It’s not often easy to find answers to commonly asked questions in the heavy industry bolting field. Most fastener houses adopt a jack-of-all-trade approach, making them a great place to buy your drywall screws and carriage bolts, but not a great place to help with your industrial fastener needs! Technical Threads is committed to being the experts in fastener distribution, and we will be continually updating this page with commonly industrial fastener questions & answers.

I don’t understand why Class II is a big deal for B8 and B8M fasteners? Why are these specified in the piping flanges I’m dealing with?
B8 and B8M fasteners are the stainless cousins for B7 carbon (alloy steel) fasteners. These fall under the ASTM A193 spec and the ASME SA193 spec. In short, the stainless B8 and B8M fasteners (typically studs, but also commonly heavy hex bolts, and less commonly other styles) require more torquing than B7s. Class II fasteners are strain-hardened, so they can successfully seal the gasket easier and better than standard B8 and B8M.

I ordered bolts/studs for my butterfly valve, and found out that the bolts don’t screw on! Is my valve tapped wrong?
In short, no. This is probably our most common question. In the butterfly valve world, “commerical” resilient-seated valves are commonly UNC above 1″ diameter bolts, while “industrial” high-performance butterfly valves are usually UN8. Typically, the high performance butterfly valves are either UN8 ASTM 193 B7 studs or bolts for carbon systems, and ASTM A193 B8/B8M studs or bolts for high performance systems.
This same issue is common for knife gate valves, where manufacturers use a mix of thread pitches.
Please contact us for assistance with this issue– we can help!

When I need stainless steel for my piping system, can I just ask for it by name?
This is a common question we get, but it very complex! Many factors dictate what type of stainless grade should be ordered, including application, pressure, temperature, environment, etc. Most of the time we get the question, we direct people to either the ASTM F593 or ASTM/ASME A193/SA193 (specifically B8/B8M) specs as a starting point.

What is the difference between A307 and Grade 2 fasteners?
Both A307 and Grade 2 are commonly referred to as “low carbon” fasteners. A307 is an ASTM spec, while Grade 2 is an SAE spec (SAE J429). Functionally, A307 is typically supplied as a hex bolt, while Grade 2 is supplied is as hex cap screw (see below question to address the difference between the two). The overall strength ranges of A307 and Grade 2 overlap significantly, and they are often used interchangeably, even though they are technically different. A307 bolts are marked as such on the head, while Grade 2 screws are only required to display the mill head marking. In short, ASTM A307 is an “industrial” spec and SAE Grade 2 is an “automotive” spec.

I need a heavy hex bolt in carbon steel. What do I need?
The piping world uses two specs commonly for heavy hex bolts. ASTM A307 Grade B is common for hvac/waterworks/low pressure systems. ASTM A193 B7 is common for general industrial use and high temperature/pressure.