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Fastener
and Thread Design Menu
ISO
Hardware Engineering Data
ANSI
Hardware Engineering Data
Mechanics / Strength of Materials Menu
Thread classes are specified to designate the
amount of tolerance allowance and installation fit desired.
Thread classes are derived from formulas which the pitch diameter
tolerances are based on increments of the major (nominal) diameter, the pitch, and
the length of engagement of the thread. These formulas and the class identification
only apply to Unified threads series. There
are three classes of external (1A, 2A, and 3A) and internal
threads (1B, 2B, and 3B). The characteristics and typical
application of each class are as follows: Classes
1A and 1B: These classes are the loosest fit, in that,
the largest amount play or clearance in assembly. These are
used for work where ease of assembly and disassembly is most
important, such as stove bolts and other general rough bolts
and nuts. Classes 2A and 2B: These
classes are specified for the typical fit grade of commercial
products, such as machine screws and fasteners, where interchangeability
and reasonable quality and fit predictability is required.
These classes are the most commonly used for commercial and
aerospace applications. Class 2AG: This
application is specified to allow for rapid assembly, and to
permit the application of a lubricant or for residual growth due to high-temperature expansion.
In these applications, the G is specified to indicate, that
when the thread is coated, the thread tolerance and size may
not be exceeded by such coating. Classes 3A
and 3B: These classes are specified for high quality
commercial, precision applications and products, such
as machine screws, where a close or snug fit is required. Thread
specification examples: 4-40 UNC-2B or
.112-40UNC - 2B 1⁄420 UNC-2A or 0.25020 UNC-2A 1032 UNF-2A
or 0.19032 UNF-2A 7⁄1620 UNRF-2A or 0.437520 UNRF-2A 212
UN-1A or 2.00012 UN-1A 1⁄420 UNC-3A-LH
or 0.25020 UNC-3A-LH (LH = Left Hand)
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