To reset the combination on a Master Lock model 643 lock, open the lock, turn the shackle one-quarter turn, and set the new combination while holding the shackle down. Release the shackle, align it with the lock opening, press it into place, and scramble the dial.

The instructions for resetting Master Lock locks vary by model number. Some require the use of a special lock reset tool that is included in the package with the lock. Resetting the lock requires the owner to know the current combination. Owners cannot reset any of these locks unless they are open.

A reset tool is needed for padlocks model numbers 175, 176, 177 and 178. With these models, the current combination must be known. "Set Your Own" locks have a preset combination depending on which model is owned. P-L-A-Y is the combination for model #643DWD.

Since the owner selects the combination, the Master Lock Corporation cannot provide help for resetting it or opening the lock if the combination is lost. The company does offer a service that allows lock owners to store their combinations online in a secure website. If the combination to a lock with a resettable combination is unknown, the company recommends replacing the lock with a new one.

Most combination locks are factory set, and users cannot reset them. The user should store the combinations to these locks in a safe place. The manufacturer has access to combinations of locks with serial numbers but requires using a special process to retrieve these locks to ensure the items they are protecting remain secure.