HKS 50003-M40XL Super Fire Racing Iridium Spark Plug XL Type HR8 12mm x 26.5mm

Vehicle Fitment

• Universal Fit - Compatible with any vehicle requiring a 12mm x 26.5mm XL-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 XL-type thread before ordering
• XL-Type uses a 12mm thread diameter with a 26.5mm long-reach length and 16mm wrench - do not confuse with the HL-Type (50003-M40HL), which shares the same 12mm x 26.5mm thread but uses a 14mm wrench; these are not interchangeable
• The 12mm thread diameter is significantly smaller than the 14mm used on ISO, JIS, G-Type, and LF-Type M-Series variants - do not interchange across thread sizes
• 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 and high-load 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 conditions
• Spark Support Gap between the insulator tip and outer electrode base prevents carbon bridging and maintains reliable ignition under high load
• 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-M40XL
• Series: Super Fire Racing M-Series
• Type: XL-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: 16mm
• Sold: Individually (per piece)

Included Components

• 1x HKS Super Fire Racing M40XL Iridium Spark Plug (XL-Type)

Product Description

The HKS Super Fire Racing M40XL (50003-M40XL) is the XL-Type variant of HKS's M-Series iridium plug line, built for engines requiring a 12mm long-reach thread with a 16mm wrench size. While it shares the same 12mm x 26.5mm thread dimensions as the HL-Type (50003-M40HL), the XL-Type uses a larger 16mm wrench - making the two non-interchangeable despite identical thread specs. Its Heat Range 8 (NGK equivalent) delivers reliable, cool-running ignition performance for moderately to heavily tuned applications demanding a colder plug than stock.

Combining a 0.6mm iridium alloy center electrode with a platinum-tipped outer electrode, the M40XL ensures lower ignition voltage requirements, sharper spark delivery, and improved combustion efficiency. The Thermal Edge insulator and Spark Support Gap work together to prevent carbon fouling under both high-load and extended idle conditions, while the shortened anti-vibration outer electrode maintains durability at elevated RPM. Plugs are sold individually - verify XL-type thread compatibility, confirm the 16mm wrench requirement, and order the correct quantity to match your engine's cylinder count.