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Threaded Products and Their Specifications Explained

Posted by Keystone Threaded Products on Apr 1, 2021 4:11:38 PM


Threaded-Rod-Assortment-1Threaded products connect componentry or are utilized in any number of applications common within manufacturing and construction that require heavy loads to be lifted or moved.

There are many types of threaded products, which can make choosing the right one for your industry-specific application challenging.

To help you make an informed decision the next time you need to choose a threaded product, we have put together an overview of what you need to know about the types of products available and their specifications.

Threaded Product Types 

Unified & Metric Threaded Rods & Nuts

Unified threads are the most widely used thread form in the U.S, and their features are defined by the ANSI/ASME B1.1-1989 specification. Unified thread forms can be found on machine screws and nuts, as well as other types of fasteners. In fact, they are the preferred thread form for adjoining the components of an assembly. 

However, unified threads (and especially unified fine [UNF] and unified extra-fine [UNEF]) are not well-suited for applications where there is continual action under compressive or tensive forces. They tend to wear quickly or cross-thread in these conditions.

Defining Characteristics of Unified Threads

These threads have an included angle of 60°. A fully realized thread of this form will terminate in a point at the crest (the prominent part of the thread) and in a vee at the root (the bottom of the groove between the two flanking surfaces). In the case of UNJ threads (used primarily in aerospace technologies), the root is held at a specific radius.

Unified Thread Specifications to Pay Attention To

Unified threads are specified by: 

  • Nominal size (major diameter)
  • TPI (threads per inch)
  • Form (UN, UNC, UNF, UNEF, and UNJ)
  • Class (1, 2, and 3)
  • Exterior (A)
  • Interior (B)

When choosing a unified thread, it is most important to pay attention to its class, which determines both precision and fit. For general-use fasteners and connections, class 2 is the most common due to its cost-effectiveness and ease of production. Class 3 unified threads are preferred in high-precision applications. You should also be sure to choose the same class for both mating components. 

Metric Threaded Rods

Metric threads are exactly the same as unified threads, except they are measured using metric units instead of imperial units. The specifications for metric threads are ANSI/ASME B1.13M (U.S.) and ISO 68 (International).

Acme and Trapezoidal Threaded Rods & Nuts

Acme threads are defined by the ANSI/ASME B1.5-1988 specification and come in many types. The height of an Acme thread is half the pitch, which makes them able to sustain much higher compression and tension forces. As a result, Acme threads are much less susceptible to cross-threading and breakage and are preferred in conditions where constant actuation under a load is expected. 

Defining Characteristics of Acme Threads

Acme threads have an included angle of 29°, which makes them much more robust compared to unified or metric thread forms. However, the corners of Acme thread profiles are not typically sharp, allowing for smoother operation. With proper lubrication and rust prevention measures in place, Acme threads will last longer.


Acme Thread Specifications to Pay Attention To

These threads can be made to have either general purpose (G class) or centralizing (C class) geometries. This distinction refers to the way mating screws and nuts interact. 

A G class Acme thread wedges under a load, which means premature wear-and-tear may be seen at the point of contact if adequate lubrication is not used. To counter the wedging, C class Acme threads limit the clearance between the major diameters of the mating screw and nut. This geometry allows the flanks of each thread to act as a radial load-bearing surface. 

Acme threads are also made in stub form, where the thread height is less than half the height of the pitch. 

Trapezoidal Threaded Rods

Trapezoidal threads are defined by the ISO 2804-1977 specification. They are essentially the metric version of an Acme thread, the only difference being that trapezoidal thread forms have an included angle of 30°. 

They are suitable for the same applications as Acme threads, their imperial unit counterparts.

Specialty Profiles

At Keystone, specialty profiles are those with knurling, serrations, or grooving rolled into them. The benefit of roll forming profiles like these is the work hardening (or strengthening of the material) that occurs during the manufacturing process. 

Similar to threading, when a work-hardened material is formed into one of these profiles, the component's exterior is hardened and burnished, which increases the longevity of the affected area.

Thread Diameters 

  • Unified thread diameters range from .0625" to several inches. At Keystone, we regularly produce unified threads from .25" up to 1", although we have manufactured threads in unified form up to 10.25" in diameter.
  • Acme threads range similarly, and at Keystone, we commonly roll these threads with diameters between .25" and 6".

It is important to note that friction increases with thread size and fineness, as there is more surface-to-surface contact between the mating components in both cases. 

Thread Stock Lengths

Threaded products typically come in lengths of 3', 6', and 12'. 

When you purchase threaded products at one of these lengths they are typically at a lower cost than those that have been saw-cut or machined to be a specific length, as there is less processing required to make them. 

The process of rolling threads tends to elongate the material, so you can expect to see threads that are 1–3" longer than, say, your 12' threaded product, unless the tolerance is specified.

Threaded Rod Materials

There are many different types of materials that can be thread rolled, and each has its own unique performance properties. 

C1018 Steel (Mild/Low Carbon) 

  • Easy to form and machine
  • Low work-hardenability
  • Inexpensive
  • High modulus of elasticity 
  • Prone to oxidization
  • Excellent weldability
  • Well-suited for high-volume production
  • Ideal metal for carburization

C1045 Steel (Medium Carbon) 

  • High mechanical strength and impact properties
  • Good machinability
  • Formable
  • Affordable
  • Prone to oxidization
  • Weldable
  • Well-suited for axles, bolts, and forgings

4140 B7 Steel (Key Alloy)

  • High strength and work-hardenability
  • High fatigue strength
  • Machinable
  • Formable
  • Affordable
  • Prone to oxidization
  • Weldable
  • Well-suited for structural applications

Duplex Stainless Steel (Ferritic-Austenitic) 

  • High mechanical strength
  • Good corrosion resistance (including chlorides)
  • Weldable
  • Low coefficient of thermal expansion
  • Well-suited for chemical applications 

17-4 PH Stainless Steel (Martensitic) 

  • High mechanical strength
  • Good corrosion resistance
  • Minimal deformation from heat treatment as compared to similar metals
  • Tough
  • Good hardenability and machinability
  • Weldable in a controlled environment
  • Low fatigue
  • Well-suited for components of intricate geometry and chemical application

304 CRES Stainless Steel (Austenitic) 

  • High mechanical strength
  • Excellent formability
  • Good machinability
  • Excellent resistance to corrosion (except in chlorides)
  • Good work-hardenability
  • Weldable
  • Well-suited for food and chemical applications

316 CRES Stainless Steel (Austenitic) 

  • Excellent mechanical strength and corrosion resistance (especially to chlorides)
  • Good formability
  • Fair machinability
  • Good work-hardenability
  • Well-suited for aqueous environments at lower temperatures (continuous use at 425–860°C is not recommended)

C36000 Brass (Free Machining)

  • Good strength
  • Low hardness
  • Poor formability
  • Excellent machinability
  • Good for brazing and soldering
  • Welding not advised
  • Excellent corrosion resistance
  • Well-suited for aqueous applications and valves

C46400 Brass (Naval)

  • Good strength
  • High hardness
  • Poor formability
  • Fair machinability
  • High corrosion resistance
  • Excellent hot workability
  • Bright finish
  • Weldable (MIG or TIG)
  • Well-suited for marine fittings and industrial applications 

C93200 Bronze (Bearing) 

  • Excellent strength and wear properties
  • Fair formability and machinability
  • High corrosion resistance
  • High impact resistance
  • Low coefficient of friction
  • Well-suited for marine applications, bearing surfaces, and light-duty gears

6061 T6 Aluminum (Structural) 

  • High strength
  • Lightweight
  • Good workability
  • Good corrosion resistance
  • Good machinability
  • Weldable (TIG)
  • Brazable
  • Well-suited for structural applications, tooling, marine, and aerospace applications

Grade 2 Titanium (Pure) 

  • Lightweight
  • Good strength and wear properties
  • Fair machinability
  • Good formability
  • Excellent corrosion resistance
  • Excellent impact resistance
  • Well-suited for marine, cryogenic, medical, and aerospace applications

Inconel 625

  • Excellent strength properties
  • Good work-hardening
  • Good workability
  • Fair machinability
  • Excellent corrosion resistance (especially in cold, hot, chloride, and alkaline environments)
  • Weldable
  • Well-suited for cryogenic, elevated-temperature, marine, and chemical applications

Inconel 718

  • Excellent strength properties, including impact resistance, precipitate hardening, and weldability
  • Good workability
  • Fair machinability
  • Excellent corrosion resistance in cryogenic applications up to 1400°C
  • Well-suited for aerospace, marine, and industrial applications 

Class 40 Gray Iron (Ductile) 

  • Good strength properties
  • Excellent impact resistance
  • Weldable
  • Excellent machinability
  • Good wear properties
  • Fair corrosion resistance (with adequate rust prevention)
  • Dampening
  • Suitable for powertrain and machine components, fittings, and valves

Thread Profiles

Unified National Coarse (UNC) Threads

These thread profiles, also referred to as simply "coarse," have fewer threads per linear distance compared to other unified thread forms like fine or extra-fine. 

Coarse threads are the standard in machined bolts and general fasteners. They are less likely to gall (i.e., seize) or foul in assembly and require less thread engagement compared to fine threads.

Unified National Fine (UNF) Threads

UNF thread profiles, also referred to as simply "fine," have more threads per linear distance compared to coarse threads. UNF threads are also stronger than coarse threads due to the fact that they have a larger minor diameter and a shallower thread profile (i.e., a larger cross-sectional area). 

These threads are optimal in cases where finer adjustments are required and where larger side loads or tension forces are expected.

Unified National Extra-Fine (UNEF) Threads

These threads, also referred to as simply "extra-fine," are often found in gauges and aerospace components. They have finer adjustment and greater load-bearing capacities compared to fine or coarse threads. Their tolerances are also more restrictive. 

Extra-fine threads have a better tension-locking ability compared to both fine and coarse threads.

8-Pitch and 12-Pitch Threads

"8-Pitch" is a common way to indicate that a thread has eight threads per linear inch. "12-Pitch" similarly refers to a thread that has 12 threads per linear inch. These terms are more commonly used when referencing unified threads, but this is not always the case. 

Be thorough when specifying 8- and 12-pitch threads, as these standard thread profiles come in different sizes and thread profiles, which makes specificity important.

Full Acme Threads

Full Acme threads are fully realized threads of an Acme form. Their included angle is 29° and their height is half that of the pitch. 

Stub Acme Threads

Stub Acme threads have thread heights that are less than half the height of the pitch. These thread profiles are formed with special dies that are specifically designed for their production.


Worm-threaded screws, or "worms," are used in conjunction with worm gears. The geometry of a worm thread profile is often similar to that of a trapezoidal or Acme thread, however, exact profiles depend on the specific geometry of the application's worm gear.

Thread Class Fits

1A, 2A, and 3A

These class fit designations are for unified threads only and indicate a thread's precision and fit. Class 1 unified threads (1A) are considered "loose fit," class 2 (2A) are "medium fit," and class 3 (3A) are "tight fit."

The tighter the fit, the more precise the thread, and the less clearance there is between the major diameters of the screw and nut.

2C and 2G

2G is the general-purpose class 2 fit designation for Acme threads. The primary characteristic of a 2G Acme thread compared to a 2C (centralizing) Acme thread is the additional clearance between the mating and component major diameters. 

Notably, 2G class threads allow a tolerance of .001" per linear inch of pitch variation, whereas 2C threads allow a tolerance of .0007" per linear inch.

3C and 3G

3C is the class and fit designation of a general-purpose Acme thread with a smaller tolerance applied to its pitch diameter than that allowed with the 2C Acme thread. Similarly, 3G is an Acme thread with a smaller tolerance applied to its pitch diameter than that allowed with the 2G Acme thread.


6H is the class fit designation of an internal, medium-grade thread that has a normal length of engagement on a metric thread. 


If a thread is designed with an alteration to its geometry to avoid conflict with the geometry of a mating part, the thread will have the term "modified" associated with it. This simply means that the thread does not strictly adhere to the typical specification. The nonconforming feature of the thread will also be noted in this case.

Thread Directions

Right-hand threads engage when the screw is turned in a clockwise direction and disengages when turned counterclockwise. The opposite is true with left-hand threads.

Thread Starts

Single-Start Threads

Single-start threads only have one starting point and, correspondingly, only one helix. For each rotation of the screw or nut, the linear distance moved for either is the pitch of the thread (1/TPI). 

For example, a single-start, 4-pitch thread has a linear movement of .250"  per rotation.

Multiple-Start Threads

Multiple-start threads have multiple starting points and the same number of helices. For each start the thread has, the distance of travel is increased by that amount. 

For example, a 2-start, 4-pitch thread has a linear movement of .5" per rotation.

Thread Finishes and Surface Treatments

The finish of a rolled thread is far superior compared to that of a cut thread, as the rolling action leaves a burnished finish on all thread surfaces that have been affected by the die that produced it.

When using carbon steels that are prone to oxidization or corrosion, surface treatments like black oxide or zinc should be applied to combat premature failure. 

Other coatings, like Teflon and nitrile, can also be applied. Each of these surface treatments is application-specific and should be well understood prior to ordering a threaded rod or product.

Thread Tapers

Thread tapers are acceptable modifications in so far as the diametrical tolerances of the given thread are adhered to. Intentional, controlled tapers are primarily applicable with pipe threads. 

Thread End Connections

End connections are highly specific to the designer of the threaded rod or product and the application. 

At Keystone, we have CNC machining centers that are able to fabricate most end connections to complement the thread-rolled rods and products we manufacture.

Thread Tolerances

While precision is always important to thread functionality, it is important to note that cost is always increased when the precision is more exacting. 

It Pays to Work With the Best

As you can see, there is a lot to consider when selecting threaded rods and products, and the right choice is often application-specific. 

At Keystone Threaded Products, we have the knowledge and skills to help you find the threaded rods and products that are right for your specific needs. Since 1920, we have been manufacturing unsurpassed parts that are recognized industry-wide for their quality. 

If you have questions about any of the information in this post or would like to start a conversation about your next project, fill out the form below. We would love to hear from you

Topics: Acme Thread, Unified Thread, Trapezoidal Thread, Threaded Product Specifications, Metric Thread

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