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MerCruiser 300HP 4-Stroke Marine Engine Parts

(28 products)

This page focuses on MerCruiser 4‑Stroke 300 HP sterndrive/inboard engines and helps you zero in on compatible parts and service information. Inside you’ll find maintenance tips, common replacement parts, and troubleshooting guidance. You can also browse the main MerCruiser collection. Scroll to the bottom of this page to start browsing parts for this engine. 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 300 HP Model

  • Locate the serial number label on the engine (commonly on the side of the engine or on top of the flame‑arrester cover). The sterndrive unit and transom assembly also have their own serial numbers.
  • Model naming examples include 6.2L 300 (ECT) or legacy 350 MAG MPI 300. Suffixes can indicate emissions package, cooling configuration (open/closed), and other options.
  • Distinguish platforms: newer 6.2L V8 EFI vs. older 5.7L (350 MAG) MPI. Parts differ between platforms and by drive type (Alpha/Bravo).
  • For precise parts, reference the engine serial number first; drive and transom serials help confirm gear lube, anodes, impeller/sea‑pump, and prop compatibility.

Engine/Platform Overview

MerCruiser 300 HP models are commonly found in two platform families. Use serial numbers to verify exact fitment.

6.2L V8 300 (ECT) – Current Platform

Feature Description
Configuration V8 4‑stroke EFI sterndrive/inboard, typically paired with Bravo drives
Common Models 6.2L 300 ECT variants with Bravo One/Two/Three; open or closed‑cooling options
Advantages Strong mid‑range torque, modern fuel efficiency, lighter package vs. legacy small‑block
Notes Cooling and emissions options impact thermostats, hoses, and anodes. Sea‑pump impeller service intervals are critical for raw‑water setups.

Legacy 350 MAG MPI 300 – Prior Generation

Feature Description
Configuration 5.7L V8 MPI sterndrive, commonly with Alpha or Bravo drives
Common Models 350 MAG MPI 300 HP; open or closed‑cooling variants
Advantages Proven package with broad parts availability; strong aftermarket support
Notes Ignition, filters, belt routing, and impeller kits differ from 6.2L engines. Verify by engine and drive serials.

Routine Maintenance & Service Parts

Intervals below are general guidelines for 4‑stroke MerCruiser sterndrive/inboard engines. Always follow your official service manual for model‑specific procedures.

Item Interval* Remarks Parts Link
Engine oil & filter 100 hours or annually Warm engine before draining; replace filter and inspect for metallic debris MerCruiser Oil Filters
Drive gear lube (Alpha/Bravo) 100 hours or annually Check for milkiness/water intrusion; replace gaskets and crush washers as needed MerCruiser Collection
Spark plugs 300 hours or 3 years (inspect annually) Verify heat range and gap by engine family; replace sooner if fouled Spark Plug
Fuel filter / water‑separating filter 100 hours or annually Drain or replace more often with ethanol fuel or contamination MerCruiser Fuel Filters
Raw‑water pump impeller (sea pump) Every 2–3 years or 200 hours Replace sooner in sandy/silty water or after overheat events MerCruiser Impellers
Anodes (engine & drive) Inspect monthly; replace when 50% consumed Choose zinc, aluminum, or magnesium based on water type MerCruiser Anodes
Serpentine/accessory belt Inspect every 100 hours; replace 300–400 hours or as needed Check for cracking or glazing; verify tensioner operation Belt

Frequently Used Parts Categories

Common Symptoms & Quick Checks

Symptom Check First Related Parts
Rising temperature or overheat alarm Inspect sea‑water intake screens on drive, sea strainer (if equipped), and impeller condition; check for collapsed hoses or stuck thermostat Water pump impeller kit, thermostat, intake screens, hoses, anodes
Poor acceleration or can’t reach WOT rpm Verify fuel quality, replace water‑separating filter, check propeller damage/pitch, confirm throttle/ECT calibration Fuel filter, fuel pump, propeller, spark plugs
Hard starting or rough idle Load‑test battery, inspect spark plugs and ignition leads/coils, check for vacuum leaks and restricted air/fuel filters Spark plugs, air filter, fuel filter, ignition components
Gearcase noise or metal in gear lube Check lube level/color, pressure test for leaks, inspect prop shaft seals and u‑joints; verify correct gear lube Lower unit seal kit, bearings, gear lube, propeller hardware
Heavy steering or belt squeal Check power‑steering fluid level, serpentine belt wear/tension, and alignment; inspect steering actuator Serpentine belt, power steering pump, tensioner, pulleys

Tip: After any overheat or water‑intrusion event, change fluids and re‑inspect wear components.

FAQs

Are parts interchangeable between different years or platforms?

Often no. The 6.2L 300 and the legacy 350 MAG MPI use different filters, ignition parts, belts, and cooling components. Drive type (Alpha/Bravo) and cooling configuration further affect anodes, impellers, gaskets, and hose kits. Verify by engine, drive, and transom serial numbers.

When should the raw‑water impeller be replaced?

As a rule of thumb, every 2–3 seasons or about 200 hours, and sooner if you run sandy or silty water or have experienced any overheat. Always inspect annually for set‑memory, cracking, or missing vanes.

How often should gear lube be changed?

At least every 100 hours or annually. If the lube appears milky, shows metal, or you suspect water intrusion, service immediately and pressure‑test the drive to find the source of leakage.

What information do you need to confirm part compatibility?

Provide the engine serial number (and, if possible, the drive and transom serials), the model designation (e.g., 6.2L 300 or 350 MAG MPI), drive type (Alpha/Bravo), and whether the cooling system is open or closed. Photos of the nameplate and the existing part help ensure a correct match.

Ready to find parts? Browse the main MerCruiser collection and filter by horsepower, year, and model to locate compatible components for your 300 HP engine.

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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.
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