Master Lock No 175

= Master Lock No 175 =

Master Lock No 175 is a resettable combination padlock made by Master Lock. It has four dials with the digits 0-9. This results in total of 10,000 theoretical permutations.

The Master Lock No 175 series includes solid brass bodied locks. Master now makes the same lock in steel as the M176 and a model of M175 that has a different style of internals. The locks described below take a change key, the M175 combination is designed to be changed by specific manipulation of the shackle(like the Master Lock No 975)

Operating Instructions

 * 1) Move each of the four dials to the correct digit
 * 2) Push down slightly on the shackle and release
 * 3) If the combination is correct the shackle will be free

Internals
An actuator that sits between the pawls and has fingers which rest on wheels. The wheels lock in to the dials the user interacts with. Each wheel needs to have the flat of the wheel facing the front of the lock, 5 steps off of the center of the flat is a will that allows the wheel to pass over part of the body during the combination change. When all the flats are facing the front of the lock the fingers can go down and allow the top of the actuator to move out from between the pawls.

The are variations with pin tumbler core on the side to allow supervisor access. The core has a long half cylinder as the tail which lifts the actuator when the tumbler is turned.

Vulnerabilities
The Master Lock No 175 series may be vulnerable to one or more of the following:


 * Decoding - The combination can discovered by using a piece of feeler gauge or other thin metal to feel for the flat on the wheel. Test at .5 of every number. The flat should be positioned -2.5 from the center of the flat.
 * Bypass - Multiple tools exist to bypass the inputting the correct combination. The are inserted next to the wheels, pushed toward the top of the lock, and levered upward to lift the actuator. Then operate the shackle normally.  In earlier versions(and possibly still, but more difficult) a hook pick could also be inserted to push the fingers of the actuator down.

Gallery
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