WikiHow.com: How to Crack a “Master Lock” Combination Lock
This is kind of scary if you use Master Locks, but the instructions actually work.
I just cracked a lock that I had from high school. With no memory of the combination and following the directions, it took me about 30 minutes to crack it. The method is part mechanical, part mathematical, and part trial and error.
Cracking the lock gave me a strange sense of accomplishment, opening something that was not supposed to be able to be opened in this way. It’s not quite as sexy as cracking a safe with a stethoscope or picking a lock, but I think my dance with the dark side will end here.
The question is, should one use a combination lock like this, even though they are not completely secure. For most short term uses like a locker at a gym or at school it’s probably ok. With practice one could probably learn to open a combination lock in 10-15 minutes, but doing so is a pretty conspicuous task.