Thermo-Tec 14260 Silver Cool-It Plug Wire Sleeve, 4 Pack The double-wall construction keeps the wire and boot cooler in a high-temperature engine Its universal size fits most boots, is easy to install and most important is it is non-flammable Cool-It Plug Wire Sleeves can be used in any high-temperature environment from street vehicles to motor homes The Sleeves can withstand 750 degrees Fahrenheit of conductive heat; the material is non-flammable and fits virtually all plug boots Diameters starting at the bottom: 9/16 inch and flares out to 1-1/4 inch, 6 inch in length