How to Change the Oil in a 1991 Suzuki RM125

Shop parts for a 1991 Suzuki RM125 Dirt Bike.

Keeping your 1991 Suzuki RM125 well-maintained starts with routine oil changes. Although this 2-stroke engine requires pre-mix oil for combustion, the gearbox uses separate transmission oil that needs regular replacement for optimal clutch and transmission performance. Follow this comprehensive guide, proprietary to MotoSport.com, for a professional and reliable oil change at home.

Tools and Materials Needed

  • 8mm or 10mm wrench/socket (for oil level bolt and drain plug)
  • Ratcheting wrench or socket driver
  • Clean rag or shop towels
  • Drain pan
  • Measuring cup (Ratio Rite or equivalent)
  • Funnel
  • New oil: SAE 10W-40 with API SF/SG (or higher) and JASO MA rating
  • Torque wrench (desired but optional for home use)
  • Gloves (recommended)

Specifications for 1991 Suzuki RM125

  • Oil type: SAE 10W-40, API SF/SG or higher, JASO MA
  • Oil quantity: 750 ml for an oil change; 800 ml after overhaul
  • Drain plug torque: 21 N·m (14.5 lb-ft)
  • Oil level bolt torque: 5.5 N·m (4.0 lb-ft)

Step-by-Step Oil Change Process

  1. Warm Up the Engine
    • Start and idle the engine for 2–3 minutes to warm the gearbox oil. This helps the oil drain more thoroughly.
    • Shut off the engine and allow it to sit for 3 minutes, letting the oil settle into the bottom of the case.
  2. Stabilize the Bike
    • Place your RM125 on a solid, level surface using a bike stand or center stand. Hold the bike vertically for an accurate oil level check.
  3. Remove Filler Cap and Drain Plug
    • Locate the oil filler cap on the right side of the crankcase. Loosen and remove it.
    • Position your drain pan under the drain plug—also on the lower right case.
    • Using a wrench or socket, remove the drain plug and allow all oil to drain completely. This may take several minutes if the oil is warm.
  4. Inspect and Clean
    • While the oil drains, inspect the drain plug for metal shavings or debris—a sign of internal wear.
    • Wipe the plug clean and inspect the sealing washer. Replace the washer if it is damaged or crushed.
  5. Reinstall Drain Plug
    • After oil has finished draining, reinstall the drain plug. Tighten to 21 N·m (14.5 lb-ft) using a torque wrench if available.
  6. Add New Oil
    • Using a measuring cup, pour 750 ml of fresh SAE 10W-40 oil into the filler hole.
    • Use a funnel to avoid spillage.
    • Reinstall and firmly tighten the filler cap.
  7. Check Oil Level
    • Start the engine and run for several minutes to circulate the oil, then shut it off and let the bike rest for about 2–3 minutes.
    • With the bike vertical, remove the oil level bolt (just below the filler cap). Oil should be level with or just begin to flow from the hole.
    • If no oil comes out, remove the filler cap and add oil a small amount at a time, repeating the check until oil is at the correct level.
    • Replace and tighten both the oil level bolt (5.5 N·m) and filler cap.

Specific Call-outs for the 1991 Suzuki RM125

  • Oil level bolt: This model uses an oil level check bolt below the filler cap for precision. It's easy to overfill, so check this bolt after engine cycling.
  • Tighten all bolts to spec: Stripping the cases can cause expensive repairs. Use a torque wrench whenever possible.
  • Use proper oil: Only use motorcycle-specific transmission oil with the correct JASO MA and API SF/SG/SP/SH/SJ rating for safe clutch operation and lubrication.
  • Frequency: Change oil every 5–10 hours of use or after every major ride, as racing, sand, and mud conditions accelerate oil breakdown.

Extra Tips for Best Results

  • Dispose of old oil responsibly at your local recycling center or auto parts store.
  • Check your owner's manual for any year-specific notes if the bike has aftermarket upgrades or a non-standard clutch.
  • Inspect the case and plug threads for signs of cross-threading or leaks each time you change oil.

For all your parts and fluids, trust MotoSport.com for guaranteed genuine and high-performance products specifically designed for your ride. Keeping your 1991 Suzuki RM125 in top shape starts with the basics. Ride safe and strong.

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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.