How to Change the Oil in a 1978 Yamaha YZ100: Complete Step-by-Step Guide
Shop parts for a 1978 Yamaha YZ100 Dirt Bike.
Changing the oil in your 1978 Yamaha YZ100 is a key maintenance task that ensures smooth shifting, optimal clutch function, and long-term durability of your two-stroke motocross bike. Unlike four-stroke engines, the YZ100 uses transmission oil (sometimes referred to as gear oil or crankcase oil), since engine lubrication is handled by the pre-mix fuel. Here's a detailed, proprietary MotoSport.com guide on how to perform this important job.
Tools and Materials Needed
- Socket wrench set (typically 12mm or 14mm for drain and fill bolts)
- Drain pan (low-profile recommended)
- Funnel or squeeze bottle with nozzle
- Gloves
- Clean rags or shop towels
- Measuring cup (such as a Ratio Rite)
- Replacement oil (Consult manual; typically 10W-30, 10W-40 motorcycle gear oil, or a specified transmission fluid)
- Torque wrench (for precise re-tightening of drain and fill bolts)
- Replacement drain bolt washer, if needed (some recommend replacing to prevent leaks)
Step-By-Step Oil Change Instructions
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Warm Up the Engine
- Start the bike and let it idle for 1-2 minutes. Warming the transmission oil allows it to drain more easily and removes more contaminants.
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Stabilize the Bike
- Place the YZ100 on a level stand or lift to keep it upright and secure. This ensures complete oil drainage and safety.
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Locate the Drain Plug and Fill Cap
- The drain bolt is positioned beneath the engine, near the center. Reference diagrams from your Clymer or OEM manual if unsure .
- The fill cap is typically on the right engine cover or just above the clutch cover.
- Important YZ100 Call-Out: Verify you are removing the correct drain bolt (center-bottom, straight into the case) and not the shift detent bolt (angled bolt). Mistakenly removing the shift detent bolt can release a spring and plunger—which could fall into your oil pan and require retrieval .
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Remove the Fill Cap
- Opening the fill port allows air in and helps the oil drain faster.
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Drain the Old Oil
- Place your drain pan under the drain bolt.
- Use the appropriately sized socket wrench to remove the drain bolt. Allow the transmission oil to fully drain. It may only be around 600–800ml (refer to your service manual for the exact oil capacity for the 1978 YZ100)
- Wipe the magnetic tip of the drain bolt (if equipped) to inspect for metal debris.
- Check the old oil for signs of dramatic clutch wear or metal flakes, which could indicate internal issues.
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Inspect and Replace the Washer
- Inspect the drain bolt washer/gasket. Replace it to ensure a good seal and prevent leaks.
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Reinstall the Drain Plug
- Tighten the drain bolt by hand to seat, then torque to the manufacturer's specification (generally 14–20 ft-lbs, but confirm with your manual for the 1978 YZ100 ).
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Add Fresh Oil
- Using a measuring cup, pour in the exact capacity of fresh transmission oil through the fill hole. For the 1978 YZ100, typical oil quantity is between 600ml and 800ml, but confirm your bike's manual for the specified volume .
- A funnel makes this step cleaner and more precise.
- Wipe any spills and reinstall the fill cap by hand.
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Check Oil Level (if equipped with a sight plug)
- Some YZ100 models have a small inspection bolt on the side case. Remove the level bolt—once oil begins to seep from this hole, the transmission is correctly filled. Reinstall the level bolt and torque appropriately.
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Start and Run
- Start the bike, let it idle a few minutes, then shut down and check for leaks at the drain and fill plug. Top off oil if necessary.
Model-Specific Notes for the 1978 Yamaha YZ100
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Drain Bolt Identification:
- Select the center, straight-up drain bolt to drain oil. The angled bolt nearby is the gearshift detent; do not remove unless necessary. If accidentally removed, inspect for the small plunger and spring in your drain pan .
- Oil Capacity: Typically in the 600ml–800ml range for the transmission. Confirm with a factory or Clymer manual to avoid overfilling or underfilling .
- Oil Specification: Use two-stroke gear/transmission oil formulated for wet clutches. Avoid automotive engine oil which can cause clutch slippage or wear.
- Watch for Leaks: Common leakage points on vintage YZ100s are the shift shaft and countershaft seals. Inspect for leaks during your oil change—heavy oil loss here could indicate a need for seal replacement.
- No Oil Filter: The 1978 YZ100 doesn't use a traditional spin-on or internal oil filter. Oil quality and change intervals become extra important.
Final Tips
- Dispose of old oil responsibly at a designated recycling center.
- Change transmission oil every 5–10 hours of racing, or 10–20 hours of recreational use for best results.
- Keep a maintenance log with date and amount/type of oil used.
Regulary changing your YZ100's transmission oil is an easy way to keep this classic motocrosser racing strong for years to come. MotoSport.com has all the tools and fluids you need for any DIY project on your vintage Yamaha.
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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.