Thread Identication Chart

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).

CHART I

CHART 2



 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.

2. 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.

3. 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.

4. 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

5. 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.

6. THE U.S.A. MINIATURE THREAD SERIES (UNM) IS INTERCHANGEABLE WITH THE CORRESPONDING SIZES OF ISO METRIC THREADS WHERE THE NOMINAL DIAMETERS ARE EQUAL.

7. 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. IT MAY BE MEASURED IN DECIMAL INCHES OR MILLIMETERS. NOTE THAT THE MAJOR DIAMETER IS OFTEN SLIGHTLY UNDERSIZED FOR CLEARANCE, BUT IS NORMALLY NEVER OVERSIZED. (SEE DIAGRAM III).

2. BY USING A RULE OR CALIPER, DETERMINE THE NUMBER OF THREADS PER INCH. (SEE DIAGRAM III). OR IF AVAILABLE, A MORE ACCURATE METHOD IS TO USE A PITCH GAGE OR A THREAD GAGE.

3. GO TO 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.

4. IF THERE IS ONLY ONE THREAD IN THIS ROW, 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 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 THREADS; NOMINAL OD 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 (OD) - 1.5 (PITCH) = 8.5 mm (TAP DRILL SIZE).

(CHART VIII): HOW TO CALCULATE THE APPROXIMATE NOMINAL MAJOR DIAMETER (OD) 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 (ID) 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 (ID) TO THE RESULT (R) WHICH WILL YIELD THE NOMINAL MAJOR DIAMETER (OD) OF THE FEMALE THREAD. ID + R = OD. Example: ASSUME MEASURED VALUE 10.1036 (ID) + 1.8944 (R) = 12 (OD OR APPROXIMATE NOMINAL MAJOR DIAMETER).

CHART III -- DEFINITIONS OF SECTION (A) COLUMN HEADINGS

1. The British thread system is fractionally based, but all thread vee angles are different than the American fractionally or decimally based threads. Most thread per inch values are different than in the American System.
2. Taper pipe thread major diameters are measured at the guage point.
3. (H)= BSPT copper pipe thread.
4. Dimensions within ( ) in the M FINE column are obsolete French or Japanese threads.
5. Dimensions within ( ) in the BSW column are obsolete British coarse threads.
6. The 6.3 mm dimension; marked GM is an obsolete General Motors thread.