HKS 50003-M40HL Super Fire Racing Iridium Spark Plug HL Type HR8 12mm x 26.5mm

Vehicle Fitment

• Universal Fit - Compatible with any vehicle requiring a 12mm x 26.5mm HL-type spark plug with Heat Range 8 (NGK equivalent) / Heat Range 24 (Denso equivalent)

Fitment Notes

• This is a universal spark plug sold individually - confirm your engine requires a 12mm x 26.5mm HL-type thread before ordering
• HL-Type uses a 12mm thread diameter - significantly smaller than the 14mm thread used on JIS (50003-M40), ISO (50003-M35i), and G-Type (50003-M40G) variants; do not interchange
• Long-reach 26.5mm thread length must be verified against your cylinder head specification before installation
• Wrench size is 14mm - confirm socket availability before installation, as this differs from the 16mm and 20.8mm used on other M-Series variants
• Heat Range 8 (NGK) / Heat Range 24 (Denso) - suited for moderately to heavily tuned forced induction or high-compression naturally aspirated engines

Features

• 0.6mm Iridium Alloy Center Electrode reduces voltage requirement and delivers consistent, high-energy spark under advanced tuning conditions
• Outer electrode incorporates a square platinum chip that improves wear resistance and boosts ignition efficiency when paired with the iridium center
• Short-type anti-vibration outer electrode is mass-reduced and shape-optimized to resist high-RPM vibration for extended service life
• Tapered outer electrode tip design maintains optimal combustion chamber space volume for improved flame kernel development
• Thermal Edge ceramic insulator actively discharges to reduce carbon deposits after prolonged idling or variable combustion events
• Anti-carbon build-up construction maintains stable, misfire-free ignition across extended service intervals
• High iridium melting point of 2,454°C tolerates elevated compression and minor detonation without electrode degradation
• Nickel-coated housing resists corrosion and prevents premature housing deterioration

Specifications

• Part Number: 50003-M40HL
• Series: Super Fire Racing M-Series
• Type: HL-Type
• Heat Range: 40 (HKS) / NGK #8 equivalent / Denso #24 equivalent
• Center Electrode: 0.6mm Iridium Alloy
• Outer Electrode: Platinum Square Chip
• Thread Size: 12mm x 26.5mm
• Wrench Size: 14mm
• Sold: Individually (per piece)

Included Components

• 1x HKS Super Fire Racing M40HL Iridium Spark Plug (HL-Type)

Product Description

The HKS Super Fire Racing M40HL (50003-M40HL) is the HL-Type variant of HKS's M-Series iridium plug line, purpose-built for engines requiring a smaller 12mm long-reach thread. With a 12mm x 26.5mm thread specification and a compact 14mm wrench size, this plug is dimensionally distinct from every other M-Series variant and must not be substituted across types. Its Heat Range 8 (NGK equivalent) makes it well matched to moderately to heavily tuned applications, including boosted engines or high-compression builds that generate sustained elevated combustion temperatures.

Like all M-Series plugs, the M40HL combines an iridium alloy center electrode with a platinum-tipped outer electrode to deliver lower ignition voltage requirements, sharper spark initiation, and improved combustion efficiency. The Thermal Edge insulator and shortened anti-vibration outer electrode further address carbon fouling and vibration wear under demanding conditions. Plugs are sold individually - verify your engine's thread specification and order the correct quantity to match your cylinder count.