A mechanic is reviewing a technical parts manual to replace damaged hardware on a heavy eq… — Armed Services Vocational Aptitude Battery (ASVAB)

A mechanic is reviewing a technical parts manual to replace damaged hardware on a heavy equipment mounting plate. The specification calls for a $3/8\text{-}16\text{ UNC} \times 1\text{-}1/2\text{ in}$ hex cap screw. In this standardized Unified National fastener callout, what does the number $16$ represent?

A
The outer diameter of the bolt head in sixteenths of an inch
B
The recommended metric wrench size of $16\text{ mm}$ required for installation
The number of threads per inch along the bolt shankAnswer
D
The minimum tensile yield strength rating of $16\text{ ksi}$

Answer

The number of threads per inch along the bolt shank

In standard imperial fastener nomenclature (Unified Thread Standard), the number immediately following the nominal diameter indicates the thread pitch, defined as the number of threads per inch (TPI). In

3/8\text{-}16\text{ UNC} \times 1\text{-}1/2\text{ in}

16

specifies that there are

16

complete thread crests per inch along the shank.

Step-by-Step Solution

Analyze the standard SAE Unified National thread callout format

The standard nomenclature is formatted as: [Nominal Diameter] - [Threads Per Inch] [Thread Series] x [Fastener Length].

Deconstructing the callout reveals the distinct mechanical property defined by each segment.

Map each value from

3/8\text{-}16\text{ UNC} \times 1\text{-}1/2\text{ in}

to its corresponding dimension

3/8\text{ in}

is the nominal major diameter,

16

is the thread pitch (threads per inch),

\text{UNC}

indicates Unified National Coarse, and

1\text{-}1/2\text{ in}

is the overall length measured from under the bolt head.

The second parameter in imperial fractional fastener callouts always specifies the thread density per linear inch.

Key Concept

Standard Unified National (SAE) fastener callout nomenclature

Estimated Time:1m 0s

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