Until 1840 no standard screw thread existed anywhere in the world.

Sir Joseph Whitworth then made a serious attempt to rationalise the screw   thread ìformî . Over the next 20 years the Whitworth thread displaced the existing  collection of different threads in general use.

The Whitworth thread form was based on a 55o angle  between the ìveeî this form was used for the British standard threads in both large and fine pitch  threads.

Twenty years later in America a thread form called the Sellers form was   introduced later to be known as the American National Coarse [A.N.C.] and  National fine [N.F.] threads. The Sellers thread used an angle of 60o between the  ìveeî form.

In Europe at this time Metric threads were being adopted. After some    changes in the ìveeî angle the international [Metric] standard eventually evolved  as 60o.

In general engineering around the world most British Countries used the    Whitworth thread form.Some other British threads were also developed namely    British Association[B.A.] thread with a 471/2o angle and the British cycle thread [B.S.C.] In America a number of equivalent threads were also developed.

A century after Whitworth introduced his standard thread the number of standard threads had increased to to eight it became apparent that a unified thread was needed such a thread was agreed upon by Britain the United States and Canada. Unified threads came to be adopted as the standard for new productions  in a number of major industries in Britain and America. In Europe metric threads remained the common standard until 1963 when I.S.O. metric standards were agreed upon. The I.S.O. metric thread comes in coarse and fine threads.

In our hobby all these threads can be found, as a general rule British cars used Whitworth form threads until the late 1940s, American cars used the Sellers  thread and the National Standard threads and the European cars used Metric threads.

I have often been asked how does one cope with all the different thread  forms used on our cars, My answer is a good thread table. Something to  measure the number of threads per inch namely a thread gauge and a tool to    work out the outside diameter of the thread.

EXAMPLE

Working on a 1936 Dodge car Screw No 1 threads per inch is 13 and the   diameter is 1/2 inch. A check of the chart the thread is 1/2  National Coarse [N.C.] Screw No 2 threads per inch is 24 and the diameter is 3/8 inch the thread is 3.8 National fine [N.F.]

Some special examples:-

Electrical threads British cars mainly used British Association[B.A.]threads with a 471/20 included angle. The problem with this series of screw threads is the  number of threads per inch these are odd the sizes most used are ì0,2,4,6,and 8.   As the number of size increases the diameter of the thread decreases.

A rule of ìthumbî often used with B.A.threads is ì0î, B.A. is just smaller than a 1/4   of an inch [0.250îdia.] 2 B.A. just below 3/16 thread [0.1875] these slight  differences raise a number of problems with the people say a standard 1/4 thread  in an ì0îthreaded hole result in a tight fit and possible damage to some electrical device.

Damaged threads on electrical devices  can be ìfixedî by the use of  Metric threads.

EXAMPLE 1:
Damaged 2 B.A. thread.  2B.A.=0.1850î diameter / 25.4 =4.7 mm use 5 mm Metric=A.P.
2B.A. =31.4 tpi since pitch =1/tpi =1/31.4 pitch = 0.03î
Pitch in mm =0.03\25.4 = 0.8mm
Looking at table tap to use 5mm diameter and 0.8 pitch.

EXAMPLE 2:
Damaged 6B.A. thread  6B.A.=0.1102 diameter \25.4 = 2.279 use 3mm tap
6B.A. =47.9  pitch = 1/tpi  =  1/47.9 pitch = 0.02 / 25.4 =0.5 use 3mm /0.5mm pitch

Small thread damage can also be fixed using American National coarse    threads number series.The number used is the American wire size.

i.e.:-
2-56 Means gauge size.
2=0860 diameter 56 threads per inch.

Example   Damaged 2 B.A.thread
2B.A.= 0.1850 diameter 31.4 threads per inch.
Use10-32 =0.1900 diameter 32 threads per inch.

SUMMARY

British Standard Whitworth form threads 55o thread angle.
BSW = Coarse threads
BSF  = fine threads
BSB  = Brass threads 26 threads
BSP  = Pipe threads all sizes
BS Cond =Conduct pipe
BSC = British standard cycle sizes
ME   =  Model engineers
BA    = British association

AMERICAN
NC  = national coarse
NF  =  national fine
NPT = pipe thread

METRIC
ISO metric coarse and fine thread and metric specials
 

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