1955 Johnson 5.5 HP Sea-Horse Outboard Specs & Part Numbers
Introduction
The 1955 Johnson 5.5 HP Sea-Horse (Model CD-12) is widely regarded by outboard enthusiasts and restorers as one of the finest small outboard motors ever built. Known for its quiet operation, smooth trolling capabilities, and rugged dependability, this model helped define postwar recreational boating. This guide provides a detailed technical overview, specifications, and maintenance procedures required to keep this classic two-stroke running smoothly today.
Overview & Specifications
The CD-12 model features Johnson’s “Aero-Dynamic” styling with a signature deep-green cowing and polished aluminum accents. Mechanically, it is an alternate-firing twin-cylinder two-stroke powerhead utilizing a reed-valve induction system.
| Specification | Value / Detail |
|---|---|
| Model Number | CD-12 (1955) |
| Horsepower | 5.5 HP @ 4000 RPM |
| Cylinders | 2 (In-line, alternate firing) |
| Displacement | 8.84 cubic inches (145 cc) |
| Bore & Stroke | 2.0625″ x 1.332″ (52.4 mm x 33.8 mm) |
| Fuel/Oil Ratio | 24:1 (8 oz TC-W3 2-stroke oil per 1 gallon regular unleaded gasoline) |
| Fuel Tank Type | 2-Line pressurized “Mile-Master” (4-gallon capacity) |
| Weight | 47 lbs (21.3 kg) dry |
| Transom Height | 15 inches (Standard Shaft) |
Common Maintenance & Tune-Up Specs
Keeping a vintage CD-12 running like new requires attention to three primary systems: ignition, fuel delivery, and cooling. Below are the precise technical specifications needed for a standard tune-up.
1. Ignition System
The CD-12 utilizes the classic OMC “Universal Magneto” system located under the flywheel. This system is highly reliable but prone to cracked ignition coils over time.
- Spark Plug Type: Champion J6C (or modern equivalent NGK B6S)
- Spark Plug Gap: 0.030 inches (0.76 mm)
- Breaker Point Gap: 0.020 inches (0.51 mm) at high point of cam lobes
- Ignition Coils: Inspect for cracking in the plastic casing. If cracked, replace with modern epoxy-coated replacement coils.
2. Carburetor & Fuel System
The motor is equipped with a float-feed carburetor with manual high- and low-speed needle adjustments.
- High-Speed (High-Speed Needle): Controls fuel mixture at wide-open throttle (WOT). Initial setting: 1 turn open from gently seated.
- Low-Speed (Idle Needle): Controls fuel mixture at trolling/idle speeds. Initial setting: 1.5 turns open from gently seated.
- Fuel Mixture Note: Do not run this motor at 50:1. The internal wrist pin and main bearings are bronze bushings and require a rich 24:1 fuel-to-oil mixture to prevent premature engine failure.
3. Cooling System (Water Pump & Impeller)
The water pump is located inside the lower unit gearcase housing, driven directly off the driveshaft.
- Impeller Replacement Part Number: OEM 277181 / Sierra 18-3001
- Thermostat: This model does not feature a thermostat; it utilizes a continuous-bypass water cooling design.
- Service Interval: The impeller should be inspected annually and replaced every 2 to 3 years. Running the motor out of water even briefly will destroy the impeller blades.
4. Lower Unit (Gearcase) Lubrication
- Lube Type: OMC/Evirude Hi-Vis or standard marine 80W-90 Gear Lubricant (Non-corrosive to yellow metals/bronze bushings).
- Capacity: Fill from the bottom drain screw hole until oil flows out of the top vent hole. Approximately 5-6 oz.
- Seal Replacement: Ensure the spaghetti seal and driveshaft seal are pliable to prevent water intrusion into the gearcase.
Notable Features & History
The Fisherman’s Friend
In 1954, Johnson introduced the 5.5 HP platform as a replacement for the older 5 HP models. By 1955, the CD-12 refined this design. It quickly earned a legendary reputation among anglers due to its incredibly slow, smooth idle. This made it perfect for “backtrolling” and keeping lures at precise depths without stalling or shaking the boat.
Suspension Drive Design
One of the key engineering breakthroughs featured in the 1955 Sea-Horse was the “Suspension Drive.” Instead of bolting the powerhead directly to the transom bracket, Johnson isolated the entire engine block and exhaust housing inside a vibration-absorbing rubber-mount system. This design drastically reduced noise and vibration transmitted to the boat hull, resulting in an exceptionally quiet ride.
The “Mile-Master” Pressurized Fuel System
The CD-12 does not have a mechanical fuel pump on the engine block. Instead, it relies on a pressurized two-line fuel tank system:
- One line pumps air pressure from the engine crankcase down into the fuel tank.
- The resulting pressure in the tank pushes fuel up through the second line into the carburetor float bowl.
Safety Warning: When restoring this outboard, ensure that all hoses on the pressurized “Mile-Master” tank are rated for modern ethanol fuels. Many collectors choose to convert these motors to a single-line fuel system by installing a vacuum-driven fuel pump (such as a Mikuni or OMC fuel pump) to allow the use of standard modern fuel tanks.
Full Gear Shift
Unlike many light outboards of its era that required the operator to spin the entire motor 180 degrees to reverse, the CD-12 features a full mechanical gear shift lever offering Forward, Neutral, and Reverse (F-N-R). This addition significantly improved safety and maneuverability near docks and shallow waters.