MST MST Outboard Engine Service & Gearcase 1 — Questions and Answers
Question 1: What is the correct procedure for checking the gear oil level in an outboard lower unit?
- Remove the lower vent screw and check that oil is at the level of the vent hole with the engine upright (Correct answer)
- Dipstick check at the top drain plug only
- Remove the drain plug and collect oil in a container to measure volume
- No check is needed — gear oil is a lifetime fill
Correct answer: Remove the lower vent screw and check that oil is at the level of the vent hole with the engine upright
With the engine in the upright position, the vent screw (lower of the two screws) is removed and oil should be level with the opening, confirming proper fill.
Question 2: Water in the outboard gear oil is most commonly caused by what condition?
- A failed propeller shaft seal or damaged prop shaft (Correct answer)
- An overfilled oil reservoir
- Ethanol contamination in the fuel
- Normal condensation during operation
Correct answer: A failed propeller shaft seal or damaged prop shaft
Water enters the lower unit most often through a failed propeller shaft seal, especially if the prop has hit an obstruction and damaged the seal.
Question 3: When removing an outboard propeller for service, what must be inspected before reinstallation?
- The prop shaft splines, thrust washer, spacers, and prop seal for wear or damage (Correct answer)
- The ignition timing only
- The fuel filter condition
- The tilt/trim fluid level
Correct answer: The prop shaft splines, thrust washer, spacers, and prop seal for wear or damage
All hardware components at the prop shaft must be inspected for wear, corrosion, or damage — incorrect assembly can cause prop loss or shaft seal failure.
Question 4: What is the purpose of the shear pin (or rubber hub) on an outboard propeller?
- To absorb shock and protect the gearcase from damage when the prop strikes an obstruction (Correct answer)
- To improve fuel efficiency
- To reduce vibration at high RPM
- To connect the shift linkage to the prop shaft
Correct answer: To absorb shock and protect the gearcase from damage when the prop strikes an obstruction
The shear pin or rubber hub insert is a sacrificial component designed to fail before gearcase damage occurs when the prop hits a solid object.
Question 5: What does milky or frothy gear oil in the lower unit indicate?
- Water has entered the lower unit, likely through a failed seal (Correct answer)
- The gear oil is fresh and new
- The lower unit is overheating
- Normal aeration from high-speed operation
Correct answer: Water has entered the lower unit, likely through a failed seal
Milky gear oil is an emulsion of gear oil and water, indicating a seal failure that allows water intrusion into the lower unit.
Question 6: What is the correct direction to fill gear oil into an outboard lower unit?
- Fill from the bottom drain plug hole until oil appears at the upper vent hole, then reinstall the vent screw first (Correct answer)
- Pour from the top vent hole with the engine tilted up
- Use a gravity fill from the top with the engine horizontal
- Either direction is acceptable
Correct answer: Fill from the bottom drain plug hole until oil appears at the upper vent hole, then reinstall the vent screw first
Filling from the bottom ensures complete purging of air, and installing the vent screw first (while oil is still present) locks in the full fill before removing the fill tube.
MST MST Outboard Engine Service & Gearcase 1
What is the correct procedure for checking the gear oil level in an outboard lower unit?