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A thread on a drawing is labeled 1/2-20 UNF - 1B. The thread is

  • bi-directional.
  • external.
  • internal.

Explanation from PART66Online

In a Unified thread callout the class suffix letter shows the type of thread: A denotes an external thread and B denotes an internal thread, so 1/2-20 UNF-1B is an internal thread of class 1 fit. The pitch and diameter give size, but it is the B that confirms it is internal rather than external.

M

MridulDev asked · 23 Sep 2013

how come, please explain :)

Community Comments (2)

P
PART66Online 23 Sep 2013
Letter B is for internal thread, but letter A is for external thread.
Here is very good document: aviacionulm.com/archivospdf/Drafting.pdf
M
MridulDev 24 Sep 2013
thank you :)

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