HKS M-Series Super Fire Racing Spark Plug (50003-M40XL)


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HKS M40XL Super Fire Racing Iridium+Platinum Spark Plug

The fires and explosions in the engine of your performance vehicle, which send you speeding down the track, start with a spark. Drivers who custom tune their vehicles for higher performance need a plug that generates a strong spark without fail at the perfect time.

The HKS M40 Super Fire Racing Spark Plugs are made to keep your engine firing strongly. Each plug features an iridium alloy center electrode that reduces voltage requirements while improving ignition performance and durability. The outer electrode is equipped with a platinum chip that also helps ignition performance, and has been shortened into an ideal anti-vibration shape for more longevity. The thermal edge of the ceramic insulator helps decrease carbon deposits that can build after long idles or other conditions.

These plugs are better firestarters than Prodigy. At your next tune, discover the difference high-performance spark plugs can make for your vehicle. Our staff of gearheads is on call to help you determine if these will work for your engine.

M-Series Super Fire Spark Plug Features

  • 0.6mm Iridium Alloy Center Electrode
  • Iridium Alloy Center Electrode reduces voltage requirement as well as improving ignition performance and durability.
  • Outer Electrode with Platinum Chip
  • The Outer Electrode incorporates a square platinum chip which improves durability, and when combined with the iridium center electrode boosts ignition performance.
  • Short Type Outer Electrode (Anti-Vibration)
  • Discharges will occur in the Spark Support Gap (space between the tip of insulator and the base of the outer electrode) to help prevent carbon buildup.


  • The outer electrode has been shortened and engineered with the ideal shape to decrease weight for resistance against vibration.  This anti-vibration design improves longevity of the outer electrode.
  • Thermal Edge
  • After long idling or various combustion conditions carbon buildup can occur causing misfires. The Thermal Edge of the ceramic insulator will discharge to help decrease carbon deposits.
  • Spark Support Gap
  • M Series is recommended for reliable ignition, durability and anti-carbon buildup under high load.


  • Heat Range: NGK #8
  • Thread Size: 12 x 26.5mm
  • Wrench Size: 16mm 

What's in the box?

  • (1) HKS M40 Spark Plug

Customer Reviews

Based on 7 reviews
David Palacios
Best Spark Plugs yet

Superb quality always expected from HKS. Smoother idle and power band all throughout. Easy install just drop right in.

Anthony Psalidas

Great spark plugs noticeably smoother idle with them

Shawn Currie
Great product

Can notice smoother idle and acceleration

Great spark plugs

The plugs are hard to find, but worth it


Perfect for evo x!