Electro Hardware S521-M10-F21-E Circuit Board Spacer No. 4

Product Description

The ELECTRO HARDWARE S521-M10-F21-E is a round, female unthreaded circuit board spacer designed to create precise standoff clearance between a PCB and a mounting surface or adjacent board. Machined from Grade 2 steel with a zinc-plated finish per ASTM B633 and QQ-S-633 Type 1, this spacer delivers corrosion resistance suitable for electronics assembly and panel-mount applications. It accepts a No. 4 screw and measures 0.375 inches overall length, 0.25 inches outside diameter, and 0.116 inches inside diameter. Electronic assembly technicians, panel builders, and controls specialists use this hardware during circuit board installation and enclosure work.

This spacer is used in residential and commercial electronics assemblies where consistent board standoff height and secure mechanical support are required. Typical applications include HVAC control board mounting, electrical panel PCB installations, and industrial enclosure builds. The female unthreaded design allows a mating screw to pass through or seat against the spacer, providing repeatable stack height across multi-board assemblies or between a board and a chassis surface.

Key Features

• Female unthreaded round body accepts No. 4 screw for straightforward board-to-chassis mounting
• Grade 2 steel construction provides reliable mechanical strength for repeated assembly cycles
• Zinc-plated finish meets ASTM B633 and QQ-S-633 Type 1 for corrosion resistance in enclosure environments
• 0.375 inches overall length with tight length tolerance of -0.005 to 0.005 inches for consistent standoff height
• 0.116 inches inside diameter with inside diameter tolerance of 0.000 to 0.010 inches for precise screw fitment
• Compact 0.25 inches outside diameter suits high-density board layouts with limited clearance

Compatibility

Designed as an OEM-style replacement or new-install standoff spacer for circuit board assemblies requiring a No. 4 screw, female unthreaded round spacer with a 0.375 inches overall length and 0.25 inches outside diameter.

Installation Notes

Installation is performed by electronic assembly technicians or licensed controls specialists. Always de-energize the equipment and discharge any stored voltage before handling circuit boards or installing standoff hardware. Use appropriately sized No. 4 fasteners and a calibrated torque driver to avoid cracking the PCB substrate. Follow applicable enclosure and panel-wiring standards such as NEC guidelines when mounting boards inside electrical enclosures.