These guide blocks are designed to help the user maintain proper alignment of a tap into a pilot hole that is to be manually threaded. Trying to visually estimate perpendicularity can be very difficult. These blocks eliminate the guess work for a perfectly threaded hole. They are machined from 1018 steel and come in both standard and metric sizes. These fit all taps up to 1/2" / 12mm. Contact Us if you are interested in ordering.