With the Wall Street Crash of October 1929 and the resulting Great Depression, Harley-Davidson had already been working on the development of a new engine to replace the aging F-Head that had served so well but just wasn’t going to be able to be competitive against the Indian and Excelsior opposition. Harley-Davidson faced some steep challenges in developing their new engine and building a motorcycle around it, challenges that were both financial and technical.
Back in 1929 the science of metallurgy was not as advanced as it would become over the coming war years in which aircraft, marine and vehicle technology were put into fast lane development. But prior to that time creating an overhead valve engine that would be affordable and reliable was not so easy to do. The design principles were understood, but the actual metal to put those design ideas into practice was not inexpensively available and it was for this initial reason that Harley-Davidson did not simply move to create an overhead valve engine immediately.