HFS(R) Hex Brass Nut 3/8 in Wrench Flats High Pressure Clamp

These nuts are versatile, designed to be installed at the end of a hexagon bolt or other precision screw, effectively holding a fastener in place when used with an unthreaded hole. [Size]: Thread size: 3/8"-16; width across flats:: 0.625"/15.875mm; nuts height: 0.472"/12mm; hex drive size: 5/8".

eBay