Mercury 300HP 2-Stroke Outboard Marine Engine Parts

(32 products)

This page is dedicated to Mercury 2‑Stroke 300HP outboard engines. Use it to find compatible parts, routine service guidance, and troubleshooting tips. You can also browse the main Mercury collection for a wider selection. Scroll to the bottom of this page to start browsing parts for this engine, and on the collection page use the “Filter & Sort” controls to narrow results by horsepower, year range, and part category/section.

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How to Identify Your 300HP Model

Locate the serial/nameplate on the transom bracket or swivel bracket (typically port side). Record the full serial number and any model code/suffix.
Understand suffixes: shaft length (L/XL/XXL), gearcase style (Fleetmaster/Torquemaster/Sportmaster), and control type can be encoded in the model code.
Platform cues: conventional 2‑stroke EFI uses fuel rails/injectors but no air compressor; DFI (Optimax/XS) adds an air compressor and distinct fuel/air rail layout.
Oil system: most 300HP 2‑strokes are oil‑injected. Inspect the remote oil tank and lines to confirm configuration.

Platform Overview

V6 3.0L EFI (Conventional 2‑Stroke)

Feature	Description
Configuration	V6 loop‑charged 2‑stroke, EFI fuel system, oil‑injected
Common Models	300HP EFI offshore‑class V6 models (many from the 1990s–early 2000s)
Advantages	High power‑to‑weight, strong mid‑range, straightforward maintenance
Notes	Confirm gearcase type (Fleetmaster/Torquemaster/Sportmaster) and shaft length. EFI fuel/ignition parts differ from DFI platforms.

V6 3.0–3.2L DFI (Optimax/XS)

Feature	Description
Configuration	V6 direct‑injected 2‑stroke (DFI), air compressor with separate fuel/air rails
Common Models	300 Optimax/XS variants (commonly early‑to‑mid 2000s)
Advantages	Improved fuel economy and emissions, crisp throttle response
Notes	Serial number is essential to match injectors, rails, and PCM. Cooling and gearcase parts often overlap with EFI models; fuel/ignition parts are platform‑specific.

Routine Maintenance & Service Parts

Intervals below are general guidelines for 2‑stroke V6 outboards. Always follow the official service manual for your exact model and serial range.

Item	Interval*	Remarks	Parts Link
Lower unit gear oil	Every 100 hrs or annually	Inspect for water intrusion/metal. Replace seals if contamination is found.	Mercury Lower Unit Seal Kits
Spark plugs	Inspect every 50 hrs; replace ~100 hrs	Use correct heat range and gap for EFI vs DFI configurations.	Spark Plugs
Fuel filter(s)	Inspect 50 hrs; replace 100 hrs	Replace inline and/or water‑separating filters; check for debris and water.	Fuel Filters
Water pump impeller / kit	1 year or 100 hrs (more often in silt/sand)	Critical for cooling; renew complete kit periodically for best reliability.	Mercury Water Pump Repair Kits
Anodes (zinc/aluminum/magnesium)	Inspect every 50 hrs; replace when 50% worn	Choose alloy for your water (salt/brackish/fresh). Keep clean for proper protection.	Anodes
Thermostats	Test annually; replace 2–3 seasons	Cold‑running (stuck open) can increase smoke and fuel use.	Mercury Outboard Parts
Oil‑injection system check	Inspect 100 hrs or annually	Inspect/replace brittle oil lines and inline oil filter; verify tank cap/vent and alarm.	Hoses
Propeller & hub inspection	Each service	Check for bent blades, line behind prop, and hub slippage.	Propellers

Frequently Used Parts Categories

Routine Service – Mercury Outboard Parts
Cooling System – Mercury Water Pump Repair Kits
Gearcase & Seals – Mercury Lower Unit Seal Kits
Fuel System – Fuel Filters | Fuel Pumps
Ignition / Electrical – Electrical Items
Corrosion Protection – Anodes
Propulsion – Propellers

Common Symptoms & Quick Checks

Symptom	Check First	Related Parts
Weak tell‑tale or overheat alarm	Inspect intake screens for blockage; test thermostats; verify impeller condition and pump housing.	Water pump impeller/kit, thermostat(s), intake screens, anodes
Hard starting or rough idle	Check fuel freshness and primer bulb; replace fuel filter; inspect plugs and plug leads.	Fuel filter, spark plugs, fuel lines/hoses, EFI/DFI fuel components
Poor acceleration or power loss	Look for restricted fuel flow; verify prop hub integrity; read plugs for fouling.	Propeller/hub, fuel filter, spark plugs, fuel pump
Excessive smoke (2‑stroke)	Confirm oil system operation; ensure thermostats close properly; avoid prolonged cold‑running.	Thermostat(s), oil lines, spark plugs
Gearcase noise or metal in gear oil	Drain gear oil; inspect for water/metal; pressure‑test and reseal if needed.	Lower unit seal kit, prop shaft seals, drain/fill gaskets

FAQs

Are 300HP 2‑stroke parts interchangeable across years and platforms?

Many cooling and gearcase components interchange, but EFI vs DFI (Optimax/XS) fuel and ignition parts are not the same. Always verify by serial number and gearcase style.

When should I replace the impeller?

Annually or every 100 hours is a common guideline, and sooner if you run in sandy or silty water. Replace the full pump kit periodically for best reliability.

How often should I change gear oil?

Every 100 hours or at least once per year. If you find milky oil or metal debris, pressure‑test the gearcase and renew seals.

What information helps confirm part compatibility?

Provide the engine serial number, model code/suffix (shaft length, gearcase), platform (EFI vs DFI/Optimax), and any photos of the part or rigging.

Ready to find parts for your Mercury 300HP 2‑Stroke? Browse the main Mercury collection and filter by horsepower, year, and category to dial in the right components.

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If you’re unsure about your model, send us a photo of the nameplate and the part you need and we’ll help verify compatibility.