Taps and dies are tools used to repair stripped or damaged threaded fasteners. Taps and dies are hand tools that precisely cut metal threads. Taps looks like bolts and are used to thread holes. Dies are metal disks with holes in the center that cut holes in threads, rods and pipes. While these tools are marked with numbers to let you know what size they work with, it doesn't label what the numbers represent.
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Look at the threaded fastener you are repairing. Measure the outside diameter of the thread.
Count how many threads there are per inch.
Find out if your thread is National Fine or National Coarse. These are the two most common thread categories within the American National Thread system, which nearly all threaded fasteners conform to. By looking at the American Standard thread identification chart (see Resources), you can find out if your thread is National Fine or National Coarse.
Compare the numbers and thread type you just gathered to the numbers written on the taps and dies. A tap stamped "1/4-20 N.C" means it cuts threads to measure 1/4-inch diameter fastener with 20 National Coarse threads per inch. The same is true for a die.