Machine screws and nuts are used in conjunction with one another in order to fasten metal parts together securely. To ensure proper fit and quality of life, make sure to use machine screws with nuts of similar thread size and finish. Variants included in this assortment feature a #4-40 coarse thread size. The included screws run 1/4" to 1-1/2" in length. Full assortments of #1961 contain sixteen (16) refills comprised of Hillman item numbers #1962 - #1977.