NGK BR6FS Spark Plug #4323

The NGK BR6FS is a resistor-equipped marine spark plug engineered for dependable ignition performance in classic carbureted and early electronic-ignition marine engines. Featuring a copper-core center electrode, durable nickel-alloy firing surfaces, and NGK’s corrosion-resistant trivalent metal plating, the BR6FS is designed to withstand the demanding operating conditions commonly found in sterndrive and inboard marine applications.

Widely recognized as a direct replacement for the classic AC Delco MR43T, the BR6FS has long been used in MerCruiser, Indmar, PCM, Crusader, and other GM-based marine powerplants where reliable cold starts, smooth idle quality, and resistance to fouling are essential.

Key Features

• Genuine NGK marine spark plug
• NGK Part Number: BR6FS
• NGK Stock Number: 4323
• Built-in resistor helps suppress radio frequency interference (RFI)
• Copper-core center electrode for excellent heat transfer
• Nickel-alloy firing surfaces for long service life
• Corrosion-resistant trivalent metal plating
• 14 mm thread diameter
• 11.6 mm (0.460″) reach
• 5/8″ (16 mm) hex size
• Tapered-seat design (no sealing washer required)
• Heat Range: 6
• Factory preset gap: 0.032″ (0.8 mm)
• Solid terminal configuration

Engine Compatibility

Commonly specified for numerous marine gasoline engines requiring NGK BR6FS or equivalent plugs, including:

MerCruiser Sterndrive & Inboard Engines

• MerCruiser 140
• MerCruiser 3.0L
• MerCruiser 4.3L V6
• MerCruiser 5.0L V8
• MerCruiser 5.7L V8
• MerCruiser 7.4L V8
• Various carbureted and early TBI-equipped MerCruiser applications

Other Marine Applications

• Indmar carbureted inboard engines
• Pleasurecraft Marine (PCM) carbureted engines
• Crusader gasoline marine engines
• Legacy tournament ski and wake boat engines
• Additional marine applications specifying AC Delco MR43T equivalents

Always verify spark plug requirements using the engine serial number, service manual, or engine specification decal before ordering.

Fitment Notice

Spark plugs should always be matched to the exact engine model and serial number. Even engines with identical displacement may require different heat ranges, electrode designs, or gap settings depending on ignition system configuration and production year.

When This Part Needs Replacement

Replace spark plugs when experiencing:

• Hard starting
• Rough idle
• Misfires under load
• Poor acceleration
• Increased fuel consumption
• Carbon fouling
• Damaged electrodes
• Corrosion or excessive wear

Marine engines that spend extended periods trolling, idling, or in seasonal storage should have plugs inspected regularly as part of scheduled maintenance.

Installation Notes

• Confirm the required spark plug gap for your exact engine before installation.
• Many marine ignition systems utilize gap specifications ranging from 0.032″ to 0.045″ depending on engine model and ignition system.
• Install into clean cylinder head threads and tighten to manufacturer specifications.
• NGK’s corrosion-resistant plating eliminates the need for anti-seize compound unless specifically required by the engine manufacturer.
• Use a spark plug socket and torque wrench for final installation.

Cross Reference Numbers

• NGK BR6FS
• NGK 4323
• AC Delco MR43T
• BR6FS-10 (catalog reference format)

Parts Desk Insight

The BR6FS remains one of the most trusted replacement plugs for classic marine GM-based engines because it combines excellent resistance to fouling with proven durability in high-load boating environments. When replacing plugs, inspect distributor components, ignition wires, and cap/rotor assemblies at the same time to ensure the ignition system performs at its best.

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