| Parameter | Specification |
|---|---|
| Production Years | 1958-1964 |
| Engine Type | 74 cu in (1200cc) Panhead V-Twin |
| Oil Capacity | 3.8 L (4.0 US qt) |
| Tire Size (F/R) | 5.00-16 / 5.00-16 |
| Spark Plug | Champion No. 3 |
| Battery | 66006-29 (6-Volt) |
The 1958 Harley-Davidson Duo-Glide represents a pivotal moment in American motorcycling history: it was the first “Big Twin” from Milwaukee to feature a true rear suspension system. By replacing the rigid “Hydra-Glide” frame with a swingarm and twin coil-over-shock absorbers, Harley-Davidson transitioned from the era of the hardtail into the modern age of long-distance touring comfort.
The Panhead Legacy: Engineering Analysis
The FL-74 is powered by the iconic 74-cubic-inch Panhead engine, so named for its chrome rocker covers that resemble inverted cake pans. This overhead-valve V-twin utilized hydraulic lifters—a rarity at the time—to reduce mechanical noise and maintenance requirements. While the Duo-Glide added significant weight over its predecessor (reaching approximately 600 lbs), the improved ride quality made it the undisputed king of the highway during the late 50s and early 60s.
Maintenance Essentials
Lubrication System
The Panhead engine is a dry-sump system that requires specific viscosity based on ambient operating temperatures. At MotoSpecsLab, we emphasize using high-quality straight-weight mineral oils for these vintage units.
- Above 24°C (75°F): Use SAE 50 or SAE 60.
- 0°C to 24°C (32°F-75°F): Use SAE 40.
Total oil capacity is 3.8 Liters. Regular intervals are crucial as these engines lack modern filtration efficiency.
Tires and Chassis
The Duo-Glide utilizes a symmetric 5.00-16 tire setup. Interestingly, the factory-recommended pressures are significantly lower than modern standards, reflecting the bias-ply construction of the era.
- Front (5.00-16): 0.8 bar (12 psi)
- Rear (5.00-16): 1.2 bar (18 psi)
Operating at modern pressures (30+ psi) on original-style rims can result in a harsh ride and reduced contact patch stability.
Electrical and Ignition
The FL-74 typically utilizes a 6-volt electrical system (Battery 66006-29). For optimal combustion, the Champion No. 3 spark plug remains the historical standard. Maintaining the points-and-condenser ignition system is vital for the “potato-potato” idle characteristic that defines the Panhead’s acoustic signature.
Editorial Verdict
The Duo-Glide FL-74 is more than a motorcycle; it is a mechanical bridge between the primitive post-war machines and the refined Electra-Glides that followed. For the collector or technician, it offers a pure mechanical experience that demands respect for its specific maintenance tolerances and historical engineering quirks.