FAQ and common discussions

Hex bolts Vs. Cap Screws
One of the most common requests we get is “cap screws”. In the piping fastener world, the terms is often misued and rarely understood.

MYTH- Cap screw is a fully threaded bolt (usually specified by somebody with a butterfly valve application in mind).
REALITY- The term cap screw doesn’t have anything to do with thread length! If you want a fully threaded bolt… it is called a TAP BOLT (or, “fully threaded cap screw” or “fully threaded hex bolt”).

There is a much longer technical explanation that gets into SAE and ASTM specs at a detailed level, but in short– the term CAP SCREW should not be used to refer to a fully threaded cap screw (i.e. TAP BOLT). The main difference between cap screws and hex bolts has to do with a very thin washer face usually prevalent on cap screws– it has zero to do with thread length.

**If you want to buy a fully threaded hex headed bolt/screw, order a TAP BOLT, or call out FULLY THREADED. This is the best, and correct way, to get what you want**

Thread pitch
The fastener world has a tremendous level of variety in thread pitches, both in the US and abroad. For the vast majority of piping applications, UNC is the standard through 1″ bolt size. Above 1″, UN8 fasteners (8 TPI), are the industry standard for A193/A194 material (i.e. “industrial”) and UNC is the standard for A307, A325, Grade 5, Grade 8 (i.e. “commercial” and “automotive” and “structural” applications).

Above 1″, please clarify thread pitch! This issue most often comes up when discussing valves (typically butterfly and less commonly knife gate valves).

What is the deal with A194 Grade 4 Nuts?
Grade 4 and Grade 7 nuts are a commonly supplied nut for both high and low temp service in A194 applications. Grade 4 nuts are being phased out in lieu of Grade 7. Grade 7 nuts are now the industry standard. Additionally, most nuts readily available in the market will often be marked “7L” instead of “7” to denote additional testing.

How do I measure studs?
Honestly, this could be an entire technical white paper of it’s own. In common, day to day industrial practice, most customers measure studs end-to-end. Specs calls for studs to be measured first-thread-to-first-thread (FTF), but most studs are procured (ETE).

Can you supply grade 36 rod?
This is kind of the wrong question to ask. Grade 36 is a steel spec. Low carbon steel ASTM A307 is the spec for low carbon steel typically used in industrial applications. HOWEVER, recently a change was made to ASTM F1554 specification clarifying their nomenclature for grades (F1554 is a structural/construction spec). They cover F1554 grade 36, which is a 36 ksi mininum yield strength spec. So, a better to question to ask is what spec is better for your low carbon application, A307 or F1554 Grade 36 (and if neither of those is correct, we can guide you to other specs).

Do you sell 304 or 18-8 stainless bolts?
18-8 is a generic descriptor for a variety of stainless-es with a certain chemical content, including 304. All 304 bolts are 18-8. Not all 18-8 bolts are 304.
However, again, this isn’t a great question. Both 304 and 18-8 are material descriptions. If you are looking for a bolt for a 304 flange or other industrial application, ASTM F593 and ASTM A193 B8 bolts are both 304 (and 18-8) specs that meet the material grade, which offering additional mechanical and chemical guidelines.

Do I need a heavy nut?
Honestly, we probably cannot answer this question. We can tell you that a HEAVY nut almost ALWAYS goes with a HEAVY HEX Bolt (or stud, or socket, etc). However, a HEAVY nut is also fairly commonly used with a FINISHED headed bolt. What is uncommon is a HEAVY headed bolt with a FINISHED nut.
(A heavy nut is a larger size than for the same diameter than a finished nut and uses a different wrench).

What is the difference between A307 and Grade 2?
SAE governs grade 2 and ASTM governs A307. Commodity fasteners companies often make no distinction between the two. They are NOT the same however. Grade 2 bolts don’t have a grade head mark. Also, grade 2 and A307 have different hardness and wedge tensile marks. They are similar, but not identical. Often they are manufactured from the same grades of steel.