Mercury 125HP 4-Stroke Outboard Engine Parts

(53 products)

This page is for Mercury 4-Stroke 125 HP outboard engines. Use it to find compatible parts and practical service information, including maintenance tips, common replacement items, and troubleshooting guidance. You can also browse the main Mercury collection. Scroll down 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.

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How to Identify Your 125 HP Model

Find the serial/nameplate on the transom bracket or swivel bracket (starboard side on most models). Take a clear photo for reference.
Mercury 4‑Stroke model info is typically verified by the serial number. Suffix letters may indicate shaft length (L/XL), steering/control type, or region.
Confirm it’s a 4‑Stroke: look for an engine oil fill cap/dipstick and an external fuel system with an EFI fuel rail (no premix fuel required).
If parts differ by platform, note shaft length (20 in L or 25 in XL), starting/charging options, and whether you have tiller or remote control.

Engine Overview

Inline‑4 EFI 4‑Stroke

Feature	Description
Configuration	Inline‑4 EFI 4‑Stroke outboard designed for mid‑range torque and smooth operation.
Common Models	125 HP FourStroke EFI variants in long (L) and extra‑long (XL) shaft lengths.
Advantages	Good fuel economy, quieter operation than 2‑strokes, and reliable mid‑range performance.
Notes	Many routine‑service parts overlap with other Mercury FourStroke inline‑4 models. Always confirm by serial number before ordering.

Routine Maintenance & Service Parts

Intervals below are general guidelines for typical recreational use; always follow the official Mercury service manual for your specific serial number.

Item	Interval*	Remarks	Parts Link
Engine oil + filter	After break‑in, then every 100 hours or annually	Use marine 4‑stroke oil meeting Mercury specs; replace filter at each oil change.	Oil Filters
Gear oil (lower unit)	Every 100 hours or annually; also before winter storage	Inspect for water intrusion and metal debris; replace seals if contamination is found.	Mercury Lower Unit Seal Kits
Spark plugs	Inspect every 100 hours; replace as needed or per manual	Check gap and look for fouling; replace as a set for consistent performance.	Spark Plugs
Fuel filter / water‑separating filter	Every 100 hours or annually; sooner with poor fuel	Replace canister or inline filter to protect EFI components.	Fuel Filters
Water pump impeller / kit	Every 2 seasons (or 1 season in sandy/silty water)	Key to cooling performance; replace housing if worn or scored.	Mercury Water Pump Repair Kits
Anodes (zincs)	Inspect every 50 hours; replace when ~50% consumed	Match anode material to your water (aluminum/brackish, magnesium/fresh, zinc/salt).	Anodes
Thermostat inspection	Check annually or if overheat/low temp occurs	Replace if corroded, sticking, or out of spec; verify proper operating temperature.	Mercury Outboard Parts

Frequently Used Parts Categories

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

Common Symptoms & Quick Checks

Symptom	Check First	Related Parts
Weak tell‑tale stream or overheat alarm	Inspect intake screens for blockage; verify tell‑tale hose is clear; check impeller age and housing wear.	Water pump kit, thermostat, intake screens/gaskets
Hard starting or rough idle (warm or cold)	Confirm battery voltage and clean connections; check primer bulb and fuel quality; inspect/replace spark plugs and fuel filter.	Spark plugs, fuel filter, injector service, starter/battery cables
Poor acceleration or power loss	Check prop for damage or hub slip; verify compression; replace clogged filters; inspect plugs and coils.	Propeller, fuel filters, spark plugs, ignition components
Gearcase noise or metal in gear oil	Drain oil and inspect; check for fishing line on prop shaft; pressure/vacuum test seals.	Lower unit seal kit, shims/bearings, gear lube, prop shaft seals

FAQs

Are parts interchangeable between different years or platforms?

Some routine‑service items may cross over among Mercury FourStroke inline‑4 models, but many parts are serial‑number specific. Always verify using your serial number and measure shaft length and control type if applicable.

When should the impeller be replaced?

Replace the impeller every 2 seasons in clean water, or annually in sandy/silty conditions. If the tell‑tale is weak or the pump housing is grooved, install a full water pump kit.

How often should gear oil be changed?

Every 100 hours or at least once per year. If the oil looks milky or metallic, service the seals and re‑test before returning to service.

What information helps confirm part compatibility?

Provide your Mercury serial number (from the bracket plate), shaft length (L or XL), control type (tiller/remote), and any notable options (electric start/charging). Photos of the nameplate and the part location are also helpful.

Ready to find parts for your Mercury 125 HP FourStroke? Browse the main Mercury collection and filter by horsepower, year, and model.

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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.