In the picture above, the thumbwheel has been turned and the flat edge of the cam is toward the bottom of the lock. The feeler inserts further. Note that the line on the feeler is closer to the thumbwheel edge, indicating that the feeler is inserted further into the lock.

The thumbwheel is between numbers when the flat edge of the cam is parallel with the bottom of the lock. Once you find a position where the feeler inserts deeper than at other numbers, check again with the thumbwheel half way to the next number. The half way point is at a groove between numbers. If the feeler doesn't insert as far as it did, move the thumbwheel half way to the other closest number. At one of these positions, you should find the deepest insertion, deeper than at the number. (With practice, when you insert the feeler with the thumbwheel at a number, you should be able to feel which way the thumbwheel needs to turn a half turn to make the deepest insertion.)

After you find the deepest insertion, remove the feeler, move the thumbwheel up to the number just below the groove, then 2 more numbers up to get the correct number (flat edge of the cam aligned under the pawl.) For example, if we found the flat edge of the cam when the thumbwheel was between 0 and 1, we would move the thumbwheel up to line up 0 and then two more numbers up, to 8. Then, we try the next thumbwheel. Once we found 3 numbers this way, to save time, we just tried the fourth thumbwheel in each of the ten positions, pushing the shackle in and releasing it at each position to find the position that opens the lock.

On our lock, one of the thumbwheels was harder to probe than the other three, so we got the other 3 numbers and saved the tough thumbwheel to last. We didn't probe, but tried all ten positions for the "tough" thumbwheel and one of them opened the lock.

If you are careful, you can leave the feeler inside the lock while changing the thumbwheel position. However, we found it better to remove the feeler between tries. Also, we found it quicker to test the either the even or odd numbers first, and then go back and try the remaining numbers if we hadn't found the flat cam edge.

Opening the similar Sesamee thumbwheel padlock

For cut-away views showing the cams and their flat edges, please click here. The pictures help show how the method works.

For instructions how to reset the combination without using the change tool sold with the lock, please click here.

Second hand stores: Combination change tools, which look like small keys, are available with new locks, and may be available from locksmiths. It may also be possible to have a keymaker make copies of the change tool. After opening donated locks with misplaced combinations, you may reset the combinations to the factory default of 0000. Combination change tools may be kept with the cashiers to permit customers to change the combinations before leaving the store. Instructions how to change the combination can be posted near the locks and also kept with the cashiers. Do not keep the change tools near the locks, since customers may change the combinations and create extra work for you recovering the combinations again! If you get a supply of change tools to sell, keep them with the cashiers.

Limitation of the Method: Can the Feeler Be Inserted on All Locks?

The method works because Master Lock left enough space on our locks to slip a 0.0015 or 0.002 inch feeler between the thumbwheel and the side of the thumbwheel's slot in the plate. Thumbwheels coming off the production line will vary in thickness, and so will the widths of the slots in the bottom plate. The production variance from the designed dimensions is called "tolerance." The widest thumbwheel meeting the manufacturing tolerance should fit into the narrowest slot within tolerance, with some space to spare to prevent "binding." In production, the tool which cuts the slots in the bottom plate might start out cutting wide slots and cut narrower and narrower slots as the cutting head wears until it is time to replace the cutting head.

The bottom line: When the widest production thumbwheel and the narrowest slot are found on a lock, it may not be possible to insert a 0.0015 inch feeler through the gap, but we think that's very unlikely. We recommended the 0.0015 inch feeler over the 0.002 feeler to give ourselves extra margin for tight fits.

Commercially Available Tools for Nondestructive Opening of Thumbwheel Combination Locks

Peterson Miniknife claims to be capable of opening the Master 175 as well as Sesamee and other thumbwheel locks. Search for "peterson miniknife" on the internet to find a web page with pictures of the tool being used on several locks, as well as descriptions of how to use the tool. As shown, the tool opens the lock, but does not recover the combination. The Sesamee decoder appears on the internet without a description of how to use it. It appears to work similarly to the automotive feeler gauge.

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Description of How the Lock Works (including pictures)

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Non Destructive Method for Opening Model 175 (concluded)