11pc 3/8" Drive 1/2 Size Socket Set - 9.5 - 19.5mm for Undersized, Swollen or Damaged Hardware Ever grab for example a 13mm socket and it's too tight, but 14mm feels loose? And it's not SAE?. Not every bolt and nut on vehicles remain factory perfect to the millimeter over time and service life. Use 0.5mm under sockets for rounded and damaged bolts and nuts. Use 0.5mm over for swollen or corroded bolts and cap covered nuts. For hand tools and impact wrenches, use a hammer to fit 1/2 size under sockets where needed 78311