Below is a general manufacturer product group description. Please note the information below is not item specific. Item specifics are above in product details.

ALL PLUGS SOLD INDIVIDUALLY
HKS S-Series Super Fire Racing Spark Plugs
HKS S-Series Super Fire Racing Spark Plugs are high performance Iridium plugs designed to handle advanced levels of tuning and still offer easy start-up and smooth idling.

0.4mm diameter Iridium plug electrode core
U-shaped grooved electrode
S-Series Super Fire Spark Plugs are recommended for smooth cold starting, response and acceleration.

M-Series Super Fire Spark Plugs
HKS M-Series spark plugs are high performance Iridium plugs designed to provide improved ignition performance, durability and anti-carbon build-up.

0.6mm Iridium Alloy Center Electrode
Outer Electrode with Platinum Chip
Short Type Outer Electrode (Anti-Vibration)
Thermal Edge
Spark Support Gap

*M Series is recommended for reliable ignition, durability and anti-carbon buildup under high load.
The Iridium alloy is used because of its durability and reliability. On an average plug, a narrow core typically shortens life span of plug but with the intense durability of iridium there is no sacrifice in any longevity. With a high melting point of 2454 degrees C, opposed to the 1769 degrees C of white gold, the electrode can tolerate the high levels of compression and minor knocking without any deterioration. A nickel coating on the housing also prevents the spark plug from premature deterioration. The Iridium plugs are available for multiple applications and offered in “colder” heat ranges of 7 and up and specifically for rotary, heat ranges of 9 and up are offered.