How to Change the Oil in a 1977 Suzuki RM125

Shop Parts for a 1977 Suzuki RM125 Dirt Bike.

Changing the transmission oil in your 1977 Suzuki RM125 is essential maintenance that prolongs engine and transmission life, ensures smooth shifting, and helps prevent costly damage. MotoSport.com brings you a proprietary, detailed guide for this classic two-stroke motocross bike.

Tools and Supplies Needed

  • Socket set or appropriate wrenches (for drain and filler plugs)
  • Drain pan
  • Measuring cup or graduated container (for accurate oil volume)
  • Clean rags
  • SAE 10W-40 or SAE 20W/40 multi-grade motor oil (API SF/SG or SH/SJ with JASO MA standard)
  • Replacement washer for drain plug (recommended but not essential)
  • Gloves (optional, for cleanliness)

Preparation Steps

  • Run the engine for a few minutes to warm up the transmission oil; this helps the oil drain more thoroughly and clears debris.
  • Stand the bike upright on level ground. The 1977 RM125 does not have a center stand, so use a sturdy bike stand to keep it vertical.
  • Gather all necessary tools and supplies within easy reach.

Step-by-Step Oil Change Procedure

  1. Remove the Oil Filler Cap:
    Locate the oil filler cap on the right side of the engine case. Remove it to allow the old oil to drain freely and avoid creating a vacuum.
  2. Drain the Oil:
    Position your drain pan underneath the drain plug located at the bottom of the transmission case.
    Using the correct socket or wrench, unscrew the oil drain plug carefully. Allow the oil to drain completely.
    Tip: Let the bike sit for a few minutes to ensure all old oil has exited.
  3. Inspect and Clean:
    While the oil is draining, inspect the drain plug for any metal particles that might indicate excessive wear. Wipe it clean and inspect the sealing washer; replace it if damaged.
  4. Reinstall the Drain Plug:
    Replace the drain plug and tighten it to 21 N·m (14.5 lb-ft). Do not overtighten, as this can damage the threads or crush the washer.
  5. Pour in New Oil:
    Measure 750 ml (0.8 US qt) of SAE 10W-40 or SAE 20W/40 multi-grade motor oil.
    Pour the fresh oil steadily into the filler hole. Be careful not to overfill.
    For transmission overhauls (after full disassembly), the volume increases to 800 ml (0.85 US qt).
    Suzuki recommends their branded transmission oil, but any quality oil matching the specifications is suitable.
    • Do NOT use automotive engine oils not labeled JASO MA. This can cause clutch slippage or premature wear.
    • Oil level can be checked with the inspection bolt or level hole (if equipped).
  6. Reinstall the Filler Cap:
    Tighten the filler cap securely. Do not over-tighten.
  7. Check Oil Level:
    Start the engine and let it idle for a few minutes, then shut it off and let it sit for about three minutes.
    Remove the oil level inspection screw (if equipped). Oil should just begin to seep out; if not, add a small amount until it does. Reinstall and tighten the inspection screw to 5.5 N·m (4.0 lb-ft).
  8. Final Inspection and Cleanup:
    Check for any leaks around the drain plug and filler cap. Wipe off any excess oil from the engine and cases.

Important Model-Specific Call-Outs: 1977 Suzuki RM125

  • This bike is pre-mix only; the oil you’re changing is transmission oil, not two-stroke premix or crankcase oil. The engine itself is lubricated via premix in the fuel.
  • Recommended oil is SAE 20W/40 if genuine Suzuki oil is not available, per factory specification.
  • Never overfill. Too much oil can cause excessive pressure, leaks, or clutch issues. Too little oil can lead to transmission and clutch failure.
  • The RM125 design of this vintage is basic, with no oil filter. Oil changes are the primary method of removing debris from the clutch and transmission.
  • If you see excessive metal shavings on the drain plug, consider a more thorough inspection of your clutch and gears.
  • For the first oil change after rebuild or purchase, change oil more frequently (after first ride or 100 km/60 miles) to remove break-in residue.

Additional Tips from MotoSport.com

  • Always use a new drain plug washer if available to prevent leaks.
  • Keep a maintenance log with date, hour meter reading, and oil brand/type used.
  • Use genuine Suzuki parts when replacing any seals, plugs, or fasteners.
  • Ride responsibly. Refreshing your transmission oil regularly is a cheap insurance for enjoying your vintage RM125 for years to come.

For all your parts, oil, and maintenance needs, trust MotoSport.com—your authority in motocross parts and service for classic machines.

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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.