Mercury 40HP 4-Stroke Outboard Motor Parts

(114 products)

This page focuses on Mercury 4‑Stroke 40HP outboard engines. Use it to find compatible replacement parts and practical service information. Below you’ll find routine maintenance guidance, frequently used categories, and troubleshooting tips. You can also browse the main Mercury collection, and at the bottom of this page you’ll find a quick link to start shopping 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 40HP Model

Locate the serial/nameplate: typically on the transom/swivel bracket (stbd. side) or on the engine clamp bracket. Record the full serial string.
Decode the model/serial: suffixes may note shaft length (S/L/XL), control type (tiller vs. remote), and region. Serial ranges help match correct parts.
Confirm platform details: most 40HP four-strokes in this range are inline‑4 carbureted. Some units are “BigFoot” (larger gearcase, 2.31:1 ratio) vs. standard (1.83:1).
Check controls and charging: tiller vs. remote, electric start, and alternator output can change harnesses and brackets.

Platform Overview

Standard Gearcase (1.83:1) – Inline‑4 Four‑Stroke

Feature	Description
Configuration	Inline‑4, carbureted 4‑Stroke; standard 3.44" torpedo gearcase.
Common Models	Mercury FourStroke 40HP (standard gearcase), tiller or remote variants.
Advantages	Balanced all‑around performance and fuel economy; common service parts.
Notes	Impeller/water pump components and props differ from BigFoot versions. Verify gear ratio by serial and gearcase diameter.

BigFoot Gearcase (2.31:1) – Inline‑4 Four‑Stroke

Feature	Description
Configuration	Inline‑4, carbureted 4‑Stroke; larger 4.25" gearcase (“BigFoot”) with 2.31:1 ratio.
Common Models	Mercury FourStroke 40HP BigFoot (remote or tiller control).
Advantages	Stronger thrust for heavy loads; better low‑speed control.
Notes	Uses different propellers/hubs and a different impeller/water pump kit vs. the standard gearcase. Confirm by serial and gearcase size.

Routine Maintenance & Service Parts

Intervals below are general guidelines for 4‑Stroke outboards; always follow the official Mercury service manual for your exact serial/model.

Item	Interval*	Remarks	Parts Link
Engine oil + filter	100 hours or annually	Use marine‑rated oil; warm engine, change filter and crush washer as applicable.	Oil Filters
Gear oil (lower unit)	100 hours or annually	Inspect for water or metal; replace seals if contamination is found.	Mercury Lower Unit Seal Kits
Spark plugs	100 hours or annually	Inspect and gap; replace if fouled or worn.	Spark plug
Fuel filter (inline or under‑cowl)	100–200 hours	Shorten interval for stale/contaminated fuel. Replace clamps if corroded.	Fuel Filters
Water pump impeller/kit	100 hours or annually	Critical for cooling; BigFoot vs. standard gearcase use different kits.	Mercury Water Pump Repair Kits
Anodes (sacrificial)	Inspect every 50 hours; replace when 50% worn	Choose alloy for your water (zinc, aluminum, or magnesium).	Anodes
Thermostat	Every 2 seasons or as needed	Replace if overheat or cold‑running issues persist.	Mercury Outboard Parts
Battery/charging checks	Each service	Verify battery health, grounds, and regulator output.	Electrical Items
Propeller & hub inspection	Seasonally or after impact	Remove, grease shaft, check for line, and confirm correct pitch.	Propellers

Frequently Used Parts Categories

Routine Service & Tune‑Up – Mercury Outboard Parts
Cooling System (impellers, pump kits) – Mercury Water Pump Repair Kits
Fuel System (filters) – Fuel Filters
Fuel System (pumps) – Fuel Pumps
Ignition / Electrical – Electrical Items
Gearcase Seals & Reseal – Mercury Lower Unit Seal Kits
Propellers & Hardware – Propellers

Common Symptoms & Quick Checks

Symptom	Check First	Related Parts
Weak telltale or overheat alarm	Inspect intake screens for debris; confirm strong telltale; verify impeller and thermostat condition.	Water pump kit, thermostat, intake screens, anodes
Poor acceleration or power loss	Check prop for damage/hub slip; replace fuel filter; verify carb sync/enrichener and spark plugs.	Propellers/hubs, fuel filter, carburetor repair kit, spark plugs
Hard starting or rough idle	Confirm fresh fuel; check choke/enrichener operation; inspect for air leaks and plug condition.	Spark plugs, fuel pump, fuel lines, carburetor kit
Gearcase whine or metal in gear oil	Change gear oil; look for water (milky oil); pressure/vacuum test for seal leaks.	Lower unit seal kit, bearings, shift shaft/driveshaft seals
Trim/tilt slow or inoperative	Verify battery voltage/grounds; check relays and fluid level; inspect connectors for corrosion.	Trim motor, relays, wiring/connectors

FAQs

Are parts interchangeable across years or between BigFoot and standard gearcase models?

Many core engine components are shared, but gearcase‑specific items (impeller kits, seals, props) differ between standard (1.83:1) and BigFoot (2.31:1). Always confirm by serial number and gearcase type before ordering.

When should I replace the impeller?

As a rule of thumb: every 100 hours or annually, sooner if you run sandy/shallow water or see a weak telltale. Replace the full pump kit periodically for best reliability.

How often should lower unit gear oil be changed?

At least once per season or every 100 hours. Inspect for water (milky oil) or metal—both indicate seal or gear issues requiring further service.

What information helps confirm part compatibility?

Provide the engine serial number, shaft length, control type (tiller/remote), gearcase style (standard vs. BigFoot), and any notable options. Photos of the nameplate and the part being replaced are very helpful.

Ready to find the right parts? Browse the main Mercury collection and use Filter & Sort to narrow by horsepower, year, and part category.

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