HKS 50003-M40i Super Fire Racing Iridium Spark Plug ISO Type HR8 14mm x 19mm

Vehicle Fitment

• Universal Fit - Compatible with any vehicle requiring a 14mm x 19mm ISO-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 14mm x 19mm ISO-type thread before ordering
• Heat Range 8 (NGK) / Heat Range 24 (Denso) - one step colder than the M35i (HR7); recommended for moderately to heavily tuned forced induction or high-compression naturally aspirated engines producing 250+ whp
• ISO-type thread standard - do not substitute for JIS-type (50003-M40) or G-Type (50003-M40G) variants of the same heat range
• For long-reach ISO applications requiring a 26.5mm thread, see 50003-M40iL

Features

• 0.6mm Iridium Alloy Center Electrode reduces voltage requirement and delivers sharper, more consistent spark initiation under high load and advanced tuning
• 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 secures 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
• 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-M40i
• Series: Super Fire Racing M-Series
• Type: ISO
• Heat Range: 40 (HKS) / NGK #8 equivalent / Denso #24 equivalent
• Center Electrode: 0.6mm Iridium Alloy
• Outer Electrode: Platinum Square Chip
• Thread Size: 14mm x 19mm
• Wrench Size: 16mm
• Sold: Individually (per piece)

Included Components

• 1x HKS Super Fire Racing M40i Iridium Spark Plug (ISO Type)

Product Description

The HKS Super Fire Racing M40i (50003-M40i) is the ISO-type, colder heat range step-up from the M35i, carrying a Heat Range 8 (NGK equivalent) suited for moderately to heavily tuned engines. Sharing the same 14mm x 19mm ISO thread as the M35i, the M40i is the natural upgrade for builds that have outgrown a Heat Range 7 plug - including forced induction setups producing over 250 whp and track-driven naturally aspirated engines under sustained high-load conditions. Its iridium alloy center electrode and platinum-tipped outer electrode deliver reliable, low-voltage spark initiation even under the most demanding combustion environments.

The Thermal Edge insulator and Spark Support Gap design actively combat carbon fouling that can occur during variable combustion or extended idling, while the shortened anti-vibration outer electrode ensures longevity at high RPM. As with all M-Series plugs, the M40i is manufactured to NGK's top-tier iridium and platinum double performance standard. Plugs are sold individually - confirm ISO-type thread compatibility and order the correct quantity to match your engine's cylinder count.