Before hammering the terminal down onto the battery post, remove its bolt (especially if it's very difficult to remove) and spread the terminal until it can drop all the way to the BOTTOM of the post without effort. Make sure the bolt hole is clean (drill it if necessary), and apply a thin layer of electrical grease (NOT dielectric, chassis, or thermal greases, or anti-seize, or red battery-spray slime) to the entire terminal & inside the bolt holes, including any exposed wire strands. When tightening the bolt, use a tool to prevent its square head from turning against the lead terminal's stop. ONLY tighten the bolt enough to prevent the terminal from rotating on the post by hand.
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