MST MST Outboard Engine Service & Gearcase 2

Question 1

An outboard engine's water pump impeller should be replaced at what interval as a best practice?

Accepted Answer

Every 100 hours or annually, whichever comes first

Answer

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?

Accepted Answer

Overheating, reduced or absent water stream from the tell-tale, and possible engine alarm

Answer

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?

Accepted Answer

A prop holding tool (prop wrench) and a torque wrench

Answer

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?

Accepted Answer

Above 90 psi and within 15% variation between cylinders

Answer

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?

Accepted Answer

It slides axially on the prop shaft to engage the forward or reverse gear by locking to the bevel gear

Answer

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.

(MST) Marine Service Technician Practice Test

(MST) Marine Service Technician Practice Test

MST MST Outboard Engine Service & Gearcase 2