Prototype Harley-Davidson Servi-Car with an XA Powerplant
Here's another interesting prototype motorcycle from the World War II era: a Harley-Davidson Servi-Car fitted with the drivetrain from an Experimental Army XA motorcycle.
Instead of using the normal Servi-Car setup, Harley-Davidson replaced the standard 750cc side-valve V-twin engine and chain-drive transmission with the XA drivetrain. That meant the three-wheeled Servi-Car chassis was fitted with a 750cc side-valve opposed-twin engine and an enclosed shaft drive instead of the usual chain-drive arrangement.
The XA drivetrain borrowed heavily from the German BMW R71, the same motorcycle that influenced Harley-Davidson's wartime XA program. The opposed-cylinder engine offered better cooling because both cylinders stuck out into the air stream, while the enclosed shaft drive offered less exposed maintenance than a chain in dirty or dusty conditions. Those were the same basic advantages that made the XA attractive as a military desert motorcycle.
On paper, the idea makes sense. The Servi-Car was already a useful utility machine, and the XA powerplant offered a cooler-running engine with a more protected drivetrain. For a commercial or service machine that might spend long hours in use, that could have been a good setup. It also shows that Harley-Davidson did not simply abandon the XA engine the moment the Army motorcycle program ended. The factory was still looking for possible uses for that unusual flat-twin drivetrain.
This prototype is generally identified as the Servi-Car Model K, a postwar experiment built around 1946. Unlike the later production K-model motorcycle, this Model K was not a new lightweight sport motorcycle. It was a three-wheel Servi-Car prototype using the XA-style opposed-twin engine. That distinction is important because "Model K" can mean very different things depending on whether someone is talking about the Servi-Car prototype or the later 1952 K-model motorcycle line.
The exact production details are limited, but this appears to have been a one-off factory prototype rather than a planned production model. It did not receive the kind of documentation that the Army XA motorcycle received, and it was never offered as a civilian production Servi-Car. No confirmed surviving example is publicly known.
The original explanation often repeated with this machine is that copyright or legal issues connected to the XA drivetrain helped keep it from production. That may have been part of the story, especially given how closely the XA followed the BMW R71 layout, but the surviving public documentation is thin. It is safer to say that the XA-powered Servi-Car never advanced beyond prototype form, and that Harley-Davidson returned to its familiar 45-degree V-twin layout for production machines.
The same opposed-twin branch also reached the Willys WAC, a four-wheel prototype powered by a related Harley flat-twin engine.
The XA-powered Servi-Car remains an interesting dead end in Harley-Davidson history. It connects the wartime XA program to Harley's long-running service-machine line, and it shows how far the factory was willing to experiment with shaft drive, opposed cylinders, and nontraditional drivetrains during and immediately after World War II. Like the two-wheel-drive XS sidecar prototype, it is one of those machines that never became a production Harley, but still helps explain what the factory was exploring behind the scenes.