Short answer
Thread shape alone does not settle screw versus bolt: the sources treat thread geometry and fastener product identity as different subjects. NASA also calls fasteners bolts when discussing tapped-hole engagement, so “no separate nut means screw” is not a reliable universal rule. The supplied ASME page identifies the distinction but does not publish its formal definitions. 1 2
On this page
At a glance
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| Question | Screw | Bolt |
|---|---|---|
| Separate product family? | Yes, in ASME’s description | Yes, in ASME’s description |
| Formal definition supplied? | No | No |
| Installation evidence here? | Limited | Nuts and tapped holes |
| Does thread appearance establish the name? | Not established | Not established |
These are evidence boundaries, not a complete classification standard. 1 2
What each thing is
ASME presents screws and bolts as fastener families within a broader terminology system. That system separately addresses characteristics, head styles, dimensions, and manufacturing. Accordingly, a product name and a description of its visible features answer different questions; this excerpt does not supply the criteria connecting them. 1
Key differences
The useful distinction here is between product classification and thread specification. NASA discusses thread forms through features such as root radius and internal versus external form. It also reproduces a specification naming a hexagon-head machine bolt. Those descriptions provide different kinds of information, rather than a single visual naming test. 2
How to tell them apart
Start with an applicable product specification or documented designation; use installation details as additional evidence, not an automatic renaming rule. NASA’s reproduced aerospace specification illustrates this approach. 2
Where they overlap
Both belong to the mechanical-fastener vocabulary covered by ASME. Shared descriptive categories, such as head style and dimensions, matter without necessarily deciding the family name. A particular thread form is not, by itself, enough to classify every fastener as a screw or bolt. 1 2
Edge cases
A bolt engaging a tapped hole is the most useful boundary case: it defeats a nut-only identification shortcut without proving that every such fastener is a bolt. Specialized terminology adds another complication. NASA describes threaded Hi-Lok lockbolts and generally serrated Huckbolt stems, so even bolt-related names need product context. 2
Why the distinction exists
The naming distinction supports communication about parts, not just description of their appearance. ASME states that standardized terminology is intended to reduce confusion among producers and users, and its glossary organizes product families alongside characteristics. Keeping those categories separate helps preserve what a specification actually identifies. 1
Common misconceptions
Avoid three shortcuts: that thread profile alone determines the name; that a separate nut is mandatory for anything called a bolt; and that this ASME webpage contains the operative definitions. NASA’s tapped-hole usage challenges the second claim, while ASME’s public description leaves the formal boundary unavailable here. 1 2
Examples
Example 1: A part identified there as a hexagon-head machine bolt has a documented bolt designation—not merely a recognizable head.
Example 2: In NASA’s tapped-hole discussion, the fastener remains a bolt. Applying “tapped hole equals screw” would contradict that usage. Neither example supplies a universal definition. 2