Piping system require either bolts or studs, along with nuts, to join the flanges together.

Technical Threads supplies all of the fasteners & gaskets necessary to make sure the flange seals correctly.

Some basic information about piping system fasteners is below. Please contact us with any questions you might have.

    A307 grade A Hex Bolts

This is the standard grade of fastener for commerical HVAC and lighter duty piping flanges. A307 bolts are usually supplied with zinc finish (also referred to as blue zinc or clear zinc, to distinguish from the yellow zinc commonly applied on SAE-spec fasteners), but are also common in plain finish or hot dip galvanized (HDG). Nuts are typically grade A/2 finished hex nuts, or A563 heavy hex nuts. These bolting systems are referred to as carbon, or low carbon, steel. A307 grade A systems are usually UNC systems, meaning in larger size bolts (over 1″) their use is limited, as most piping fasteners are UN8 over 1″.

    A307 grade B Heavy Hex Bolts

The big brother of A307 grade A, grade B A307 bolts have a larger, heavier head (requiring a different size wrench), and have a generally higher upper range in tensile strength. These bolting systems are referred to as carbon, or low carbon, steel. They are most commonly supplied in plain finish, but are also available in hot dip galvanized. Zinc plating on A307 B is a newer product that has begun to gain acceptance in the piping industry. Typically A563 heavy hex nuts, and occasionally A194 2H heavy hex nuts are supplied with A307 B. Like A307 grade A, A307 B fasteners are UNC. This specification is very common in piping flanges in water and wastewater plants.

    A193 B7 Heavy Hex Bolts

A193 B7 is the other type of heavy hex bolt typically found in piping systems. The A193 spec is typically found in heavy industry, larger piping systems where bolts are required rather than studs, and in when alloy steel is preferred over low carbon steel. Plain finish is the most common type of finish, but cad plated and Xylan® coatings are frequently provided on carbon systems where corrosion is still a concern. A194 2H heavy hex nuts are the nut of choice.

    18-8/304 Hex Bolt

18-8 is one of the most common type of stainless steel bolts in the commerical piping industry. The 18-8 refers to 18% chromium and 8% nickel found in the chemical composition. Several grades of stainless steel are composed of 18-8, including 302, 303, 304 and 305. However, in layman’s terms, for the vast majority of applications 18-8 and 304 are used interchangeably in the piping industry at large. Typically they are paired with an 18-8 finished hex nut. One important note regarding 18-8 is that ASTM does not recognize “18-8” as a “spec”. Instead, please see our information on ASTM F593 here .

    316 Hex Bolt

316 is the second most common type of stainless steel for fasteners in the piping industry. It’s increased resistance to chlorides makes it popular for salt, brine and conditions that have more chlorides. The bolts are paired with 316 nuts. One important note regarding 316: ASTM recognizes the spec F593 here, rather than calling out “316” as a “spec”.


F593 (hex bolts, hex cap screws and studs) and F594 (nuts) are stainless steel ASTM specs that place most stringent requirements on many common stainless grades, including 304 and 316. For more information, please click here.

    B8 Hex Bolt

B8 is the heavy hex version of 304 stainless. It falls under the ASTM A193 specification. The corresponding nut is Grade 8. Also, the SA193 ASME specification covers B8. For more information, please click here .

    B8M Hex Bolt

B8M is the heavy hex version of 316 stainless. It falls under the ASTM A194 specification. The corresponding nut is a grade 8M. Also, the SA194 ASME specification covers B8M. For more information, please click here .

    A193 studs

Studs are the most common type of piping fastener for heavy process industry. ASTM A193 is the most common governing specification for studs. B7 alloy steel is the single most common stud or bolt from ASTM A193, although many other grades exist. Also known as continuous rod or fully threaded rod, they are unique from the headed product in that they use a nut at each end. Similar to A193 B7 bolts, studs are most often provided plain, with Xylan® gaining in popularity. The A193 spec also covers a variety of other materials, which are detailed in the link below. They are paired with A194 heavy hex nuts.

Washers, which rarely provided with piping system fasteners, are more common on studs than headed bolts. F436 and USS are the most common supplied washers.

Find more information on studs, please click here.