Mercury 300HP 4-Stroke Outboard Marine Engine Parts

(44 products)

This page is dedicated to Mercury 300HP 4-Stroke outboard engines. Use it to find compatible parts and service information, including maintenance intervals, common replacement parts, and troubleshooting guidance. You can also browse our main Mercury collection. 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.

Browse All Mercury Parts

How to Identify Your 300HP Model

Locate the serial number plate: typically on the starboard side of the transom/swivel bracket or on the engine block under the cowling. Mercury parts lookup is based on the serial number.
Decoding suffixes: labels may note shaft length (L/XL/XXL), control type (mechanical vs Digital Throttle & Shift), and charging/alternator options.
Platform clues: 300HP 4‑Stroke models are commonly V8 and V6 platforms. Cowling badging and intake styling can help distinguish platforms and trim levels.
Record key details before ordering: serial number, model year range, shaft length, steering type, and whether your controls are mechanical or DTS.

Platform Overview

Mercury 300HP 4‑Stroke V8

Feature	Description
Configuration	4‑Stroke V8, EFI, available with mechanical or Digital Throttle & Shift (DTS)
Common Models	300HP V8 outboards in multiple shaft lengths (L/XL/XXL) with various gearcase options
Advantages	Strong mid‑range torque, refined NVH, compatible with modern digital rigging
Notes	Some ignition, fuel, and cooling parts are platform‑specific; confirm by serial number for exact fit.

Mercury 300HP 4‑Stroke V6

Feature	Description
Configuration	4‑Stroke V6, EFI, model‑specific control systems (mechanical or DTS) and gearcase choices
Common Models	300HP V6 outboards used across center consoles and multi‑engine rigs
Advantages	Lightweight for output, crisp throttle response, efficient cruising
Notes	Cooling system, ignition components, and sensors may differ from V8 versions—verify by serial number.

Routine Maintenance & Service Parts

Intervals below are general guidelines for Mercury 4‑Stroke outboards. Always follow your official service manual and operating conditions.

Item	Interval*	Remarks	Parts Link
Engine oil + filter	100 hours or annually	Warm engine before draining; replace crush washer where applicable	Oil Filters
Gear oil (lower unit)	100 hours or annually	Inspect for water intrusion/metal dust; replace seals if contamination is found	Mercury Outboard Parts
Spark plugs	100 hours or annually	Gap to spec; replace sooner if misfire or fouling occurs	Spark Plugs
Fuel filter / water separator	100 hours or seasonally	Drain or replace; check for water and debris	Fuel Filters
Water pump impeller / kit	2–3 years or sooner if stream is weak/overheats	Replace impeller and key; inspect housing and wear plate	Mercury Water Pump Repair Kits
Anodes (internal & external)	Inspect every 50 hours; replace when 50% depleted	Choose zinc (salt), aluminum (salt/brackish), or magnesium (fresh)	Anodes
Thermostat inspection	Annually	Replace if sticking or temperature control is erratic	Mercury Outboard Parts
Propeller & hub inspection	Each 50–100 hours	Check for dings, bent blades, and worn hub; torque prop nut to spec	Propellers

Frequently Used Parts Categories

Routine Service Kits – Service & Maintenance Kits
Cooling System – Mercury Water Pump Repair Kits
Gearcase & Seals – Mercury Lower Unit Seal Kits
Fuel System Filtration – Fuel Filters
Ignition / Electrical – Electrical Items
Propellers & Hubs – Propellers
Corrosion Protection – Anodes

Common Symptoms & Quick Checks

Symptom	Check First	Related Parts
Weak tell‑tale stream or overheat alarm	Inspect intake screens for blockage; check impeller condition and thermostat	Water pump kit, thermostat, intake screens, anodes
Poor acceleration or power loss	Verify prop condition; replace fuel/water separator; inspect spark plugs	Propeller, fuel filter, spark plugs
Hard starting or rough idle	Confirm battery voltage; check for stale fuel; inspect plugs and fuel lines	Spark plugs, fuel filter, hoses, injectors
Gearcase noise or metal in gear oil	Change gear oil; examine for water intrusion; pressure/vacuum test seals	Lower unit seal kit, bearings, gearset
Erratic idle speed or stalling when shifting	Check for air leaks and kinked hoses; verify throttle/throttle body cleanliness and cable/DTS setup	Hoses and clamps, sensors, control cables or DTS components

FAQs

Are 300HP V6 and V8 parts interchangeable?

Only some hardware overlaps. Cooling, ignition, electrical, and many fuel components are platform‑specific. Use your serial number to confirm compatibility before ordering.

How often should I replace the impeller?

Inspect annually and replace every 2–3 years, or immediately if water flow is weak, the engine overheats, or the impeller shows wear.

When should gear oil be changed?

Change gear oil every 100 hours or once per year. If water or metal debris is present, service the seals and investigate further before continued operation.

What information helps confirm part compatibility?

Provide the engine serial number, horsepower (300HP), platform (V6 or V8 if known), shaft length (L/XL/XXL), control type (mechanical or DTS), and any notable accessories or rigging.

Ready to find the right parts? Browse the main Mercury collection by horsepower, year, and model to locate exact‑fit components for your 300HP outboard.

Shop Mercury Outboard Parts

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.