Allis Chalmers went bankrupt in 1988. The electrical division was purchased by Siemens USA at that time. The division was known as Siemens-Allis for several years, but was eventually merged into the Siemens brand.
The best place to start your search would be the Siemens USA motor website at http://www2.sea.siemens.com/Products/Electric-Motors/. There are options at the top right of the page for contacting customer service.
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Actually it depends upon the size of the motor you are referring to. Siemens did NOT buy the "large motor" group from Allis Chalmers, they went independent and eventually were sold off piecemeal to other companies.
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