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Dixon Sanitary 2016
800.789.1718
Technical
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Identifying Threads
It is important to identify the threads required before ordering couplings.
Identifying threads can sometimes be the most difficult and frustrating part of coupling selection. However, without the right
combination of threads, you may not provide a functional or safe connection.
The diameters, threads per inch (TPI) and thread pitch, etc. are necessary to completely identify a thread. Ring, Plug and GO/
NOGO gauges are required to accurately gauge or identify threads. In the field, in the absence of these gauges, thread leaf
gauges can be used to identify the Threads Per Inch (TPI) and the thread pitch. On threads you have determined to be straight
threads, a caliper can be used to measure the Outside Diameter of the Male (ODM) or the Inside Diameter of the Female
(IDF). A caliper can also be used to take measurements of tapered thread diameters. However, these are more difficult to
define because of the taper. Fortunately, there are few tapered threads to deal with and these can usually be identified from the
nominal ODM and the TPI.
However, identifying the thread may not fully identify what is needed in a mating fitting. The application is the primary
limiting
factor on the thread type used.
Dixon offers products with a wide variety of threads used with hose, pipe and hydraulics.
When attempting to choose a fitting, it is always advisable to first identify the thread to which it must connect. This may entail
checking with a fitting or equipment manufacturer.
When it is not possible to identify the thread:
1) Determine the number of threads per inch by measuring
the distance from peak of thread to peak of thread across
the largest number of whole threads. Then divide the
number of threads by the measurement
(This will provide the TPI).
2) Check to see if the thread is straight or tapered or flat
thread.
a) Straight Threads
Measure the Outside Diameter of the Male (ODM)
or the Inside Diameter of the Female (IDF), from
peak of thread to peak of thread.
b) Tapered Threads
Measure the Outside Diameter of the Male (ODM)
at the large end and the small end, or the Inside
Diameter of the Female (IDF) at the large end
and the small end, from peak of thread to peak of
thread. Then measure the Outside Diameter (OD)
of the unthreaded pipe.
c) Flat (ACME) Threads
Acme threads are used for bevel seat and John
Perry fittings in sanitary installations. Sizes 1"
to 3" (tube size) have 8 threads per (TPI) and
the 4" has 6 TPI. There is not a sharp point on
this thread and it is a straight thread. The tops
of the threads are flat and there is typically a 29°
included angle between the threads. Measure
the outside diameter of the male (OD) or inside
diameter of the female (ID)
Once the application and these two pieces of information have
been determined, the thread can generally be determined.
When in doubt, contact the factory.
Straight Thread
Tapered Thread
29°
29°
ACME Thread