The NGK BU8H is a non-resistor surface-gap spark plug designed for selected high-performance Mercury and Mariner two-stroke outboards. Its surface-discharge electrode arrangement eliminates the conventional projecting ground electrode, helping resist fouling in demanding two-stroke marine applications. BU8H uses an M14 × 1.25 thread, 12.7 mm reach and 20.8 mm hex and is specified for numerous older Mercury V6 engines.
Marine & Outboard Spark Plugs
NGK Surface Gap Spark Plug BU8H (6431)
SKU: GB-BU8H
$10.85
Key benefits
Key Benefits
- - Surface-gap marine spark plug
- - Designed for selected two-stroke outboards
- - No conventional projecting ground electrode
- - Helps resist fouling in oil/fuel two-stroke environments
- - M14 × 1.25 thread
- - 12.7 mm / ½" reach
- - 20.8 mm / 13/16" hex
- - NGK heat range 8
- - Non-resistor design
- - Commonly used in older Mercury/Mariner V6 outboards
Specifications & Dimensions
- - Brand: NGK
- - Part Number: BU8H
- - Spark Plug Type: Surface-gap
- - Thread Diameter: 14 mm / 0.551"
- - Thread Size: M14 × 1.25
- - Thread Reach: 12.7 mm / 0.500"
- - Hex / Spanner Size: 20.8 mm / 13/16"
- - NGK Heat Range: 8
- - Resistor: No
- - Electrode Design: Surface discharge
- - Conventional Ground Electrode: No
- - NGK published spark-gap dimension: 1.5 mm / 0.059"
- - Adjustable conventional gap: No
- - Seat Type: Flat / gasket
Compatible Outboard Motors
- Mercury Outboards
- - Mercury V135 / 135 HP
- - Mercury V150 / 150 HP Carburettor
- - Mercury V150 EFI
- - Mercury XR2 150
- - Mercury XR4 150
- - Mercury XR6 150
- - Mercury Magnum II
- - Mercury Magnum III
- - Mercury V175 / 175 HP Carburettor
- - Mercury V175 EFI
- - Mercury V200 / 200 HP Carburettor
- - Mercury V200 EFI
- - Mercury V225 2.4L
- - Mercury V250 3.4L
- - Mercury V275 3.4L
- - Mercury V300 3.4L
Mercury 135 HP
- - V135 – USA serial 0A904646 & up, within the applicable production generation
Mercury 150 HP
- - V150 Carb – USA serial 4868998–0G960499
- - V150 EFI – USA serial 0D082000–0G960499
- - XR2 / XR4 / XR6 / Magnum II / Magnum III – USA serial 4868998–0G960499
- From serial 0G960500 onward, Mercury's chart changes these applications to BPZ8HS-10.
Mercury 175 HP
- - V175 Carb – USA serial 4301235–0G960499
- - V175 Carb – Australian serial 8058870 & up, applicable generation
- - V175 EFI – USA serial 0D000750–0G960499
Mercury 200 HP
- - V200 Carb – USA serial 4839034–0G960499
- - V200 Carb – Australian serial 8061595–8065672
- - V200 EFI – USA serial 0C291521–0G960499
- NGK also directly lists Mercury 200 HP, 1979–1999 as a BU8H application.
Mercury 225–300 HP
- - V225 2.4L – USA serial 5628028 & up, applicable generation
- - V250 3.4L – USA serial 0C101891 & up, applicable generation
- - V275 3.4L – USA serial 0C101891 & up, applicable generation
- - V300 3.4L – USA serial 5611648 & up, applicable generation
- These should not be confused with later 3.0L EFI/DFI/Pro XS engines using completely different plugs.
Mariner Outboards
- - Mariner 105 HP Jet, 6-cylinder – 1995–1999
- - Mariner 135 HP – 1986–1999
- - Mariner 135 Marathon – 1986–1999
- - Mariner 135 Work – 1986–1993
- - Mariner 140 HP Jet – selected production
- Additional Mercury-built Mariner V6 engines can share Mercury applications, but model/year or serial number should be checked before listing fitment.
Cross-Reference / Part Numbers
- - NGK: BU8H
- - Mercury / Quicksilver: 33-13686
- - Mercury / Quicksilver: 33-13686 2
- - Mercury: 136862
- - Mercury: 13686Q
- - Related resistor/suppressor plug: NGK BUZ8H
- - Mercury suppressor reference: 33-14103568
Signs the Spark Plug Needs Replacing
- - Difficult starting
- - Rough idle
- - Engine misfire
- - Poor acceleration
- - Reduced engine power
- - Poor high-RPM performance
- - Heavy carbon deposits
- - Oil or fuel fouling
- - Corrosion
- - Damaged ceramic insulator
- - Persistent ignition problems
Shipping
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