How to Change the Oil in a 1999 Suzuki RM125: Step-by-Step Guide

Shop parts for a 1999 Suzuki RM125 Dirt Bike.

Keeping your 1999 Suzuki RM125 in peak condition requires regular transmission (gearbox) oil changes. This do-it-yourself guide, exclusive to MotoSport.com, shows you all the tools, steps, and Suzuki-specific details you need to take care of your bike’s transmission oil like a pro.

Tools and Supplies Needed

  • 10mm socket or wrench (for oil drain plug and oil level bolt)
  • Ratchet or driver compatible with socket
  • Oil catch pan
  • Funnel (helps during refill)
  • Measuring cup (at least 750 ml capacity)
  • Shop rags or paper towels (for cleanup)
  • New transmission oil (SAE 10W-40, API SF/SG or SH/SJ with JASO MA rating, as specified by Suzuki)
  • Torque wrench (for proper tightening of bolts)
  • Latex gloves (optional, for mess-free work)

Preparing Your Suzuki RM125 for an Oil Change

  • Work on a level, hard surface. Place the bike on a stand or keep it upright with the help of a friend.
  • Before starting, warm up the engine for a few minutes. This helps the oil drain out smoothly and removes more contaminants.
  • Let the engine cool for about 3 minutes after shutting it off, ensuring the oil isn’t scalding hot but still flows freely

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Step-by-Step Transmission Oil Change Process for 1999 RM125

  1. Locate the Drain Plug and Filler Cap
    • The oil drain plug is under the engine. The oil filler cap is typically on the right side of the crankcase. The oil level bolt (used to check correct oil fill level) is also on the right side, just below the filler cap, and is typically a 10mm bolt.

  2. Drain the Old Oil
    • Place your oil catch pan under the drain plug.
    • Using a 10mm socket or wrench, remove the oil drain plug
      (Tip: Position the pan to cover any splashes as the oil streams out).
    • Remove the oil filler cap, which helps air enter and oil flow out more quickly.
    • Wait until the oil fully drains. Swirling the bike gently (while keeping it upright) may release trapped oil.
  3. Inspect Drain Plug and Threads
    • Check the drain plug for metal debris stuck to the magnet (if equipped). Clean thoroughly.
    • Inspect the crush washer; replace if deformed or damaged for a secure seal.
  4. Reinstall the Drain Plug
    • Thread the drain plug back in by hand to avoid cross-threading.
    • Use your torque wrench to tighten the plug to 21 N·m (14.5 lb-ft) as specified.

  5. Add New Transmission Oil
    • Confirm you’re using SAE 10W-40, API SF/SG or SH/SJ with JASO MA-rated oil, as recommended.

    • Using a measuring cup, pour in 750 ml (0.8 US qt) through the filler opening.

    • Replace and hand tighten the filler cap.
  6. Run the Engine and Check Oil Level
    • Start the engine and let it run for a few minutes to allow oil to circulate fully.
    • Shut the engine off and let it sit for three more minutes.
    • Remove the oil level check bolt. If oil dribbles out, your level is correct; if not, slowly add a little oil, replace the cap, and repeat the check.

    • Reinstall and tighten the oil level bolt to 5.5 N·m (4.0 lb-ft).

  7. Final Wipe Down and Inspection
    • Clean spilled oil with shop rags.
    • Visually inspect the cases for leaks at both drain and filler plugs.

1999 Suzuki RM125 Model-Specific Oil Change Call-Outs

  • No Engine Oil: The RM125 is a two-stroke; oil changes are for the transmission only.
  • Oil Level Bolt Location: On this bike, the oil level check bolt is just below the oil filler cap on the right side. If you’re not sure, consult your owner’s manual or service guide.

  • Correct Oil Quantity is Critical: Too much or too little can cause shifting issues or premature wear. Always measure exactly 750 ml for a standard oil change.

  • Use Only Specified Oil: Using incorrect oils can affect clutch performance and gearbox life.
  • Check for Leaks: Always verify both drain and fill bolts are snugged up to spec after changing oil.

Disposal of Old Oil

  • Always take your used oil to a certified recycling center for proper disposal.

When to Change your RM125 Transmission Oil

  • Change every 5–10 hours of riding under normal use, or after each race or ride in muddy conditions.

Get Your Parts from MotoSport.com

For all overhaul necessities—oil, spark plugs, wrenches, and more—MotoSport.com is your trusted source for Suzuki OEM and aftermarket dirt bike parts.

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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.