Bomag AW120 Roller Engine Failure and Flywheel Damage

[MikePhua]

The Bomag AW120 and Its Role in Light Compaction
The Bomag AW120 is a compact tandem vibratory roller designed for small-scale paving, patching, and shoulder work. Bomag, founded in Germany in 1957, has become a global leader in compaction technology, with over 250,000 rollers sold worldwide. The AW120 model, often equipped with a Deutz 2-cylinder diesel engine, is known for its simplicity, maneuverability, and reliability in tight urban environments and minor roadwork. Its hydrostatic drive and dual-drum configuration allow for smooth compaction with minimal operator fatigue.
Flywheel Bolt Shear and Crankshaft Concerns
A rare but serious failure reported in the AW120 involves the shearing of all four flywheel bolts. This condition typically prevents the engine from turning over and raises concerns about deeper internal damage. The flywheel, mounted at the rear of the engine, connects to the hydraulic pump and starter. If the bolts shear, it may indicate:

• Excessive torque stress during startup
  
• Misalignment between engine and pump shaft
  
• Fatigue due to vibration or improper torque settings
  
• Sudden engine seizure causing rotational shock
  

In some cases, the crankshaft itself may be compromised. A broken crankshaft can be diagnosed by attempting to rotate the front pulley manually or prying gently beneath it to detect vertical movement. If the crank flexes or shifts unnaturally, internal fracture is likely.
Hydraulic Pump vs Engine Seizure
Before assuming engine failure, technicians should isolate the hydraulic pump. Disconnecting the pump from the engine allows independent rotation tests. If the engine turns freely without the pump, the fault lies in the pump’s internal components—possibly a seized rotor or jammed valve plate. Conversely, if the engine remains locked, further disassembly is required.
One field mechanic reported discovering broken flywheel bolts only after separating the hydraulic pump. This suggests that the pump may have locked up first, transmitting shock back to the flywheel and shearing the bolts.
Deutz Engine Characteristics and Vulnerabilities
The Deutz 2-cylinder diesel engine used in the AW120 is air-cooled and compact, ideal for light rollers. However, it has known vulnerabilities:

• Flywheel bolt torque must be precise—typically 75–85 Nm with thread locker
  
• Starter engagement must be smooth; misaligned teeth can cause shock loads
  
• Vibration dampers must be intact to prevent harmonic stress on the crank
  

In one case, a roller used on uneven terrain developed excessive vibration. The operator ignored early signs—rattling and starter misfire—until the flywheel bolts failed. Post-mortem inspection revealed that the damper plate had cracked weeks earlier.
Repair Strategy and Component Access
To repair flywheel bolt failure:

• Remove the hydraulic pump and starter assembly
  
• Inspect flywheel for cracks or warping
  
• Check crankshaft flange for scoring or deformation
  
• Replace all bolts with OEM-grade fasteners and torque precisely
  
• Use thread locker and verify alignment before reassembly
  

If the crankshaft is damaged, engine replacement may be more economical than rebuilding. Deutz engines are available as remanufactured units, often costing $2,500–$3,200 USD depending on region and configuration.
Preventive Measures and Operator Tips
To avoid flywheel-related failures:

• Perform vibration checks monthly
  
• Inspect starter gear engagement annually
  
• Torque flywheel bolts during major service intervals
  
• Avoid abrupt starts or cold cranking without preheat
  
• Monitor hydraulic pump temperature and noise during operation
  

Final Thoughts
The Bomag AW120 is a dependable roller, but its compact design means that small failures can cascade into major damage. Flywheel bolt shear is a red flag that demands immediate inspection of both engine and hydraulic components. With proper torque practices, vibration monitoring, and alignment checks, operators can prevent costly downtime and extend the life of this versatile machine.