Intro to Millwright Practice Exam

If a threaded fastener is designated as 1/4" -20 UNC-1A X 3/4", what does "1A" indicate?

1. Class 1 Fit External Threads

2. Class 2 Fit Internal Threads

3. Class 3 Fit External Threads

4. Class 1 Fit Internal Threads

The correct answer is: Class 1 Fit External Threads

The designation "1A" in the fastener specification indicates that it is a Class 1 Fit for external threads. In the Unified Thread Standard, the designation system conveys both the size and the type of fit of the threads. Class 1 Fit external threads provide a looser tolerance, which allows for easier engagement and disengagement during assembly. This is particularly valuable in applications where quick adjustments or installations might be necessary. The "A" denotes that the fit is for external threads, as opposed to the "B" designation, which would indicate internal threads. Given this context, while other classes exist, such as Class 2 and Class 3 fits, those would indicate different tolerances that are not applicable in this specific scenario. Class 3, for instance, is tighter and used where precise alignment is critical, whereas Class 2 is a general-purpose fit that balances ease of assembly with a secure connection. However, since the fastener specified is in the Class 1 Fit for external threads category, the reference to "1A" clearly identifies it as such.