As we have discussed before, this old saying is one that has stuck with me from the mid-1990’s when I learned it a process piping course. Catchy phrase – easy to remember. And I still use it, although my 2023 post made it clear it may be a thing of the past. So I asked AI to explain the saying “3-threads showing,” and I was delighted to see its response:
In the world of piping and pressure vessel inspection, seeing “3-threads showing” is a common rule of thumb, though the technical requirements often allow for more flexibility than the field-standard “safety margin.”
Here is the breakdown of how this is typically handled in industrial codes:
The “Full Thread” Requirement
The most common engineering requirement (found in ASME B31.3 and ASME Section VIII) is that bolts must be “fully engaged.” This technically means the male threads should extend at least to the outer face of the nut.
- Code Minimum: The nut is considered fully engaged when the bolt is flush with the end of the nut.
- The “Safety” Margin: In practice, most inspectors and site standards require 1 to 3 threads to extend beyond the nut to ensure that the full height of the nut is utilized for load bearing.
Why “3-Threads”?
The preference for seeing a few threads beyond the nut serves several practical purposes:
- Verification of Full Engagement: It provides a visual guarantee that the bolt isn’t “short-bolted” (where the bolt ends inside the nut, reducing the stripping strength).
- Thermal Expansion: It allows for minor movement and expansion of the stud without losing engagement.
- Vibration Resistance: It ensures that even if there is minor settling, the nut remains fully seated on the load-bearing threads.
Is “Too Many” Threads a Problem?
While “not enough” is a safety violation, “too many” threads showing can also be an issue:
- Interference: Excessive thread length can interfere with nearby equipment or piping.
- Corrosion/Maintenance: Exposed threads are prone to corrosion and mechanical damage. If 10+ threads are showing, it becomes very difficult to remove that nut later without cutting the bolt.
- Wrong Stud Size: Usually, more than 5 or 6 threads showing indicates that the wrong length stud was used for that specific flange rating.
Technical Reference (ASME B31.3)
Per Paragraph 335.2.2, the code states that “bolts shall extend completely through their nuts.” It does not strictly mandate a specific number of threads beyond the nut, but it does clarify that any lack of engagement is a non-conformance.
Note: If you are working in a specific jurisdiction or under a specific QC program, check the Bolting Procedure or the Pipe Class Specification. Many owners (like ExxonMobil or Shell) have “Global Practices” that explicitly mandate a minimum of 2 threads and a maximum of 5 threads showing.
Source: Gemini Plus