The price of heavy-duty extension cords has jumped in recent years because of rising copper prices, so rebuilding old, damaged ones with new plugs or receptacle ends ($5) is a smart move.
Cut off the old plug, then cut back the insulation jacket with a sharp razor knife (Photo 1). Don’t push the blade in—just score the rubber jacket gently until you can tear the rubber off, so you don’t accidentally cut into one of the wires.
Strip the wires (look for a stripping gauge on the plug or in the instructions), then separate the wires and screw them into place. This step can be fussy, especially with stiff 12-gauge wire, but resist the temptation to cut the insulation jacket back—the more of the jacket you can leave inside the plug, the less likely it is to tear or pull out of the plug clamp when the cord is yanked out of an outlet.
Close the plug and screw it together tightly so the cord is locked in. Our replacement plug had a reversible gasket with a curved side for heavy cords. Other types use screws to hold the cord in place.