Products - Wrenches Index - Trox L-Key

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Torx head screws resist cam-out better than Phillips head or slot head screws. Where Phillips heads were designed to cause the driver to cam out, to prevent over-tightening, Torx heads were designed to prevent cam-out. The reason for this was the development of better torque-limiting automatic screwdrivers for use in factories. Rather than rely on the tool slipping out of the screw head when a torque level is reached, thereby risking damage to the driver tip, screw head and/or work piece, the driver design achieves a desired torque consistently. The manufacturer claims this can increase tool bit life by ten times or more. 
Sizing

Torx head sizes are described using the capital letter "T", followed by a number. A smaller number corresponds to a smaller point-to-point dimension of the screw head. Common sizes include T10, T15 and T25, although they reach as high as T100.[5] Only the proper driver can drive a specific head size without risk of damaging the driver or screw. The same series of Torx drivers is used to drive SAE, metric and other thread system fasteners, reducing the number of bit sizes required.

The "external" variant of Torx head sizes (see below) are described using the capital letter "T" , followed by a number.

Properties of various Torx drives[6]
Size Inch point to point [in] Metric point to point [mm] Maximum torque range [ft-lb] Maximum torque range [N-m]
T1 .031 .81 .01 to .02 .02 to .03
T2 .036 .93 .05 to .07 .07 to .09
T3 .046 1.10 .10 to .13 .14 to .18
T4 .050 1.28 .16 to .21 .22 to .28
T5 .055 1.42 .32 to .38 .43 to .51
T6 .066 1.70 .55 to .66 .75 to .90
T7 .078 1.99 1.0 to 1.3 1.4 to 1.7
T8 .090 2.31 1.6 to 1.9 2.2 to 2.6
T9 .098 2.50 2.1 to 2.5 2.8 to 3.4
T10 .107 2.74 2.7 to 3.3 3.7 to 4.5
T15 .128 3.27 4.7 to 5.7 6.4 to 7.7
T20 .151 3.86 7.74 to 9.37 10.5 to 12.7
T25 .173 4.43 11.7 to 14.0 15.9 to 19
T27 .195 4.99 16.6 to 19.8 22.5 to 26.9
T30 .216 5.52 22.9 to 27.6 31.1 to 37.4
T40 .260 6.65 39.9 to 48.0 54.1 to 65.1
T45 .305 7.74 63.4 to 76.1 86 to 103.2
T50 .349 8.86 97.4 to 117 132 to 158
T55 .444 11.21 161 to 189 218 to 256
T60 .519 13.25 280 to 328 379 to 445
T70 .610 15.51 465 to 516 630 to 700
T80 .690 17.54 696 to 773 943 to 1048
T90 .784 19.92 984 to 1094 1334 to 1483
T100 .871 22.13 1359 to 1511 1843 to 2048