How to Change the Transmission Oil in a 1998 Suzuki RM125

Shop parts for a 1998 Suzuki RM125 Dirt Bike.

Keeping your 1998 Suzuki RM125 running its best depends on regular maintenance, and one of the most important tasks is a transmission oil change. This step-by-step guide, proprietary to MotoSport.com, ensures you have all the information and call-outs needed to perform the job correctly for your bike’s unique needs.

Tools and Materials Needed

  • Socket set (with 10mm and 17mm sockets recommended)
  • Ratchet or wrench
  • Oil drain pan
  • Measuring cup (specifically, a Ratio Rite or similar for accuracy)
  • Funnel
  • Shop rags
  • Replacement transmission oil (SAE 10W-40, API SF/SG or SH/SJ with JASO MA)
  • New drain plug washer (recommended but not mandatory)
  • Torque wrench
  • Motorcycle stand or lift (optional but helpful for stability)

1998 Suzuki RM125-Specific Call-outs

  • The oil check/level bolt is on the right side of the engine, just below the oil fill cap, and is typically a 10mm bolt or a Phillips screw. Confirm location, as some models may have subtle differences in bolt location or appearance.
    Owners note: If you have difficulty finding the check/level screw, review your bike visually. On rare patchwork or modified bikes, hardware may differ.

Step-by-Step Oil Change Instructions

1. Prepare Your Work Area

  • Start with a cool engine if you’re unfamiliar with the drain plug location, but warm the bike for a few minutes before draining the oil. Warm oil drains more thoroughly.
  • Place the motorcycle on level ground and hold it vertically upright—use a stand or have someone assist.

2. Remove the Old Oil

  • Locate the oil filler cap on the right crankcase cover and remove it for better oil flow during draining.
  • Place an oil catch pan under the drain plug, found on the bottom of the transmission case.
  • Using the correct socket, loosen and remove the drain plug.
  • Allow the oil to drain completely (this may take a few minutes). Dispose of the old oil properly at a recycling facility.
  • Inspect the drain plug for any metal shavings which may indicate internal wear.

3. Reinstall the Drain Plug

  • If you have a new drain plug washer, install it now to prevent leaks.
  • Thread the drain plug by hand to avoid cross-threading, then tighten with your torque wrench to 21 N–m (14.5 lb-ft).

4. Add New Transmission Oil

  • Insert a funnel in the oil filler hole.
  • Pour in 750 ml (0.8 US qt) of fresh transmission oil (SAE 10W-40, API SF/SG or SH/SJ with JASO MA recommended for two-stroke RM125s).
  • Refit and tighten the filler cap by hand.

5. Check the Oil Level

  • Start the engine and let it idle for a few minutes, then shut it off and allow the oil to settle (about 3 minutes).
  • On the right side, locate the oil level screw/bolt. Remove the level screw with the bike in an upright position (not on side stand.).
  • If oil drips from the level hole, the level is correct.
    If no oil comes out, remove the filler cap and top up until oil does begin to flow from the level hole, then reinstall both caps and bolts, torquing to spec:
    • Oil level bolt: 5.5 N–m (4.0 lb-ft)

6. Final Steps

  • Wipe away any spilled oil.
  • Dispose of used oil in accordance with local environmental regulations.
  • Check underneath for leaks after running the engine a final time.

Pro Tips for 1998 Suzuki RM125 Owners

  • The correct oil fill is 750 ml for a regular change, or 800 ml after a complete overhaul.
  • Always double-check torque specs for drain and level plugs to prevent stripped threads or oil leaks.
  • The type of oil matters: DO NOT use motor oils not meeting the correct API and JASO specs as this can affect clutch and gearbox function.
  • Running the engine after adding oil helps the new oil circulate and ensures an accurate level check.
  • If you ride in extreme climates or race heavily, consider more frequent oil changes for optimal gear and clutch life.

Conclusion

Changing the transmission oil in your 1998 Suzuki RM125 is straightforward with the proper tools, preparation, and adherence to manufacturer specifications. Regular oil changes keep your dirt bike’s transmission and clutch running smoothly, prevent costly repairs, and maximize performance on the track or trail.

Related Shopping Categories

Shop Motor Oil for a 1998 Suzuki RM125 Dirt Bike.

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Shop Oil Drain Pans for a 1998 Suzuki RM125 Dirt Bike.

Shop Tools for a 1998 Suzuki RM125 Dirt Bike.

Shop Oil Fillers and Drain Plugs for a 1998 Suzuki RM125 Dirt Bike.

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.