Quick Answer

Choose a partially threaded hex head cap screw when you need maximum clamping force without thread contact in the joint. Match the unthreaded body length to the total material thickness and select a grade that meets your strength and corrosion needs.

When Partial Threading Makes Sense

A partially threaded hex head cap screw has a smooth shank between the head and the threaded portion. This unthreaded body allows the screw to act like a dowel pin, keeping parts aligned and preventing side-to-side movement. It also distributes clamping force more evenly because the threads do not bear against the hole walls.

Fully threaded screws are better when you need maximum thread engagement, such as when fastening into tapped holes or when the joint is thin. But for thick assemblies where you are bolting through multiple plates, a partially threaded screw often gives a stronger, more stable connection.

Length, Thread Engagement, and Grade Trade-offs

The key measurement is the unthreaded body length. It should be at least as long as the total thickness of the materials being clamped. If the body is too short, the threads will sit in the hole, reducing shear strength and increasing wear.

Thread engagement is also critical. You need enough threads to develop full strength, typically at least one diameter of engagement in steel. A 300mm screw with a partial thread gives you a long body for thick stacks, while an 85mm fully threaded screw suits thinner joints.

Material choice affects corrosion resistance and strength. 316 stainless steel offers superior corrosion resistance for marine or chemical environments, while 18/8 (304) is a common, cost-effective general-purpose option. Both are non-magnetic in the annealed condition, but 316 is more resistant to pitting and chloride attack.

Step-by-Step Selection Process

  1. Measure the total material thickness: Use calipers to measure the combined thickness of all parts the screw will pass through. This tells you the minimum unthreaded body length required. If the body is shorter than the stack, choose a longer screw or a fully threaded option.
  2. Decide between partial and full threading: Choose partially threaded when the joint is thick and you need alignment and shear strength. Choose fully threaded when you are threading into a tapped hole or the material is thin. When in doubt, a fully threaded screw is more versatile for short engagements.
  3. Verify thread size and pitch: Confirm the thread size (e.g., M12) and pitch (1.75mm) match your nut or tapped hole. Using the wrong pitch will damage threads and weaken the joint. Always test-fit with a nut before final assembly.
  4. Select the material and grade: Choose 316 stainless steel for corrosive or marine environments. Choose 18/8 (304) for general indoor use where cost is a factor. For high-strength applications, consider a higher-grade alloy; check the grade marking on the head.
  5. Check thread engagement length: Ensure at least one full diameter of thread engagement (e.g., 12mm for an M12 screw) is achieved after clamping. This prevents stripping under load. If the threaded portion is too short, select a longer screw or a fully threaded version.
  6. Confirm the drive type and tooling: Hex head cap screws require a wrench or socket. Verify you have the correct size to avoid rounding the head during installation. Use a six-point socket for better grip and less chance of damage.

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