Was wondering if anyone out there has copies of old ads or any other information on the Chicago companies that made these "Heinrici-type" air cooled hot air engines? The only literature I know about is a 3-page flyer from the Duplex Vacuum Motor Co. (sometimes incorrectly called "Chicago Duplex") from the 1920s. At least two engine designs were made, one with typical linkages outside the flywheel and the other (probably later) design with a more efficient 3-throw crankshaft with no external linkages (also similar to Heinrici's "HM" design and a much earlier Robinson patented design). Does anyone have ads or other information showing the later design (photos on right, below)? Thanks.
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