A double acting gas engine fires both sides of the piston with a crosshead and a sealed round rod to keep the compression from escaping. They were made with bores as large 64" (see above engraving) and used mostly for generating, natural gas pumping and blowing engines in the steel mills.

A 1912 Snow 450 HP Tandem Double Acting Horizontal Engine became available to the Flywheelers for only the cost of hauling and setting it up. Its overall length is 60 feet and it weighs 100 tons. The 12 foot flywheel weighs 12 tons and the main crankcase weighs 20 tons. There are 4 cylinders with a 20 inch bore and a 36 inch stroke with 4-cycle firing on both sides of each piston. The displacement is over 40,000 cubic inches and it will run at 120 RPM.