Bolts with left-hand threads are used in applications in which rotating parts may accidentally loosen the bolt and nut holding them together. For instance, left-hand pedals on bicycles must have a left-hand thread to keep them tightened when being turned.
Left-hand thread bolts are used widely on bicycles and other forms of wheeled transportation. These vehicles need bolts and nuts with opposing threads so that each remains tightly in place on each side of the equipment.
On traditional right-handed bolts, turning the bolt clockwise tightens it and turning the bolt counterclockwise loosens it. With left-handed bolts, the opposite is true. Left-hand bolts loosen when they are turned clockwise and tighten when turned counterclockwise.
The thread on a left-handed bolt is also sloped upward to the left as opposed to the thread on a right-handed bolt, which slopes to the right. This helps keep the bolt from loosening on its own. It should also be noted that there are external and internal bolts for bicycles. The left-hand external bolt thread slopes to the left, while the thread on a left-hand internal bolt slopes to the right.
There are many sizes and styles of these bolts used on vehicles. It is important to double-check the required bolt size and type before buying a replacement for a missing one.