MST MST Outboard Engine Service & Gearcase 2 — Questions and Answers
Question 1: An outboard engine's water pump impeller should be replaced at what interval as a best practice?
- Every 100 hours or annually, whichever comes first (Correct answer)
- Every 500 hours only
- Only when overheating is observed
- Never — impellers are lifetime components
Correct answer: Every 100 hours or annually, whichever comes first
Rubber impeller vanes harden and crack over time, losing pumping efficiency; annual or 100-hour replacement prevents overheating before failure occurs.
Question 2: What symptom would a worn or damaged water pump impeller cause on an outboard engine?
- Overheating, reduced or absent water stream from the tell-tale, and possible engine alarm (Correct answer)
- Rich fuel mixture and black exhaust smoke
- Hard starting and rough idle
- Excessive lower unit vibration
Correct answer: Overheating, reduced or absent water stream from the tell-tale, and possible engine alarm
A failed impeller reduces cooling water flow, causing overheating that triggers the engine's thermal warning system and can cause severe engine damage.
Question 3: What special tool is required to properly torque the propeller nut on most outboard engines?
- A prop holding tool (prop wrench) and a torque wrench (Correct answer)
- Only a standard socket set
- An impact wrench set to maximum torque
- No special tool — hand-tighten only
Correct answer: A prop holding tool (prop wrench) and a torque wrench
A prop holding tool secures the propeller from rotating while a torque wrench applies the OEM-specified torque to the prop nut.
Question 4: During a compression test on a two-stroke outboard, what is the acceptable minimum compression reading per cylinder?
- Above 90 psi and within 15% variation between cylinders (Correct answer)
- Above 50 psi regardless of variation
- Exactly 120 psi on all cylinders
- Compression testing does not apply to two-stroke engines
Correct answer: Above 90 psi and within 15% variation between cylinders
Healthy two-stroke outboards typically produce 90–150 psi depending on the model; the critical factor is that cylinders are within 15% of each other.
Question 5: What is the function of the shift dog (or clutch dog) in an outboard lower unit gearcase?
- It slides axially on the prop shaft to engage the forward or reverse gear by locking to the bevel gear (Correct answer)
- It drives the water pump impeller
- It connects the driveshaft to the crankshaft
- It operates the throttle from the shift lever
Correct answer: It slides axially on the prop shaft to engage the forward or reverse gear by locking to the bevel gear
The shift dog is a splined sliding coupler that engages lugs on the forward or reverse bevel gear to transmit engine power in the selected direction.
Question 6: What is the correct process for removing a seized propeller from an outboard shaft?
- Apply penetrating oil, wait, then use a prop puller or wooden block with mallet — never use heat near seals (Correct answer)
- Heat the prop hub with an oxy-acetylene torch to expand it
- Strike the prop blade directly with a hammer
- Use an impact wrench to spin the prop off
Correct answer: Apply penetrating oil, wait, then use a prop puller or wooden block with mallet — never use heat near seals
Penetrating oil and a proper prop puller safely remove a seized prop without damaging the shaft threads or seals — heat or striking can cause serious damage.
MST MST Outboard Engine Service & Gearcase 2
An outboard engine's water pump impeller should be replaced at what interval as a best practice?