GM Keys

GM Car Keys

This Technique involves a method I found that will, in most cases, allow me to make proper working key from a GM sidebar lock form a tryout key.

In most cases, tryout keys use half depths. The GM series uses three depths; a shallow depth (S) at 1-1/2, which should operate a 1 to 2 depth wafer, a medium depth (M) at 3-1/2, which is designed to operate a 3 or 4 depth wafer, and a deep depth (D)which is cut at 6 depth, to operate a 5 depth wafer. After you have gone through the tryout keys, and found a key that will operate the lock cylinder to some degree, it is usually necessary to find a proper working key. This can be done by impression or progression. The progression sequence I describe should require the least amount of time and effort on the part of the locksmith.

After going through the tryout keys we find that one operates the cylinder. Lets say the key that operates the cylinder has a code SMMMSM. The “S” sallow cut is between a 3 and a 4 depth. Using the knowledge the sum of the cuts in particular key must equal an even number, we can write a list of all the possible combinations this tryout key should operate. I find 15 possible keys for the SMMMSM tryout combination. They are listed in table five. Duplicate the tryout key over a blank, and make sure it operates the lock. Studying the table, we find that the first depth of the key can be a 1 or a 2. There is always a cut position whose lower correct cut position seems to dominate the chart. In this case it is position 1. The tryout key is cut to a 1-1/2 depth in the first position, and still operates the cylinder. Cut the first position of the duplicated tryout key to a 2depth, and try it in the cylinder again. If it does not operate the cylinder, you will have to eliminate 10 of the possible cut configurations of the chart, and will be left with only five possibilities. If the first cut is a 1 depth, the second cut can be no deeper than 3 depth, because the “maximum adjacent cut difference “ can be no more than two depths. Make another tryout key using a 1 depth in the first position and a 3depth in the second position. The remaining positions are duplicated from the tryout key. Try the key in the cylinder to ensure that it operates. Study the remaining unknown positions of the chart. Position five dominates the remaining combinations. Four of the five combinations have a possible 2 depth in this position. Cut position five o a 2 depth and try the key in the cylinder. If the key does not operate the cylinder, the true combination is found. It is the only combination with a 1 depth in this position (combination number 1 of the table is the only remaining combination with a 1 depth in position five.). If the key does operate the cylinder, the true combination will be one of the four remaining combinations in the chart with a 2 depth in position five (These are combinations 2, 3, 4 and 5.). Move on to another position with the same key. Always take the lower depth. In no time you will have a working key.

By studying and understanding the two basic rules that apply to all GM codes (cut configurations), we can eliminate a substantial number of possible combinations, and simplify the key making process. I hope this Tip helps fellow locksmiths that are using GM tryout keys.