The term "bolt," amongst climbers, usually refers to an expansion bolt and a hanger that a climber clips a draw into for protection.
Bolts are typically placed at the top of climbs, in areas where there is inadequate natural protection
to use clean protection,
in spots where a hanging belay
is necessary, and for rebelays.
When placing bolts, a hole is drilled into a solid section of rock. The diameter of the hole is equivalent to the diameter of the expansion bolt. The hole is then cleaned out by blowing air into the hole.
The expansion bolt is placed into the hole and the hanger is placed on the end of the expansion bolt. By tightening the expansion bolt with a wrench the expansion bolt widens, creating a permanent placement rated at more than 5000 pounds (presuming the rock is stable). When placed correctly, they are often the safest protection
available, but are used as little as possible because they deface the rock permanently.Full Article