Thread Identification & Measurement Procedures


Thread Identification & Measurement Procedures

Thread Identification

  1. Introduction
  2. Some Helpful Notes About Threads
  3. Thread Measurement Procedures
  4. Section A -- For major diameters from 0.3 mm to 7.7 mm
  5. Section B (Continuation) -- For major diameters from 7.8 mm to 64 mm
  6. Section B -- Pitch conversion table
  7. Chart I-- How the 3 most popular threads are identified and Chart II -- Designation examples of various threads and Diagrams IV thru VII -- Thread angles
  8. Diagram VIII -- Thread angle 47.5 deg. vs 60 deg.
  9. Diagrams II and III -- Ruler & micrometer pix
  10. Chart IV -- Tap drill formula for metric threads
  11. Chart VIII -- How to calculate the major diameter of the metric male thread when only the female thread is known


To identify an unknown thread, certain tools and data are necessary. Included in these Charts are a helpful compilation of the data needed for the identification of most external (Male) threads from the three most popular thread systems in the world, metric standards, U.S.A.(Inch) standards, and the British (Inch) standards. Also, this Chart will outline the useful tools and procedures in attempting to identify an unknown thread.

These are the three basic steps to identify a thread:

  1. Measure the major diameter of the male thread. {The major diameter is the outside diameter (OD) of the male thread}. If you are trying to identify an internal (Female) thread these data Charts will not contain sufficient information. For additional information on measuring internal (Female) threads; see the instructions in Chart VIII.
  2. Determine the number of threads per inch {Or the number of threads per centimeter (cm)}. See Section (B) for conversions from pitch to threads per inch, and/or threads per centimeter. An ideal way to accomplish this measurement is with pitch gages or with thread gages (Note: pitch gages are quite inexpensive.). However, using a rule and taking a careful count of threads is normally acceptable.
  3. Compare your measurements with the data charts in Section (A).

Note: Class of fit - such as the U.S.A. '2A/2B' or the metric '6G/6H' are beyond the scope of the data included in these charts.

Some helpful notes about threads


    1. The metric thread designation uses pitch in place of the more familiar U.S.A. Method of threads per inch. Pitch is the distance in mm from any one point on a thread to a corresponding point on the next thread, when measured parallel to its axis.



    1. Metric coarse thread does not have to have pitch specified. The absence of the pitch specification indicates that the thread will be from the coarse thread series.



    1. The nominal diameter of metric pipe threads, both tapered and parallel is the same as the major diameter (OD) of the thread. However, the U.S.A., British and Japanese nominal pipe thread diameters correspond to the approximate inside diameter (ID) of the pipe or tube. In Section (A), all the threads are listed in ascending major diameter size order. This places the U.S.A., British and Japanese pipe threads among thread diameters which are larger than their nominal size.



    1. Most metric countries in Europe and Asia use the British based inch system (ISO) for measuring non-hydraulic/pneumatic pipe and tubing threads. Some examples of these thread designations follow:
      • Threads equivalent to BSPP/BSPF (British standard pipe - parallel) are designated in Germany as: G 1/4; in France: G 1/4 cyl; in Japan: PF 1/4; in Britain: R 1/4 tr.
      • Threads equivalent to BSPT (British standard pipe - tapered) are designated in Germany as: R 1/4 keg; in France: G 1/4 co; in Japan: PT 1/4; in Britain: R 1/4.
      • The reference standards for ISO R228 parallel pipe thread are designated as: Germany - DIN 259; Britain - BS 2779; Japan - JIS B 0202
      • The reference standards for ISO R7 tapered pipe thread are designated as: Germany - DIN 2999; Britain - BS 21; Japan - JIS B 0203



    1. In the British system there is no need to indicate the number of threads per inch. Each of the British thread series have only one number of threads per inch allocated within its series and nominal diameter.



    1. The U.S.A. Miniature thread series (unm) is interchangeable with the corresponding sizes of ISO metric threads where the nominal diameters are equal.



    1. A few additional hints to thread system identification are as follows: generally, bolt heads with numerical markings such as 8.8, 9.8, 10.9, 12.9, will probably be metric; and bolt heads with line/slash markings will probably be from the U.S.A. Grade marking system.


Measurement procedures:

    1. By using a micrometer or caliper; determine the major diameter (Actual outside diameter) of the male screw thread. The major diameter may be measured in decimal inches or millimeters. Note that the major diameter value is slightly undersized for female thread clearance, but the major diameter is never greater than the nominal size of the male thread. (see Diagram III).
    2. By using a rule or caliper, determine the number of threads per inch. (see Diagram II). Or if available, a more accurate method is to use a pitch gage or a thread gage.

      Note: to properly identify a thread, you must have both a diameter measurement and a pitch value, or thread per inch value.



    1. Go to the appropriate size range of Section (A). Check the basic major diameter columns; locate the closest measurement in either the appropriate millimeter (mm) or decimal inch column for the nominal diameter that you have measured in step one. Move across the row until you have found a match for the threads per inch or pitch measurement from step two.



  1. If there is only one thread in this row with your pitch or thread per inch value, then the thread is now identified. However, if there is more than one thread in this row, you will have to determine the angle of the thread for a final identification (See Diagram VIII below plus Diagrams IV through VII for thread angles). For example, a metric screw pitch gage will readily show the difference in thread angles between 'BA' (47 1/2 degree) threads and metric (60 degree) threads.


(Chart IV) Tap drill formula for metric 60 deg. threads

Nominal O.D. Of thread (Major diameter) - pitch = Tap drill size.
(Note: this formula will yield approximately 68% - 77% of thread.)

Example: To find the tap drill size for an m10 x 1.5 thread.
10 (O.D.) - 1.5 (pitch) = 8.5 mm (tap drill size).

(Chart VIII) How to calculate the approximate nominal major diameter (O.D.) of the metric male (External) thread when only the female (Internal) metric thread is available for measurement

  1. Measure very carefully the pitch (p) using a thread pitch gage, or any other type of measuring tool which will give an accurate reading for pitch.
  2. Multiply the factor value: (f)=1.0825 by the pitch (p) which yields result (r). F x p = r. Example: 1.0825 (f) x 1.75 (p) = 1.8944 (r)
  3. Measure very carefully the (I.D.) inside diameter (Minor diameter) of the female metric thread using an internal micrometer, internal caliper, or gage pin.
  4. Add the measured value for the inside diameter (I.D.) to the result (r) which will yield the nominal major diameter (O.D.) of the female thread. I.D. r = O.D. Example: assume measured value 10.1036 (I.D.) 1.8944 (r) = 12 (O.D. Or approximate nominal major diameter).

WARNING: We are not responsible for any technical errors or typographical errors in this publication.

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