How to Change the Oil in a 2021 GAS GAS TXT GP 125

Shop parts for a 2021 GAS GAS TXT GP 125 Dirt Bike.

Maintaining your 2021 GAS GAS TXT GP 125 is straightforward, but like any trials bike, it has a few model-specific steps that are important to follow when changing the transmission oil (as the engine itself uses a premix and does not require a traditional oil change). This guide is exclusive to MotoSport.com and will walk you through the process, including all tools, recommended fluids, and care tips unique to your bike.

Tools and Materials Needed

  • Socket wrench set
  • 8mm and 13mm sockets (verify size for drain and fill bolts)
  • Flathead screwdriver
  • Drain pan
  • Funnel
  • Clean rags
  • Transmission oil (SAE 75W/80W or specific trials gearbox oil, approx. 400cc–consult your manual for exact volume)
  • Gloves
  • Parts cleaner (optional)
  • Torque wrench

Preparation Steps

  • Start your bike and let it idle for a few minutes to warm the oil, which helps it drain more thoroughly.
  • Turn off the motorcycle and allow it to cool enough to handle without risk of burns.
  • Place your TXT GP 125 on a stable stand so it’s upright and level.
  • Lay out your tools and supplies within easy reach before you begin.

Step-by-Step Oil Change

  1. Locate the Oil Drain Bolt
    • The drain bolt is typically found at the lowest point of the right engine casing. Consult your manual if unsure.
  2. Remove the Drain Bolt
    • Place your drain pan directly under the bolt.
    • Use your socket wrench to carefully remove the bolt. Allow all oil to drain fully. Inspect the drain plug magnet for metal shavings; minor debris is normal in two-strokes but excessive fragments signal the need for further inspection.
  3. Inspect and Clean Components
    • Clean the bolt threads and wiping the surface around the drain area with a rag to ensure a tight seal on reassembly.
    • If your bolt uses a crush washer, inspect and replace it to prevent future leaks.
  4. Reinstall the Drain Bolt
    • Hand-tighten first, then use a torque wrench to the manufacturer’s recommended specification (typically around 10–12 Nm for this model, but verify with your user manual or dealer).
  5. Remove the Oil Fill Plug
    • The fill plug is often on the right engine cover, higher up than the drain. Remove it using the correct socket or Allen key.
  6. Add New Transmission Oil
    • Insert a funnel and pour in the recommended volume of fresh oil (most TXT GP 125 models are around 400cc, but always confirm with your specific manual for 2021).
    • A top-quality synthetic or semi-synthetic gearbox oil offers best clutch feel and protection. A specialist trials gearbox oil is ideal, but a premium SAE 75W/80W is also suitable.
  7. Reinstall the Fill Plug
    • Tighten securely, but do not overtighten. Use a torque wrench if possible.
  8. Wipe Down and Check for Leaks
    • Clean off any oil residue from the case. Start the bike and let it run for a few minutes, then inspect around the drain and fill plugs to confirm there are no leaks.

Model-Specific Notes and Call-Outs

  • 2021 GAS GAS TXT GP 125: This model uses a premix for combustion lubrication (a mix of premium unleaded gasoline and synthetic or semi-synthetic 2-stroke oil at 1.5%—that’s about 67:1). There is no separate engine oil system, so only the gearbox (transmission) oil is changed during routine servicing. Do NOT attempt to add engine oil to a crankcase; you only change the transmission oil.
  • Check Oil Window: Some TXT GP models feature a sight glass on the engine cover to check oil level. Always confirm oil level after filling, and adjust as needed.
  • Tightening Torque: Threaded fasteners in the engine case are made from soft alloys. Over-tightening can strip threads—be gentle and use proper torque specs.
  • Oil Recommendations: High-quality trials-specific transmission oil can improve clutch feel and component longevity. Avoid automotive engine oils, as they may cause clutch slippage or poor performance.
  • Service Interval: Change the gearbox oil after the initial break-in (≈8 hours) and then every 20–30 riding hours or after any event involving heavy clutch use.

Additional Owner Tips

  • Always warm the engine before draining to help the old oil flow out with contaminants.
  • Dispose of used oil responsibly—do not dump it outside or in household waste.
  • If you ride in wet conditions and suspect water ingress, change oil immediately, as milky oil can damage internal parts.
  • If your clutch performance feels off after oil change, double-check you used the recommended oil type and volume.
  • Use only OEM or high-quality aftermarket washers and seals to prevent leaks.

By following these steps and recommendations, your 2021 GAS GAS TXT GP 125’s transmission will remain protected and your ride will continue to perform at its best. For further details, always consult your owner’s manual or a GAS GAS-trained technician.

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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.