These inserts have an unthreaded lead and a patented 1-start external thread contour for simplified assembling in wood, wood-based materials and thermoplastics. Made from steel and zinc plated, the heads have collars that are about 2mm wider than the typical pre-drill hole, so they seal as they seat. They are available with M6, M8, 1/4-20 and 5/16-18 internal threads. Made in Germany.