1948 Johnson 5 HP TD-20 Outboard Specs & Part Numbers
1948 Johnson 5 HP TD-20: Technical Guide & Restoration Reference
The post-World War II era saw an explosion in recreational boating, and leading the charge was Johnson Motors with their highly reliable, beautifully engineered small outboards. Among these, the 1948 Johnson 5 HP (Model TD-20) stands out as a quintessential vintage “fishing motor.” Known for its smooth operation, lightweight design, and robust build quality, the TD-20 remains a favorite among antique outboard collectors and restorers today.
This technical guide provides comprehensive specifications, maintenance parameters, and historical context for the 1948 Johnson TD-20 to assist in restoration, operation, and maintenance.
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Overview & Specifications
The TD-20 is an alternate-firing twin-cylinder outboard. Unlike simultaneous-firing twins of the era, the cylinders fire 180 degrees apart, resulting in significantly less vibration, easier starting, and smoother idling at low trolling speeds.
| Parameter | Specification |
|---|---|
| Model | Johnson TD-20 |
| Year of Production | 1946 – 1949 (This guide focuses on the 1948 model year) |
| Horsepower Rating | 5.0 HP @ 4,000 RPM (OBC Certified) |
| Cylinders | 2 (Alternate Firing Twin) |
| Displacement | 8.84 cubic inches (144.8 cc) |
| Bore x Stroke | 1.9375″ x 1.500″ (49.2mm x 38.1mm) |
| Fuel to Oil Ratio | 16:1 (1/2 pint of oil per 1 gallon of regular gasoline) |
| Fuel Type | 87 Octane (Non-ethanol/Rec 90 preferred) with TC-W3 two-stroke oil |
| Weight | Approximately 42 lbs (19 kg) |
| Ignition System | Flywheel Magneto (Dual Points & Condensers) |
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Common Maintenance & Tune-up Specs
To keep the TD-20 running reliably, regular maintenance of the ignition system, fuel delivery, and cooling pump is required. Because this motor relies on bronze bushings rather than modern needle bearings, never run a fuel mixture leaner than 16:1.
Ignition System
- Spark Plugs: Champion J8C (historically Champion J8 or J11J).
- Spark Plug Gap: 0.030 inches (0.76 mm).
- Breaker Point Gap: 0.020 inches (0.51 mm). Ensure the point faces are clean, free of oil, and polished.
- Coil Condition: The original metal-clad or early plastic-molded coils frequently crack over time, leading to internal shorts when warm. Modern replacement coils are readily available and can be retrofitted under the flywheel.
Carburetor & Fuel System
- Carburetor Type: Float-feed carburetor with high-speed and low-speed adjustable needle valves.
- Initial Needle Settings:
- High-Speed Needle (Bottom): Start at 1-1/2 turns open from gently seated.
- Low-Speed Needle (Top): Start at 1 turn open from gently seated.
- Float Height: Parallel to the gasket surface of the carburetor body when inverted.
Cooling & Water Pump
- Impeller Type: The TD-20 features a displacement-style water pump utilizing a synthetic rubber rotor (often called a “wobble” or positive displacement pump) located on the lower unit.
- Maintenance: These vintage rotors harden and set over decades of storage. If the motor has sat unused, the rotor must be replaced to prevent overheating. Ensure the water bypass holes in the lower leg are free of debris.
- Gearcase Lubricant: Use a non-corrosive, semi-fluid gear grease such as Lubriplate 105 (or modern equivalent Outboard Gear Grease). Do not use modern high-viscosity 90W hypoid gear oil unless the seals have been fully upgraded, as the original gearcases were designed for heavier grease/oil mixtures and lack modern lip seals.
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Notable Features & History
The “Light Twin” Legacy
The Johnson TD series was introduced before World War II and resumed production immediately after. The TD-20, manufactured from 1946 to 1949, was marketed as an easy-to-carry, dependable utility motor for fishermen and campers. Its 5-horsepower output was the “sweet spot” for propelling typical wooden and aluminum rowboats of the era.
Alternate Firing Design
Prior to the widespread adoption of alternate-firing twins, twin-cylinder outboards typically fired both cylinders at the same time (opposed twins). By staggering the firing cycles by 180 degrees, Johnson engineered a motor that was incredibly smooth. This design minimized the “paint-shaker” vibration common in early outboards, making long hours of trolling comfortable.
360-Degree Steering
The TD-20 does not feature a gearshift (Neutral/Reverse). Instead, it utilizes full 360-degree pivoting steering. To go in reverse, the operator simply rotates the motor 180 degrees using the steering handle. A automatic lock prevents the motor from tilting out of the water when running in “reverse” thrust.
The “Ready-Pull” Recoil Starter
Unlike many budget outboards of the late 1940s that required wrapping a loose rope around the flywheel plate every time you wanted to start the motor, the TD-20 came equipped with an enclosed “Ready-Pull” recoil starter. This top-mounted pull-start mechanism was a premium convenience feature that set Johnson apart from its competitors.
Restoration Tips
For modern enthusiasts, the TD-20 is highly sought after because of its aesthetic appeal—featuring a polished aluminum gas tank, brass fittings, and elegant Art Deco decals. Mechanics find them gratifying to work on due to their simplicity and the wide availability of wear-and-tear parts (such as points, condensers, and carb kits) which are interchangeable with later OMC models.