Textron's line of Torx-head screwdrivers are designed to be used specifically with Torx-head screws. The bits on these screwdrivers are six-point splines, the individual sizes of which are measured in a standard known as "T-sizes." If you ever come across a Torx bit that's mislabelled or doesn't have a T-size on it, you can figure out the exact T-size with a ruler and a chart from Textron.
Place the ruler across the head of the Torx bit and line it up with the tip of one of the points.
Line up the ruler so that you're measuring from the tip of the first point to the tip of the point directly opposite the first.
Examine the markings on the ruler to determine how far apart the two points are.
Compare the measurement with the "A" column of the first chart on the Wiha website in the Resource section to determine your Torx bit's exact T-size.
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