HOW TO MEASURE SCREWS LENGTHS

The general rules of thumb used to measure the length of a screw are as follows:

  • COUNTERSUNK FLAT HEADS - Measure from the top of the head to the end of the shank (threads).
  • OVAL COUNTERSUNK HEADS - Measure from the top rim of the countersink to the end of the shank.
  • ALL OTHER HEADS STYLES - Measure from under the head to the end if the shank (threads).

This length is express in 1/16 inch increments. Occasionally, it may be necessary to measure in 1/32 inch increments for closer tolerance work.

NOTE: For commercial applications, the angle of countersink under the head is 82 degrees as opposed to aerospace applications which often refer to 100 degree flat heads.

HOW TO SPECIFY A SCREW

The following is an example of a properly specified screw:


LENGTH TOLERANCE OF CLASS 2A COMMERCIAL
AND AEROSPACE MACHINE SCREWS
Nominal Screw Length #0000-#00 Diameter #0-#12 Diameter 1/4" to 3/4" Diameter
Up to including 1/2" -0.01 -0.02 -0.03
Over 1/2:" to 1" -0.02 -0.03 -0.03
Over 1" to 2"   -0.06 -0.06
Over 2"   -0.09 -0.09
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