The tool I have holds multiple razor blades at the same time and heats up to make it easier to cut. They're just straight blades, not loops. It's adjustable so I can change the spacing and the depth of the blades. I'm not quite sure what to set it to. The last time I sipped the tires I did it with a single blade and I didn't make that many cuts since it was kind of a pain in the ass.
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This was how I siped them last time. I'm not sure what the depth was since I was just kind of winging it before. I'm wondering if I should have more sipes on each lug compared to this.