Visual Comparison: Reference known failure points on similar models. For example, Komatsu PC750 and Cat 345C units have documented boom failures in similar locations.
  

Repair Strategies and Structural Integrity
If a crack is confirmed, repairs must go beyond surface welding:

• Cut out the affected boom section
  
• Access internal gussets and welds
  
• Rebuild with new plate and reinforcement
  
• Follow OEM welding procedures and stress relief protocols
  

A superficial weld or fishplate patch may hold temporarily but will fail under heavy use. One repair on a Cat 320CL involved cutting a three-foot section, rewelding all internal corners, and adding reinforcement—over 40 hours of skilled labor.
Safety and Operational Risk
Operating a machine with a suspected boom crack is dangerous. A full boom failure can result in uncontrolled collapse, risking injury or death. One operator recalled a Cat 245 boom snapping in –20°C weather, requiring emergency stabilization and extensive repair. Another reported a 308C boom detaching at the pin boss due to an undetected crack.
Recommendations for Buyers and Operators

• Avoid repainted machines without full inspection
  
• Request engineering drawings for boom sections from OEMs
  
• Use certified welders and follow manufacturer procedures
  
• Document repairs and monitor crack-prone areas regularly
  
• Treat any visible line near welds or pin bosses as a potential crack
  

Conclusion
A line on a boom may be paint, filler, or the start of a structural failure. Only thorough inspection and proper diagnostics can tell. In heavy equipment, ignoring a crack is never worth the risk. Whether buying, operating, or repairing, vigilance and expertise are the best safeguards against catastrophic failure.