1. Basic Information

• Part Number: 12420478 (OEM original part)
• Type: Flywheel Ring Gear
• Applicable Models: MWM TBG620 V12/V16/V20 and TBD620 series gas & diesel engines
• Material: Grade 45 steel (S45C) with high-frequency quenched teeth, hardness: 45–55 HRC
• Tooth Specification: 136 teeth (standard for 620 series)
• Installation Method: Shrink fit. Heat the gear for expansion and fit it onto the flywheel, which will be tightly secured after cooling.
• Original Reference Price (2026): Approximately €120–140 per piece (exclusive of tax)
• Related Parts:
  • 12420477: Flywheel body
  • Starter: Matched for 136-tooth specification

2. Functions & Features

• Torque transmission for starting: Meshes with the starter drive gear to transmit starting torque and rotate the crankshaft.
• Speed stabilization: Stores energy together with the flywheel to reduce speed fluctuation of four-stroke engines.
• Positioning reference: Works with the crankshaft sensor to deliver speed and phase signals, ensuring accurate fuel injection and ignition timing.
• Wear and impact resistance: Quenched tooth surface withstands instantaneous impact load during starting for long service life.

3. Common Failures & Replacement

• Broken or chipped teeth: Abnormal noise and idling during starting, usually caused by poor meshing of starter gear or excessive tooth wear.
• Wear and deformation: Long-term engagement leads to tooth profile wear and contact surface distortion, resulting in difficult starting.
• Loosening: Failure of shrink fit causes relative displacement between ring gear and flywheel, leading to vibration and abnormal noise.
• Replacement Interval: Replace every 20000–25000 operating hours, or immediately once tooth damage or loosening occurs.

4. Installation Instructions

1. Heat the ring gear to 200–250°C with even heating to avoid local overheating.
2. Align it quickly with the positioning surface on the flywheel outer rim and push in horizontally to prevent tilting.
3. Let it cool down naturally to ambient temperature. Check the fitting tightness and concentricity to ensure no gap or wobble.
4. Dynamic balance check: The dynamic balance error of the complete flywheel assembly shall not exceed 5g·cm.