10.4mm Pro Spark Plug Wires / M8

Sumax 10.4mm Pro wires with a Spiro-Pro core, precut with the boots and terminals already fitted.

A carbon core plug wire works by being resistive, and it degrades from the day it is fitted. The Spiro-Pro core replaces the carbon with a wound conductor, which Sumax rates at up to ten times the conductivity of a standard carbon core wire. Lower resistance means the wire runs cooler, and running cooler is why Sumax rates their life at three to four times a carbon core set. The silicone used in the boots and the jacket holds up to 600 degrees Fahrenheit, which is what a wire routed near a Harley-Davidson exhaust actually has to survive.

Features of the Sumax 10.4mm Pro Spark Plug Wires

• Spiro-Pro core, rated at up to 10 times the conductivity of a standard carbon core wire
• Low resistance, so the wires run cooler
• Rated to last three to four times longer than carbon core wires
• Extra high temperature silicone boots and jacket, protected to 600 degrees Fahrenheit
• Precut, with plug and coil boots and terminals already installed
• 10.4mm diameter, offered in black, red and blue
• Made in the U.S.A.

Fitment

• 2018-2021 Harley-Davidson Softail, Milwaukee-Eight
• Cut to length for the M8 Softail. A bagger routes its wires differently, so a Softail set will not fit a touring frame

Common Questions About the Sumax Pro Wires

Do I have to cut or crimp anything?
No. They arrive precut with the boots and terminals installed. It is a swap, not a build.

Will low resistance cause interference?
The Spiro-Pro core is a wound conductor designed to suppress interference while carrying current, which is what separates it from a plain solid core wire.

Will I feel a difference?
On a healthy motor with good wires, no. On a motor with tired carbon core wires, yes, and it will show up as a cleaner idle and no misfire under load in the damp.