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Coarse vs Fine Thread – What Thread Type Do I Need?

Fastener Thread Styles

Have you ever been stuck trying to figure out when to use a coarse thread vs fine thread fastener? Don’t worry you aren’t alone. Most people would be surprised to learn about the significant characteristic differences between the two. Each type of threading has its own benefits that the other style lacks. Read on to learn more about coarse vs fine thread fasteners and never choose the wrong thread type again.

u Screw threading is a uniform ridge that is wrapped around a cylinder (straight thread) or cone (tapered thread) in the form of a helix

u Screw threads vary in height and width, from fastener type to fastener type

u Threads set with “wide & deep ridges” are considered a coarse thread

u Threads set with “more narrow & shallow ridges” are considered a fine thread

What’s the Difference Between Coarse & Fine Threads

Coarse threaded fasteners are a great choice for most construction applications. One reason is that coarse threaded fasteners can be “roughed up” on the job site and still have the ability to work properly. “Roughed up” meaning, their threads can be lightly damaged or the presence of dirt and other obstructions can be in the threads, the same “roughing up” could render some fine threaded fasteners useless. Coarse threaded fasteners are also more difficult to strip or cross-thread because of the wider thread style, allowing for more material between the threads.

Fine threaded fasteners are a much better choice when specific measurements are required. Fine threads feature a higher tensile strength, making them ideal for strength applications. The shallow pitch (helical angle) of fine threads makes them much less susceptible to loosening from vibration. With less material to cut to form threads, the process of tapping holes for fine threaded fasteners is much easier.

Read on to compare the features and benefits of coarse and fine threaded fasteners side by side.

Coarse vs. Fine Thread Comparison

Coarse Threads

Fine Threads

· Stripping strengths are greater for the same length of engagement.

· Less likely to cross-thread.

· Quicker assembly and disassembly.

· Tap better in a brittle material.

· Larger thread allowances accommodate thicker plating, coatings and are therefore less likely to seize in corrosion-prone applications.

· Less prone to stripping when fastened into lower strength materials.

· Better fatigue resistance because of less concentration to stress at thread root radius.

· The height of each thread is greater than the corresponding fine thread so there is more material between each thread-making flank engagement greater.

· Less susceptible to being nicked or damaged, so they do not have to be “handled with care” as much as fine threads.

· Coarse threads are much less likely to experience thread galling than fine threads.

· Aerospace applications generally use coarse threads on sized 8–32 and smaller.

· Coarse threads are used when threaded into aluminum or cast iron because the finer threads tend to strip more easily in these materials.

· Are stronger when assembled with lower strength nut or tapped hole materials.

· Stronger for bolt/screw/nut sizes over one inch.

· They start more easily than fine thread, particularly in awkward positions and require less time to tighten.

· Are stronger in tapped hole materials normally used in the design and significantly increase joint clamping force.

· Allow for greater adjustment accuracy because of their smaller helix angle. Can be threaded closer to the head since thread die chamfer is a function of pitch.

· Are better for tapping thin-walled members because tapping torque is lower for short engagement lengths.

· Are stronger for fastener sizes one inch diameter and smaller, gaining strength advantage as size decreases.

· Maintains joint tightness and clamping force better due to the smaller helix angle of the thread.

· Aerospace applications generally use fine thread fasteners due to their increased strength.

· Stronger than the corresponding coarse threaded bolts of the same hardness.

· Stronger in both tension and shear due to having a slightly larger tensile stress area and minor diameter.

· They have less of a tendency to loosen under vibration due to their having a smaller helix angle than coarse threads.

· Shorter thread depth allows for threading in thin wall applications.

· Where the length of engagement is limited, fine threads provide greater strength.

· Their larger minor diameters develop higher torsional and transverse shear lengths.

· Fine threads require less torque to develop equivalent bolt preloads.

· Fine threads are more susceptible to thread galling than coarse threads.

· Fine threads need longer thread engagements and are more prone to damage (nicking) and thread fouling.

· They are less suitable for high-speed assembly since they are more likely to seize when being tightened.

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