Why Harley-Davidson's Switch To V-Twins Was Harder Than Just Adding Another Cylinder

Harley-Davidson has been building and selling motorcycles for over 120 years. But the first bikes, and how William Harley and Arthur Davidson built them, are far cry from how the Motor Company does things today. The two built a tiny wooden shack in Milwaukee in 1903, no bigger than a typical one-car garage. It was in that shack that William, Arthur, and Arthur's brother, Walter Davidson, set to work building motorcycles. But they didn't start by slapping a V-Twin into a chassis. No, they started with single-cylinder bikes.

In 1909, the Harley-Davidson Motor Company took a stab at its first V-Twin. So that's it, right? The company's first two-cylinder applications were an instant hit, and the Motor Company was off to the races? 

Not quite. In fact, Harley-Davidson's first attempt at a 45-degree twin didn't work quite right. As it turns out, making the shift from single-cylinder bikes to the V-Twins that would ultimately define the brand and give Harley-Davidsons their distinctive sound wasn't easy.

Let's get one thing straight: The single-cylinder bikes weren't a flop. After all, the brand's 35-cubic-inch single carried Walter Davidson to victory in a 1908 endurance run and even netted an economy record. But it didn't take long for William Harley to realize that the one-pot engines wouldn't keep up with the consumer's demand for more power. So he concluded that adding another cylinder to the mix was the best course of action. One is good, so two must be better, right?

Here's the thing, though: Adding a cylinder presented a new problem. The new 7-horsepower V-Twin in the 1909 5-D Twin used an atmospheric intake-over-exhaust application. In the previous single-cylinder applications, atmospheric inlet valves operated by using suction created by the operation of the solitary piston. Using a retreating piston to suck a valve open had worked well with singles, but not so much with the new 5-D "F-head" twin.

The extra volume (two cylinders at 49 cubic inches compared to the single's 35 cubic inches) reduced that suction's effectiveness and kept the valves from operating properly. There was a chance Harley-Davidson could have remedied the problem, but the team went a different route. Instead of trying to change things up for the F-head bikes, the MoCo recalled every one of the 27 it was reported to have built with the early twin and destroyed them.

It didn't take long for Harley-Davidson to improve after the 5-D's design flaws. By 1911, the brand rolled out an improved Model 7-D twin. Having learned from the 5-D's finicky valve operation, the new 7-D F-head featured pushrod-operated mechanical valves, still in an inlet-over-exhaust layout. Gone was the issue with the atmospheric inlet valves. Finally, Harley had a scalable V-Twin with which to increase displacement and keep up with consumer demands for increased power output. And that's exactly what the Wisconsinites did.

In the years that followed, the motorcycle builder increased the F-head V-Twin's displacement to 61 and then 74 cubic inches. But time stops for no one, not even the company's then-successful F-head twin. By 1929, Harley-Davidson had its replacement: the Flathead. Adopting a side-valve layout and a set of characteristic flat cylinder heads, the Flathead powered the brand's products for over four decades, from 1929 to 1973. 

But the Flathead was far from alone. Over the years, Harley-Davidson would continue to work on improving the V-Twin on its way to the latest Milwaukee-Eight engines, adding features like overhead-valve construction, electronic fuel injection, and even high-horsepower, liquid-cooled applications with Porsche power. But it all started with a troubled transition from a single to a twin.