MST Outboard Engine Service & Gearcase Flashcards

What is the correct procedure for checking the gear oil level in an outboard lower unit?

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.

Water in the outboard gear oil is most commonly caused by what condition?

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.

When removing an outboard propeller for service, what must be inspected before reinstallation?

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.

What is the purpose of the shear pin (or rubber hub) on an outboard propeller?

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.

What does milky or frothy gear oil in the lower unit indicate?

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.

What is the correct direction to fill gear oil into an outboard lower unit?

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.