I forgot to add that the “101” in the Atlas change gear part numbers being the same as the “101” in the Sears part numbers is just a coincidence.? Until the Clausing hands began to get the upper hand around 1957, Atlas had a pretty good part numbering system.? Unlike most companies, whatever part number was first assigned to a part stayed with it as long as it was in use on something.? There are still a handful of parts on the 3996 that were first assigned back in the early 1930’s.? The system was that the first one or more numbers and/or letters identified the first machine that it was used on.? Then there was a hyphen followed by a sequence number.? 9-1, for example, was the first part number assigned for the Atlas 9” lathe in probably the Summer of 1931.? The headstock was 9-2, and so on.? 9-101 just happened to be the next number when they got to the change gears.? And instead of the second change gear being 9-102, they added another hyphen and the tooth count. Revisions to a part got a suffix letter beginning with A.? The only thing that their system doesn’t do is tell you for sure whether or not a revised part was backwards compatible.? In most cases it isn’t because a revised part that was backwards compatible kept the same part number.? But there are a few exceptions to both rules.

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Robert Downs

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