Service intervals are published by each manufacturer and vary by family; always work from the manual for the specific engine. What follows is the general shape of four-stroke outboard servicing and, more usefully for a workshop or fleet operator, what each interval actually consumes from the parts shelf.
The intervals
Every 100 hours or annually, whichever comes first. The baseline service: engine oil and oil filter, fuel filter, spark plugs on many families, gearcase lubricant, anode inspection, and a general check of the cooling circuit and control linkages.
Every 300 hours. Adds the water pump impeller and its associated seals and gaskets, thermostat inspection or replacement, valve clearance check on applicable families, and closer inspection of the timing components.
Annually, regardless of hours. Anodes, cooling-system inspection, and full corrosion protection. Engines that sit unused still corrode, and saltwater engines that sit unused corrode faster than engines in regular service.
Why the impeller interval is not negotiable
The water pump impeller is a wear item made of flexible material that takes a set, stiffens and loses vane contact over time. It degrades on a calendar as well as on an hour meter. An impeller that fails in service can cause a powerhead overheat within minutes, and the cost of the repair is measured in thousands against a part measured in tens. Where a fleet operator gets one thing right, this is the one to get right.
Always replace the pump housing wear plate, the gaskets and the seals with the impeller. This is why these items are supplied as kits — fitting a new impeller into a worn housing returns much of the original problem.
Stocking rather than reacting
For a workshop or fleet, the economic argument is straightforward: service parts are low-value, high- frequency, and the cost of not having one on the shelf is a vessel out of service. The items worth holding in depth are impellers and pump kits, oil and fuel filters, anodes, spark plugs, thermostats, and the common gaskets and seals for the engines you actually operate.
We supply pre-assembled 100-hour, 300-hour and annual kits by engine family, which is normally simpler to stock and to count than the same items as loose lines. Send us your engine list and we will return a consolidated stocking proposal with current availability.
Last reviewed September 2026. This guide is general technical reference. Always work from the manufacturer documentation for your specific engine.