New American Bestfit Clock Key Size 7. This is a new American single end clock key size 7. This is used to wind clock mainsprings. The hole measures 4.00 mm. We work hard to update our descriptions and pictures of our merchandise. As part of a process of ongoing innovation by our suppliers, newer versions of the item shown may be shipped before we are able to complete an update. As a result there may be a slight variation in color and design.