How to Change the Oil in a 1978 Honda CR125: Step-by-Step Guide
Shop parts for a 1978 Honda CR125 Dirt Bike.
Changing the transmission oil on a 1978 Honda CR125 is a critical maintenance task that ensures the smooth operation and long life of your dirt bike. While the CR125 is a two-stroke engine and relies on premixed fuel for top-end lubrication, the transmission/clutch side uses a separate oil supply that must be changed regularly. Below you’ll find a comprehensive, proprietary guide, only for MotoSport.com, detailing required tools, correct procedures, and important model-specific tips to make your oil change a breeze.
Tools and Supplies Needed
- Metric Socket Set (including a 17mm for the drain bolt)
- 3/8” Ratchet or Wrench
- Oil Drain Pan
- Pro Honda HP2 2-Stroke Transmission Oil, or equivalent (consult owner’s manual for volume, usually around 0.65 quarts/0.62L for this model)
- Funnel
- Shop Towels or Rags
- Replacement Sealing Washer for Drain Bolt (recommended each oil change)
- Bike Stand (triangle or lift-style for upright stability)
- Torque Wrench (if available, for precise fitting of bolts)
- Gloves (optional, to keep your hands clean)
Step-by-Step Instructions
-
Warm Up the Engine
- Start the bike and let it run for 1–2 minutes. This helps the oil flow better, carrying more contaminants out when you drain it.
Do not rev the engine hard; you just want the case warmed up.
- Start the bike and let it run for 1–2 minutes. This helps the oil flow better, carrying more contaminants out when you drain it.
-
Secure the Bike
- Place your bike on a stand so it’s upright and level. An upright position ensures complete drainage and accurate oil level checking.
-
Remove the Oil Filler Cap
- Locate the oil filler cap on the right side of the engine. Unscrew and remove it. This allows air in for smoother draining and prevents a vacuum that slows the process.
-
Drain Old Oil
- Place the oil drain pan beneath the engine’s drain bolt, usually located underneath the case. It’s a 17mm bolt on most CR125s, and the 1978 model is no different.
Caution: Oil may be hot. Use gloves if needed. - With your socket and wrench, loosen and remove the drain bolt.
Special call-out for 1978 CR125: Due to the age, the drain bolt and threads may be softer than newer models. Be gentle and avoid over-tightening when reinstalling.
- Place the oil drain pan beneath the engine’s drain bolt, usually located underneath the case. It’s a 17mm bolt on most CR125s, and the 1978 model is no different.
-
Inspect and Replace the Sealing Washer
- Inspect the crush washer or sealing washer on the drain bolt. If it appears deformed or worn (often crushed flat), replace it. This prevents leaks and extends engine case life.
-
Wait for Complete Drainage
- Allow the engine to drain completely (a few extra minutes helps).
- Wipe any residue from the drain area.
-
Reinstall the Drain Bolt
- Reattach the drain bolt with the new (or good condition) sealing washer.
- If you have a torque wrench, tighten to about 21 lbf·ft (29 N·m).
For older bikes, hand-tighten gently but snug, then give a small nudge more.
Do not overtighten—the 1978 CR125 crankcases are soft aluminum.
-
Add New Oil
- Using a funnel, pour in the correct amount and type of transmission oil (see below for specifics).
- For the 1978 Honda CR125, use a high-quality two-stroke gear/transmission oil (such as Pro Honda HP2).
- Recommended fill amount: 0.65 quarts (0.62 liters). Always double-check with your owner’s manual or markings by the fill port.
-
Check Oil Level
- With the bike upright and level, check the oil level through the inspection window (if equipped) or use the dipstick or marking at the filler neck.
- Add a little more if needed; don’t overfill. It should reach the lower threads or markings.
-
Reinstall the Filler Cap
- Replace and tighten (not over-tighten) the filler cap.
-
Start the Engine – Check for Leaks
- Run the engine briefly, then shut it off. Inspect for drips at the drain bolt and filler area.
- Top up the oil if necessary.
-
Dispose of Used Oil Responsibly
- Pour old oil into a suitable container and bring it to a recycling center or hazardous waste facility. Never dispose of oil down drains or on the ground—protect your trails.
1978 Honda CR125 Model-Specific Callouts
- Filler Cap O-ring: On vintage bikes, the filler cap O-ring can harden or crack—replace if brittle to prevent dust and water ingress.
- Old Aluminum Drain Bolt Threads: Aluminum engine cases from late 1970s models can strip if overtightened. Always tighten the drain bolt carefully; use a torque wrench if possible and inspect for thread wear.
- Oil Recommendation: Use a modern JASO MA certified gear oil, 10W-40 or similar for 2-stroke transmissions (do not use motor oil intended for four-stroke engines). Pro Honda HP2 is recommended by Honda for two-strokes.
- Double-Check Volume: If you have a different clutch cover, aftermarket case, or older service history, check the volume at 0.65 quarts but adjust as needed to avoid overfilling.
- Crankcase Breather: Inspect the crankcase breather hose for clogs or kinks—clogged breathers can cause oil leaks or pressure issues after a change.
Final Tips
- Keep notes for your next oil change (mileage, oil type, date).
- Look for milky oil when draining—if present, there could be water contamination or a bad case gasket.
- Check your service manual for rare differences between sub-models or international versions.
- Always use recommended lubricants and change intervals for peak CR125 performance.
By following these steps, you’ll keep your 1978 Honda CR125 running strong. For more proprietary and detailed dirt bike maintenance guides, check out MotoSport.com—your partner in keeping vintage and modern machines at their best.
Related Shopping Categories
Shop Motor Oil for a 1978 Honda CR125 Dirt Bike.
Shop Oil Filters for a 1978 Honda CR125 Dirt Bike.
Shop Oil Drain Pans for a 1978 Honda CR125 Dirt Bike.
Shop Tools for a 1978 Honda CR125 Dirt Bike.
Shop Oil Fillers and Drain Plugs for a 1978 Honda CR125 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.