Safe locks: Group 2 vs. Group 2M
Group 2 locks meet certain requirements for mechanical tolerance and reliability. But have no special safe guards in place other than that. Group 2 locks have 3 wheels and are all turned sequentially using the dial on the front of the safe lock.
After I think the 50's or so a safe lock had to have a re-locker which would hold the safe lock shut in the event that a thief (or locksmith) drilled out the safe lock to gain relatively quick access. Safe locks can have suffixes depicting resistance to particular types of attacks. For example: a Group 2 M safe lock will resist expert manipulation usually for XX number of hours. Compare a Group 2 lock to a Group 2M lock:
On the top is the coveted Sargent and Greenleaf 6700 Group 2 lock.
On the bottom, a relatively new brand "Big Red" Group 2M lock.
Both have relockers as mentioned, the metal arm on the bottom right of the lock.
The S&G lock is the same in design as the Big Red BUT....
The Big red has an extra plastic arm on a pivot with a spring connecting it to the lever of the lock. The drive cam on the Big Red has a roller which will actuate this plastic arm, causing it to "pull" the lever of the lock down into the drop in area ONLY when the drive cam is in the drop in area.
The roller is pushing the plastic arm to the left, which is lowering the lever and fence onto the wheels. In this picture the nose touching the left contact point.
The roller has pushed plastic arm, which is pulling the lever and fence down onto the wheels. In this picture the nose is touching the right contact point.
While readings can still theoretically be taken with this style, it is MUCH more difficult. The manipulator would have to know exactly how the lock is designed, and then hold enough back tension on the dial to stop the drive cam from rolling out from under the plastic arm, then measure miniscule differences in pressure, all while getting false indications from spring tension any time the lock is in the drop in area.
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