Good idea to try other keys. GM only had 1,333 different key codes in the early to middle 60s. But I don't think that they were really all that theft resistant. I can remember helping a person with a 1964 Vette back in the middle 60s with his keys locked inside. We tried a couple of our own GM keys - they didn't work. The second stranger that we asked to try his keys opened the door.
Starting in 1967, GM went to 4,000 new codes. 2,000 of the new codes were for ignition and door locks and 2,000 for compartment locks. The cross-sections of the two keys differ so that an ignition/door key could not be inserted in a compartment lock and vice-versa. In each of the succeeding three years, these cross-sections were varied making keys for a specific model year unique. The cross-sections were then repeated. So you actually ended up with 6,000 ignition key combinations repeated over a three year period.
Also, I believe that the ignition lock cylinder was made more theft resistant starting in 1969 when it was moved from the instrument panel and installed in the steering column.
Here is what I did...I took a 1/8" bit and drilled to the side of the key hole (the side with more space). I drilled about an inch deep and this made the lock mechanism no longer functional. This made it so I could turn the cylinder without the key - I then just turned it back a click, inserted my paper clip in the little hole and it slid right out! After cleaning out the metal shavings, my new lock cyl slid right in. Task complete.
The lock cylinders that were installed in the instrument panel (up until it was placed in the steering column in 1969) had a small hole in the face. You needed the ignition key in order to rotate the lock cylinder to ACC. You then inserted a paper clip wire into a hole in the face of the lock. It then slides right out of the dash.
Starting in 1969 the lock cylinder was moved to the steering column. The replacement procedure changed. There no longer was a hole in the face of the lock cylinder. You had to remove the steering wheel, hub, and locking plate from the steering column. You then had to pull the turn signal switch up and out of the way in order to get at a small slot in the steering column head. You could then release the lock cylinder by depressing the locking tab that engages the slot. A lot more complicated procedure but one that makes the car more theft resistant.
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