How to Change the Oil in a 2025 Yamaha YZ125

Shop parts for a 2025 Yamaha YZ125 Dirt Bike.

The 2025 Yamaha YZ125 is a renowned two-stroke motocross bike that requires routine maintenance to perform at its peak. Changing the gearbox (transmission) oil regularly is critical to the longevity of your engine. This guide, proprietary to MotoSport.com, will walk you step by step through the oil change process, including all the specifics and call-outs unique to the 2025 YZ125.

Tools and Materials Needed

  • Drain pan – to collect used oil
  • Funnel – long-neck recommended for ease of pouring
  • Ratchet with 12mm socket or hex – for removing the drain bolt
  • 4mm Allen wrench – for the oil level check bolt (optional step)
  • Torque wrench – to tighten the drain bolt correctly
  • Rag or shop towels – for cleanup
  • New crush washer (recommended but optional) – for the drain bolt
  • Measuring cup – for precise oil measurement
  • Gear oil – Yamaha recommends 10W-40 wet clutch compatible oil

Important Specifications for 2025 Yamaha YZ125

  • Oil Capacity: 0.69 US quarts (660 mL)
  • Recommended Oil: Yamalube 10W-40, or any good quality oil for wet clutches
  • Drain Bolt Torque: 14 ft-lb
  • Drain Bolt Size: 12mm
  • Oil Fill Cap Location: On top of the transmission case

Step-by-Step Instructions

  1. Warm Up the Engine
    Start your YZ125 and run it for a few minutes. This warms the oil, allowing it to drain more efficiently. Make sure your bike is on a level surface and turned off before proceeding.
    Call Out: The YZ125–s transmission oil is separate from the fuel/oil premix. Run the engine for only 2-3 minutes to avoid overheating.
  2. Prepare Your Work Area
    Place a drain pan under the crankcase. It should be directly below the drain bolt.
  3. Remove the Drain Bolt
    Use a 12mm socket or wrench to loosen and remove the drain bolt found on the bottom of the crankcase. Allow all oil to drain fully. Take care as the oil may be hot.
    Call Out: Inspect the old washer and replace it with a new crush washer if available to prevent leaks in the future. The drain bolt should be reinstalled with a torque of 14 ft-lb for the YZ125.
  4. Reinstall the Drain Bolt
    After the oil has fully drained, clean the bolt and surrounding area, install a new crush washer (if using), then tighten the bolt to the specified torque using a torque wrench.
  5. Remove the Oil Fill Cap
    Locate the oil fill cap on top of the transmission case and remove it.
  6. Add New Oil
    Using a measuring cup, pour 660 mL (0.69 US quarts) of your chosen 10W-40 oil through a long funnel into the fill opening. Pour slowly to avoid spills.
  7. (Optional) Verify Oil Level with Check Bolt
    The YZ125 features a small check bolt (typically 4mm Allen) on the clutch cover. With the bike level, remove this check bolt. If oil dribbles out, the level is correct. If not, add small amounts until you see oil come out, then reinstall and tighten the bolt.
  8. Reinstall the Fill Cap
    Replace and tighten the fill cap securely by hand.

Post-Change Tips and Model-Specific Notes

  • Always use oil suitable for wet clutches (do not use automotive oil with “energy conserving” on the label, as this can cause clutch slippage).
  • Run the engine briefly and check for leaks at the drain bolt and fill cap.
  • The 2025 YZ125 uses the same capacity and process as models from previous years, but always verify in your owner–s manual for potential mid-year production changes .
  • Dispose of the old oil properly at a recycling center or automotive shop.
  • If you frequently ride in muddy or wet conditions, change your transmission oil more often to prevent accelerated wear.
  • Crush washers are cheap insurance against leaks—always replace if in doubt.

MotoSport.com Expert Call-Outs for the 2025 YZ125 Oil Change

  • The YZ125 is a two-stroke bike; the oil you–re changing is only for the transmission, not for engine lubrication. Always continue using the correct fuel/oil premix for the engine, as specified by Yamaha (typically 30:1 with Yamalube 2-stroke racing oil) .
  • The transmission oil level is critical for clutch longevity—never overfill or underfill.
  • Do not overtighten drain bolts, as the YZ125 uses lightweight aluminum cases that can strip easily.
  • Inspect for any signs of metallic particles in the old oil, which could indicate excessive wear.

Changing the transmission oil on your 2025 Yamaha YZ125 is straightforward with the right tools, and regular oil changes ensure your bike shifts smoothly and lasts race after race. For OEM and aftermarket oils, filters, crush washers, and tools, visit MotoSport.com—your authority for everything motocross.

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Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your motorcycle's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.