Views: 111 Author: Site Editor Publish Time: 2025-10-27 Origin: Site
The lower unit—also called the foot—is the bottom section of an outboard motor that houses the gears, driveshaft, and propeller. Proper lower unit maintenance is one of the most important steps in ensuring long engine life and optimal performance for your boat motor.
At Earrow, a professional outboard motor manufacturer, we recommend routine inspection, cleaning, and oil replacement to keep your lower unit performing at its best.
The lower unit transfers engine power to the propeller and converts vertical rotation into horizontal thrust.
Inside, precision-engineered gears constantly turn under high pressure. These components depend on marine gear lubricant to:
Prevent metal-to-metal contact
Resist foaming and pressure
Maintain lubrication even with water contamination
Always use a marine-grade lower unit oil, never automotive or tractor gear oil. Marine lubricants maintain performance even with up to 10% water contamination.
Follow these steps to inspect and maintain your lower unit regularly.
Before heading out, check the exterior of the lower unit for:
Cracks or dents
Leaking oil around seals
Fishing line or debris wrapped around the propeller shaft
Fishing line can damage the prop shaft seals, allowing water to enter the gear case.
If you frequently fish, inspect every 50 hours of operation or after each trip in heavy vegetation.
To check the condition of your oil:
Crack open the drain screw (bottom of the bullet) and vent screw (side of the lower unit).
Let a small amount of lubricant drain into a clean container.
Observe the color and smell.
Interpretation:
✅ Clear amber or dark oil → Normal
⚠️ Milky color → Water contamination
Burnt smell → Overheating or old lubricant
If oil appears milky, perform a pressure test to detect seal damage and replace seals if necessary.
For a new outboard, change the oil after the first 20 hours.
After that, replace it every 100 hours or once per season.
Procedure:
Place a drain pan under the lower drain screw.
Remove both the bottom drain screw and the upper vent screw.
Allow all lubricant to drain completely.
Clean and inspect the magnetic drain plug for metal particles.
⚙️ Fine metallic dust is normal. Large chips may indicate internal gear wear and should be inspected by a technician.
Replace both crush washers (drain plug gaskets) each time. These are vital to prevent leaks.
Once the oil has drained:
Refill from the bottom drain hole until oil flows from the vent hole.
Reinstall the vent screw first, then the drain screw.
Use a high-performance marine gear oil recommended for your outboard’s horsepower.
Earrow’s 15HP–40HP outboards are designed for easy lower unit oil replacement, featuring accessible screw placement and magnetic plugs.
Remove the propeller to:
Clear any debris or fishing line.
Grease the shaft to prevent corrosion.
Check the propeller key and bushings for wear.
If you notice any of the following, stop using the engine and inspect immediately:
Water in oil (milky fluid)
Grinding or knocking noises
Overheating at high speed
Oil leaking near the prop shaft
Visible metal shavings on drain plug
Always store your outboard upright to prevent oil from migrating into the exhaust.
After saltwater use, flush the lower unit thoroughly with fresh water.
Replace seals if you see air bubbles during the pressure test.
Keep a maintenance log of hours used and service intervals.
Pro Tip: When changing oil, mark the date and hours on a small tag near the fuel tank or inside the engine cover.
As a leading outboard motor manufacturer, Earrow provides:
Factory-direct 2-stroke and 4-stroke outboards (3.5HP–40HP)
OEM services for marine dealers
Durable gear cases and precision-engineered lower units
Long-lasting anti-corrosion coatings for harsh marine conditions
Explore our models:
Proper lower unit maintenance ensures smoother performance, higher efficiency, and longer engine life.
Regular inspection, timely oil changes, and attention to prop shaft seals are the keys to avoiding costly repairs.
⚙️ Keep your engine strong — maintain your lower unit with the same care that Earrow builds into every outboard motor.
Q1: How often should I change my outboard lower unit oil?
Every 100 hours or once per boating season.
Q2: What type of oil should I use?
Use marine-grade gear oil with water-resistant additives, not automotive oil.
Q3: Why is my lower unit oil milky?
Water has entered the gear case. Perform a seal pressure test immediately.
