4731A6 4731A8 Mercury Trigger | Genuine Used Part

🔧 Product Description

Genuine Used Mercury Mariner 40HP Twin-Cylinder Trigger Plate - Part Numbers: 4731A6 and 4731A8

A genuine pre-owned Mercury and Mariner trigger-plate assembly for selected 40HP and 402 twin-cylinder two-stroke outboards.

Part number 4731A6 supersedes to 4731A8. The trigger assembly is mounted beneath the flywheel and produces the ignition-timing signals required by the switchbox.

The complete trigger plate also rotates with the throttle and spark-advance linkage to alter ignition timing as engine speed increases.

⚙️ Key Features

• Genuine Mercury or Mariner used trigger plate

• Earlier OEM number 4731A6

• Later service number 4731A8

• Designed for selected 40HP and 402 twin-cylinder engines

• Mechanical spark-advance configuration

• Original Mercury ignition component

🔄 Supersession & Cross Reference

OEM Supersession Chain:

• Mercury 4731A6 - Earlier trigger-plate assembly

• Mercury 4731A8 - Later functional replacement

• The later assembly may differ visually while remaining the verified service replacement

OEM / Related Numbers:

• 4731A6

• 4731A8

• 338-4731A6

• 338-4731A8

• OEM Description: Trigger Plate Assembly

• OEM Description: Trigger Assembly

• Engine Family: Mercury 402 / 40HP Two-Cylinder

Aftermarket / Verified Equivalents:

• No verified new aftermarket equivalent

• Genuine used, new-old-stock or professionally rebuilt assemblies are generally required

🚤 Compatibility

Commonly used on:

• Mercury 402 outboards

• Mercury 40HP twin-cylinder outboards

• Mariner equivalents using the Mercury ignition system

• Capacitor-discharge ignition systems with a movable trigger plate

Verified catalogue families include:

• Mercury 402 serial-number-controlled applications

• Mercury 40HP two-cylinder applications

• United States, Canadian, Australian and European serial ranges listed for the 402 / 40HP family

Important fitment note:

• Confirm that the engine is the two-cylinder 40HP or 402 design.

• Mercury also produced three- and four-cylinder 40HP engines that use different triggers.

• Compare the wire colours, connector terminals, linkage arm and mounting diameter.

• Switchbox, stator, flywheel and mounting hardware are included only when shown or stated.

🔌 Function

Trigger coils produce electrical pulses as the flywheel magnets pass over the assembly.

The switchbox uses these pulses to discharge the ignition coils at the correct crankshaft position. Rotation of the trigger plate advances ignition timing as the throttle is opened.

Common signs of trigger failure:

• No spark on one or both cylinders

• Intermittent spark

• Spark disappears as the engine becomes warm

• Difficult starting

• Engine backfires

• Misfire during acceleration

• Engine will not reach maximum spark advance

• Cracked or brittle trigger wiring

• Trigger plate binds or does not return to idle

• Resistance or peak output is outside specification

Recommended checks before installation:

• Confirm 4731A6 or 4731A8 on the original assembly

• Compare the linkage arm and wire terminals

• Measure trigger resistance

• Check trigger peak output where specified

• Inspect the flywheel magnets and key

• Test the switchbox, stator and ignition coils

• Confirm free trigger-plate movement

• Set ignition timing and throttle synchronisation after installation

📦 Condition

• Genuine Mercury or Mariner used part

• Pre-owned electrical component

• Wire ageing, marks and light oxidation may be present

• Electrical testing before installation is strongly recommended

• Included linkage and hardware are limited to those shown or described

• Supplied in protective or equivalent packaging