How to Change the Oil in a 1979 Yamaha YZ125

Shop parts for a 1979 Yamaha YZ125 Dirt Bike.

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Tools and Materials Needed

  • Socket wrench and ratchet
  • 12mm socket (for the drain bolt)
  • 4mm Allen wrench (for the check bolt)
  • Oil drain pan
  • Long funnel
  • Measuring cup (or graduated cylinder for mL)
  • Paper towels or rags
  • New oil (SAE 10W-40, API SG or higher, JASO MA, preferably YAMALUBE)
  • New crush washer (optional, but recommended for a better seal)
  • Torque wrench
  • Owner's manual (for reference, if available)

Steps for Changing the Oil in Your 1979 Yamaha YZ125

Warm Up the Engine

  • Start the engine and let it idle for several minutes. This warms the oil, making it drain more completely.
  • Turn off the engine and let the bike sit for about five minutes—this allows the oil to settle in the crankcase before draining.

Prepare Your Work Area

  • Place the YZ125 on a level surface, ideally on a center stand or pad to keep it upright and stable.
  • Position your oil drain pan directly beneath the crankcase. The drain bolt is located on the underside of the engine.

Drain the Oil

  • Remove the oil filler cap on top of the crankcase–this allows air into the crankcase and promotes faster, smoother draining.
  • Using the 12mm socket, loosen and remove the oil drain bolt at the bottom of the crankcase. Allow the oil to drain completely into the pan. This may take a few minutes.
  • Inspect the oil drain bolt and its washer. It's a good practice to replace the crush washer at each oil change to prevent leaks.
  • Clean any metal filings or debris from the drain plug and check the oil for signs of excessive metal, which could indicate internal wear.

Reinstall the Drain Bolt

  • Install a new crush washer on the drain bolt if you have one.
  • Reinstall and hand-tighten the drain bolt.
  • Use a torque wrench to tighten the bolt to 14 foot-pounds (20 Nm)–this specification is standard for Yamaha YZ125 models and prevents overtightening, which could damage the crankcase threads.

Refill with Fresh Oil

  • Consult your owner's manual for the recommended oil type and capacity. For the Yamaha YZ125, use 0.66 liters (0.69 US quarts, 660 mL) of SAE 10W-40 engine oil, API SG or higher, JASO MA standard. For best results, use YAMALUBE or a high-quality equivalent.
  • Insert a long funnel into the oil filler hole–the exhaust pipe on vintage YZ125s often makes this a tight fit.
  • Slowly pour in the correct amount of oil.
  • If you wish to confirm the oil level, remove the check bolt (if your model has one) located near the bottom of the clutch cover using a 4mm Allen wrench. When oil starts to seep out, the level is correct–this method is common on many Yamaha models, but the 1979 YZ125 may or may not have this feature; check your specific manual.
  • Replace the oil fill cap and wipe away any spills.

Final Checklist and Cleanup

  • Inspect around the drain bolt and fill cap for leaks.
  • Dispose of the used oil properly at a recycling facility–never pour it down a drain.
  • Clean your tools and work area.
  • Start the engine and let it idle for a minute to circulate the new oil. Check again for leaks.

Model-Specific Call-Outs for the 1979 Yamaha YZ125

The 1979 Yamaha YZ125 is an air-cooled two-stroke motocross bike. There is no engine oil in the traditional sense–the oil being changed here is transmission oil, which lubricates the gearbox and clutch. This is a key distinction from four-stroke engines, which use oil for both crankcase and transmission lubrication.

Overfilling or underfilling the transmission oil can cause clutch slippage or gear shifting problems. Always use the recommended oil type and quantity. Some owners add a bit less oil (e.g., 600 mL) to compensate for residual oil in the gearbox; for most riders, following the factory spec is safest.

On vintage bikes like the 1979 YZ125, gaskets and seals may be old and brittle. Inspect the drain and fill plugs for leaks after your oil change. If you notice persistent leaking, consider replacing the seals.

If your bike has a check bolt for oil level verification, this is a reliable method to avoid overfilling, but not all years and models have this feature. Refer to your owner's manual for your specific bike's configuration.

Finally, if your bike has been sitting for a long time, expect the old oil to be especially dirty. You may want to repeat the oil change after a short ride to ensure contaminants are fully flushed.

Summary Table: Key Oil Specs for 1979 Yamaha YZ125

Specification Value
Oil Type SAE 10W-40, API SG or higher, JASO MA (YAMALUBE recommended)
Oil Capacity 0.66 L (0.69 US qt, 660 mL)
Drain Bolt Torque 14 ft-lb (20 Nm)
Special Notes This is transmission oil, not engine oil. Check for check bolt on clutch cover for level verification (if present).

Regular oil changes are a simple yet vital part of maintaining the performance and longevity of your 1979 Yamaha YZ125. Keep to the schedule, use quality oil, and your vintage dirt bike will reward you with reliability for years to come.

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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.