Non Destructive Method for Opening the Master Thumbwheel Padlock
This method was submitted by a web site visitor. In the picture above, the object in front of the lock is one feeler from a feeler gauge, sold by auto stores and hardware stores. Feeler gauges are sets of metal strips of various thicknesses (see picture below at left.) Probably, the most common use for feeler gauges by home auto mechanics is for setting spark plug gaps. Feeler gauges also are used to set valve clearances and, on older gasoline engines and, maybe, a few modern engines, ignition point gaps.The thinnest strip in the web visitor's gauge was 0.002 inch. We bought a feeler gauge that had a 0.0015 inch strip. We found that some of the feeler gauges in the auto store did not have the very thin feelers, so check the package carefully when buying a feeler gauge and make sure the gauge has a 0.0015 or 0.002 inch feeler. If you have a choice, we recommend buying the gauge with 0.0015 inch feeler, even if it costs a dollar or two more. (We paid about $5.00 for our gauge with a 0.0015 feeler.)

The strips on the site visitor's feeler gauge and ours had rounded ends. He cut the end of the 0.002 strip with scissors, slightly beyond where the rounding started, thus leaving beveled corners. The picture below at the right shows an umodified feeler and a modified feeler. The flat end should be made as square as practical with the sides of the strip. The beveling aids in inserting the feeler into the lock. With a felt tipped marker, he put a line about 1/4 inch from the end of the feeler, to help determine how far the feeler was inserted into the lock.
The feeler is used to locate flat spots on cams that are to the right of each thumbwheel. Dialing the correct combination lines up the flat spots on four cams under a pawl, allowing the shackle to release after being pushed into the lock. The feeler finds when the flat spots are toward the bottom of the lock. Afterward, the thumbwheels are moved approximately a quarter turn to move the flat spots under the pawl and open the lock,

In the picture above, the feeler is inserted into the lock, and resting on the round part of the cam to right of the thousands digit thumbwheel. The feeler has been bowed. There is a ridge on the side of the thumbwheel, inside the lock. The feeler has been worked over the ridge to drop against the cam, resulting the bowing. In this picture, the flat edge of the cam is not lined up with the bottom of the lock. The feeler doesn't insert as far as it will when the flat edge of the cam, which we are trying to locate, is lined up.

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