How to Change the Oil in a 1981 Suzuki RM100: A Comprehensive MotoSport.com Guide

Shop parts for a 1981 Suzuki RM100 Dirt Bike.

Changing the oil in your 1981 Suzuki RM100 is a key maintenance task that helps ensure a long, healthy life for your engine. While the RM100 is a vintage two-stroke dirt bike (meaning it doesn’t require the same oil changes as a four-stroke engine), it does have a separate transmission (gearbox) oil that needs regular changing. This guide covers the tools you’ll need, step-by-step instructions specific to this model, and call-outs unique to the 1981 RM100.

Tools and Materials Needed

  • Socket wrench set (you’ll need the appropriate size for your drain bolt, typically 12mm or 14mm for RM models)
  • Oil catch pan to collect the old transmission oil
  • Funnel for pouring in new oil
  • Clean shop rags for wiping spills
  • New transmission oil (SAE 10W-40 or equivalent, non-friction-modified oil designed for two-stroke transmission cases; check your owner’s manual for exact capacity, generally between 600ml and 700ml)
  • Replacement crush washer (recommended for drain plug, if needed)
  • Torque wrench (optional, but helpful for accurately tightening drain bolt)

Steps to Change the Transmission (Gearbox) Oil

  • Warm Up the Bike
    Start your RM100 and let it idle for several minutes. Warm oil flows better and helps carry contaminants out with it.
  • Place the Bike Safely
    Put your bike on a level surface, ideally on a stand. Make sure the engine is off and the bike is stable.
  • Locate the Drain Bolt
    The transmission drain bolt is typically found on the bottom of the crankcase, usually on the left side under the engine. Place your oil catch pan directly underneath the drain bolt.
  • Remove the Drain Bolt
    Using your socket wrench, carefully remove the drain bolt. Allow the old gearbox oil to drain fully into the catch pan. This may take a few minutes. Inspect the drain bolt for metal shavings, which can indicate excessive wear.
  • Replace or Inspect the Crush Washer
    Many Suzuki models use a crush washer under the bolt to help prevent leaks. If it is damaged or heavily compressed, replace it.
  • Reinstall the Drain Bolt
    Once the oil has completely drained, reinstall and tighten the drain bolt securely, but avoid over-tightening to prevent stripping the threads. Consult your service manual for the correct torque specification if available.
  • Refill with Fresh Oil
    Locate the transmission oil fill cap, usually on the side cover of the engine. Remove it and insert a funnel. Pour in the recommended amount and type of new oil (usually SAE 10W-40 motorcycle oil, about 600ml–700ml for the RM100). Do not overfill.
  • Check Oil Level
    Some models have an inspection bolt or sight glass on the side of the case; with the bike upright, check that oil is at or just below this point. Add oil as necessary.
  • Replace Fill Cap and Clean Up
    Reinstall the fill cap securely. Wipe any drips or spills from the engine and surrounding area.
  • Start and Inspect
    Start the engine and let it idle for a few minutes. Flick through the gears, then shut it off and check for leaks at the drain bolt and fill cap. Recheck oil level and top off if necessary.

Specific Call-Outs for the 1981 Suzuki RM100

  • Two-Stroke Design: The RM100’s engine itself is lubricated by mixing two-stroke oil with gasoline, so engine oil changes aren’t required. However, the transmission uses separate oil for the clutch and gears that must be changed regularly.
  • Avoid Automotive Oils: Do not use automotive oils with friction modifiers, as these can cause your clutch to slip. Always use oils meant for two-stroke transmissions.
  • Drain Bolt Gasket: The drain bolt usually has a small copper or aluminum washer that ensures a good seal. This part may be hard to find, so keep a few spares handy.
  • Clutch Fiber Life: Fresh, quality oil helps extend your clutch life. Dirty or low oil can cause clutch drag or slippage.
  • Inspection Plug: Many older Suzukis have a small inspection or check bolt a few inches above the drain bolt. With the bike level and filled, oil should just barely trickle out of this bolt.
  • Drain and Fill Capacities: Always refer to your owner’s manual, but for the 1981 RM100 plan on using 600ml–700ml of quality gear oil.
  • Frequency: For aggressive riding, change the oil every 10 to 15 hours of run time, or after every few races for maximum protection.

Tips for Success

  • Dispose of used oil responsibly—most auto parts stores offer recycling.
  • Keep a log of your oil changes for reference and to monitor gear case health.
  • When in doubt, consult the factory service manual or a reputable technician.

Regular gearbox oil changes are one of the simplest and most effective ways to protect the transmission and clutch of your 1981 Suzuki RM100, ensuring more laps, better shifting, and extended clutch life every season.

This how-to guide is proprietary to MotoSport.com.

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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.