You can do it the hard way by measuring the diameter of the screw then counting how many threads the screw has per inch. But a screw thread gauge (figure A) helps take the guesswork out of picking the right nut.The gauge offers an assortment of measurements to choose from. Try flipping through the different sizes and holding onto the screw until you find the one that fits like a glove (figure B). Now just go to the hardware store and ask for a nut that will fit the screw, such as one that's 1/4-inch in diameter and has 20 threads-per-inch.
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