Classic Harley-Davidson Motorcycle Flathead

Few engines in Harley-Davidson history are as iconic—or as misunderstood—as the Flathead. With its exposed valves, vintage charm, and indestructible reputation, the Flathead wasn't just a stepping stone in motorcycle design. It became a symbol of American resilience, wartime ingenuity, and garage-built creativity.

This article explores everything you need to know about the Harley Flathead engine family: how it evolved, what makes it unique, which models ran it, and what oil to use if you're still running one today.

What Is a Flathead Engine?

The term "Flathead" refers to the engine's side-valve design, where the intake and exhaust valves are positioned beside the piston rather than above it (as in overhead-valve engines). This simplified layout reduced manufacturing costs and improved reliability—two critical factors during the Great Depression and World War II.

But while Flatheads were easy to maintain and durable, they lacked the power of OHV engines. This led to their gradual phase-out in favor of Knucklehead and Panhead motors. Still, Flatheads earned a cult following among classic bike lovers, restorers, and military history buffs—and they're still on the road today.

Harley-Davidson Flathead Models (1929–1973)

The Flathead wasn't a single bike. It was an engine platform used across multiple Harley models:

• 45ci Flathead (WL, WLA, WLC)

  • Years Produced: 1929–1952 (civilian); through the 1950s for military/Servi-Car
  • Displacement: 45 cubic inches (737cc)
  • Applications: WL (civilian), WLA (U.S. Army), WLC (Canadian Army), WR (racing)
  • Known For: Military use, high production numbers, simple mechanics

  The WLA/WLC became legendary during WWII. Nicknamed the “Liberator,” it was Harley's workhorse on the battlefield. Many surplus WLAs were later civilianized, chopped, and customized by early postwar riders.

• 74ci & 80ci Big Twin Flatheads (Model U, UL, UH, etc.)

  • Years Produced: 1937–1948
  • Displacement: 74ci (1,207cc) or 80ci (1,310cc)
  • Applications: U, UL (high-compression), UH (80ci variant)
  • Known For: Smooth torque, heavyweight cruisers, sidecar rigs

  The big-twin Flatheads never saw the same commercial success as the 45s, but they were popular with long-haul riders who valued smooth power and repair simplicity.

• Servi-Car (G Model)

  • Years Produced: 1932–1973
  • Displacement: 45ci Flathead
  • Known For: Three-wheel utility, police and delivery service
  • Fun Fact: It outlived all other Flathead platforms by decades.

  The Servi-Car is a true oddball classic. Built for durability, not speed, it became a favorite for small-town shops and law enforcement.

Why Riders Still Love Flatheads

Even today, Flatheads draw a passionate following. Here's why:

• Simplicity: Fewer moving parts means easier wrenching.
• Style: That distinctive low-profile cylinder look is pure vintage Americana.
• History: Owning a Flathead is like riding a rolling museum piece.
• Durability: Built to run in war zones, survive abuse, and keep going.

It's not the fastest engine Harley ever made—but it's arguably the most soulful.

Flathead Engine Specs

What Oil Should You Use in a Flathead?

Flathead Harleys use a dry clutch system with separate engine, transmission, and primary cases. That means you don't need JASO MA-certified wet-clutch oils. You're free to run high-ZDDP synthetics like AMSOIL Z-ROD or Premium Protection for better rust protection and engine longevity.

Note: Harley-Davidson Flathead models used a dry clutch system with a separate transmission and primary. Oils listed here are appropriate for that configuration. Always verify your specific setup before use.

Recommended Oil Options for Flathead Models