The Yard Machine Co. began in 1975 when it was purchased by Machine Tool and Die Co. MTD liked to purchase companies that were foundering and revive them. Yard Machine had been owned by Montgomery Ward, and the brand would have disappeared had MTD not purchased the company. Thanks to MTD, Lawn Machine was able to continue making its walk-behind mowers and branch out into riding mowers as well. The11A-41MB029's 139 cc OHV engine has specs that help explain the success of the company.
The engine of the MTD 138 CC OHV engine has an auto-choke that does not require priming. You crank it up with a pull start.
The valves of the engine arranged in an overhead configuration, hence the term OHV (overhead valve). This allows the engine to produce fewer emissions than other lawnmower engines.
The displacement of the engine is 138 cc, which also explains its name.