Class 1A and 1B: "They are used on
threaded components where quick and
easy assembly is necessary and where
a liberal allowance is required to
permit ready assembly, even with
slightly bruised or dirty threads."
Class 2A and 2B: "...are the most
commonly used for general
applications, including production of
bolts, screws, nuts, and similar
fasteners."
Class 3A and 3B: "...may be used if
closer tolerances are desired than
those provided by Classes 2A and 2B.
The maximum diameters of Class 3A
(external) threads and the minimum
diameters of Class 3B (internal)
threads, whether or not plated or
coated, are basic, affording no
allowance or clearance for assembly
of maximum metal components.
In my 20+ years of owning and
operating a machine shop,
I have looked at literally thousands
of prints from various customers
including military and commercial and
I have never seen a Class 1A or 1B
thread called out. It is very rare
to see a Class 3A or 3B thread called
out. I would say probably one in a
hundred. I don't work on aircraft
parts, but I suspect you may find
more Class 3 threads called out for
these parts.
My advice is to stick with Class 2
threads. They're cheap and readily
available. If you absolutely need
Class 3 go to a specialty fastener
distributor. They might be able to
find some for you if they have access
to AN series aircraft bolts.
I hope this info helps clear up some
confusion over thread Classes.
-Ken
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