How to Size Taps & Dies

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Things You'll Need

  • Screw driver

  • National coarse standard thread size chart

  • Calipers

  • Tap set

  • Split ring die

  • Cutting oil

  • Measure tape or ruler

The threads on bolts are made with a die while taps make the threaded holes they go into.

Taps make threaded holes for screws and bolts. Dies make threads on rods and other devices so that they can screw into things. Sometimes the threads of bolts or their holes can get worn down and they need to be re-threaded. Tap and die sets are used to fix them.


Fitting a Tap

Step 1

Count the number of threads per inch on the screw for which you wish to make a threaded hole.

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Step 2

Measure the diameter with calipers to find the bolt size.


Step 3

Refer to the national coarse standard thread tap drill size chart to determine what size hole to drill. For instance, if the hole you are going to make is 3/8 inch, find the measurement on the left side of the chart and look across it's line to see that it has 16 threads per inch and the tap drill size is 5/16 inch.


Fitting A Die

Step 1

Back the locking screws off of the die. Place the rod or other item that you wish to thread into the center hole of the split ring die.

Step 2

Tighten the adjusting screw until it is secure and the gap of the ring closes shut. Tighten the locking screws.


Step 3

Press down on the split ring die and then start turning it while keeping the die straight. Make a half turn forward on the die, then a quarter of a turn backwards to create the start of the thread. Add lubrication, then proceed to cut the thread out of the rod.


The American National Form Thread Chart can be found online and printed for easy reference.

You can also use the rule of thumb measurement by holding the tap up against a light, and bringing the shaft of a drill bit in front of it. When you can just see the threads of the tap, but not the shaft itself, around the bit, it is close to the right size and you can use that one. Or you can fit the thread of a bolt with the thread of the tap, when they marry together perfectly, this is the tap that you need to use.

30-weight engine or gear oil can be used if you cannot find any cutting oil at the hardware store.

Match the thread of the tap to a larger-sized bolt when the bolt hole's threads have been stripped.


Make sure that the tap or die is square before starting a thread to avoid damaging the item with which you are working.


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