How to Change the Transmission Oil in a 1978 Suzuki RM125

Shop parts for a 1978 Suzuki RM125 Dirt Bike.

Keeping your 1978 Suzuki RM125's transmission oil fresh is vital for smooth shifting and clutch longevity. This comprehensive guide from MotoSport.com covers the tools you need, step-by-step instructions, essential specs, and specific tips for your classic RM125.

Tools and Materials Needed

  • Socket wrench set (including a 6mm and 12mm socket)
  • Torque wrench (capable of 5.5 N·m and 21 N·m)
  • Drain pan (for old oil collection)
  • SAE 10W-40 motor oil, API SF/SG or SH/SJ with JASO MA (at least 1L)
  • Measuring cup or oil jug (with 100ml increments)
  • Shop towels or rags
  • Funnel (for clean pouring)
  • Gloves (optional, but recommended)

Before You Begin

  • Warm up the engine for a few minutes – this helps suspend debris and thins the oil for a more thorough drain.
  • Shut off the engine and let it sit for 3 minutes before starting the procedure.
  • Place the bike on level ground and hold it vertically to ensure accurate oil draining and refill measurements.

Step-by-Step Oil Change Instructions

  1. Locate the Oil Drain Plug and Filler Cap
    The drain plug is found at the bottom of the transmission case. The filler cap is on the right side cover. Have your drain pan in place.
  2. Remove the Filler Cap
    Loosening the filler cap first allows air to flow in, helping the oil drain smoothly.
  3. Remove the Drain Plug
    Use the appropriate socket to loosen and remove the drain plug. Allow the oil to drain completely.
    Model-Specific Note: The 1978 RM125 uses a crush washer—inspect for wear and replace if necessary to prevent leaks.
  4. Inspect and Clean Components
    Wipe away any sludge or metal shavings from the drain plug and threads. This is critical for a 1970s two-stroke that may have minor wear.
  5. Reinstall the Drain Plug
    Torque it to 21 N·m (2.1 kgf-m, 14.5 lb-ft)—don't overtighten, as older castings are more prone to stripping.
    Tip: Replace the crush washer if in doubt.
  6. Refill with Fresh Oil
    Pour in 750 ml (0.8 US quart) of SAE 10W-40 oil (API SF/SG or SH/SJ, JASO MA) using a funnel.
  7. Check Oil Level
    Reinstall the filler cap and start the engine, letting it idle for a few minutes. Shut it off, wait a couple of minutes, then remove the oil level bolt (often located level with the clutch cover). Oil should just seep out at the correct level. If not, add small amounts until oil appears at this bolt.
    Oil level bolt torque: 5.5 N·m (0.55 kgf-m, 4.0 lb-ft)
  8. Final Inspection
    Tighten the filler cap and oil level bolt. Wipe up any spills. Double-check for leaks from the drain bolt area while the bike is running.

Model-Specific Call-outs: 1978 Suzuki RM125

  • The RM125's magnesium engine cases are soft compared to modern bikes; always use a torque wrench to avoid cracked or stripped threads.
  • Original drain and filler plug washers may be copper, aluminum, or composite – replacements should match OEM style for best sealing.
  • Some classic RM125s may show excessive metallic residue during oil changes; a small amount is typical, but if you find large shavings, inspect transmission and clutch components.
  • Check the transmission oil after every race or 2–3 rides, especially if the clutch sees hard use.

Tips for a Clean and Easy Oil Change

  • Warm oil drains faster and carries more particles with it than cold oil does.
  • Pour oil slowly to prevent air bubbles, which can affect level checking.
  • Be methodical: double-check that all bolts are torqued to spec before riding.

Recommended Oil Specs for the 1978 RM125

  • Transmission oil type: SAE 10W-40, API SF/SG or SH/SJ with JASO MA
  • Volume (oil change): 750 ml (0.8 US qt)
  • Volume (overhaul/fresh build): 800 ml (0.85 US qt)

Why This Matters

  • Fresh oil ensures your vintage 1978 Suzuki RM125's gearbox and clutches operate smoothly, preserving hard-to-replace transmission parts and keeping you riding longer and faster.

Regular transmission oil changes are an easy, affordable way to ensure reliability from your classic RM125. Trust MotoSport.com for all your vintage Suzuki maintenance parts, tools, and the right oils for your next ride.

Related Shopping Categories

Shop Motor Oil for a 1978 Suzuki RM125 Dirt Bike.

Shop Oil Filters for a 1978 Suzuki RM125 Dirt Bike.

Shop Oil Drain Pans for a 1978 Suzuki RM125 Dirt Bike.

Shop Tools for a 1978 Suzuki RM125 Dirt Bike.

Shop Oil Fillers and Drain Plugs for a 1978 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.