1956 Mercury 20 HP Mark 25 Outboard Specs & Part Numbers
1956 Mercury 20 HP Mark 25: Comprehensive Technical Guide
The 1956 Kiekhaefer Mercury Mark 25 is an iconic representation of mid-century American marine engineering. Rated at 20 horsepower, this inline two-cylinder outboard motor was prized for its high power-to-weight ratio, exceptional reliability, and aesthetic appeal. Today, it remains a favorite among vintage outboard collectors and restorers. This guide provides a detailed technical overview, vital tune-up specifications, and historical context for the Mark 25.
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Overview & Specifications
The heart of the Mark 25 is the “Hurricane” powerhead. Known for its high-revving nature and robust construction, this engine utilized advanced porting and a loop-charged design that set it apart from its contemporary competitors. Below are the primary technical specifications for the 1956 model.
| Specification | Details |
|---|---|
| Manufacturer | Kiekhaefer Mercury (Fond du Lac, Wisconsin) |
| Model Year | 1956 |
| Horsepower Rating | 20 HP @ 4,800 RPM |
| Engine Type | 2-Stroke Inline 2-Cylinder (“Hurricane” Block) |
| Displacement | 19.96 cubic inches (approx. 327 cc) |
| Bore x Stroke | 2.125 in. x 1.406 in. (53.98 mm x 35.71 mm) |
| Ignition System | Scintilla Magneto (or Phelon, depending on production run) |
| Carburetors | Single Tillotson AJ series (typically AJ-51A) |
| Starter | Manual rewind (Electric start available on the ‘Mark 25E’ model) |
| Weight | Approximately 75 lbs (34 kg) |
Fuel & Oil Mixture Requirements
Crucial Warning for Vintage Owners: Unlike modern outboards that run on a 50:1 lean mixture, the vintage Mark 25 relies on internal bronze bushings and early-generation needle bearings that require a much richer oil ratio to prevent catastrophic engine failure.
- Historical Spec: 12:1 ratio (3/4 pint of SAE 30 or 40 non-detergent outboard oil per 1 gallon of gasoline).
- Modern Recommendation: 16:1 to 24:1 ratio using high-quality TC-W3 rated two-stroke outboard oil and premium 89+ octane ethanol-free gasoline. Never run this motor at 50:1.
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Common Maintenance & Tune-up Specs
Keeping a vintage Mark 25 running smoothly requires precise adherence to clearances and gaps. Below are the critical tune-up specifications required for routine servicing.
Ignition & Electrical System
- Spark Plug Type: Champion J6C (Modern replacement for the original J8C / J6J).
- Spark Plug Gap: 0.025 inches (0.64 mm).
- Breaker Point Gap: 0.020 inches (0.51 mm).
- Timing: Set statically via the magneto stator plate rotation. Ensure points are clean, free of oil, and open exactly at the timing marks on the flywheel.
Carburetor Adjustment
The Tillotson AJ carburetor features a low-speed idle mixture needle and a high-speed main jet needle.
- Initial Low-Speed Needle Setting: Open 1 to 1-1/4 turns from gently seated.
- Initial High-Speed Needle Setting: Open 1 to 1-1/2 turns from gently seated.
- Note: Final adjustments must be made on the water under load with the engine at normal operating temperature.
Cooling System & Impeller
The cooling system relies on a rubber impeller located in the lower unit water pump housing, driven by the driveshaft.
- Impeller Service Interval: Inspect annually; replace every 2 to 3 seasons. Running a dry pump for even a few seconds will destroy the impeller blades.
- Lower Unit Gear Lubricant: Use a quality SAE 90 marine gear oil (non-corrosive to yellow metals, as the gearcase contains bronze bushings). Fill from the bottom drain hole until it vents from the top hole.
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Notable Features & History
The 1956 Mercury Mark 25 represents the pinnacle of Kiekhaefer’s mid-50s “Full Jeweled” power design—so named because the engine utilized ball, roller, and needle bearings throughout its internal rotating assembly, minimizing friction and maximizing power production.
Carl Kiekhaefer’s Engineering Legacy
Carl Kiekhaefer, the legendary and notoriously demanding head of Mercury, designed his outboards to outperform competitors like Evinrude and Johnson in both speed and visual appeal. The Mark 25 was marketed as a quiet, smooth-running engine that didn’t sacrifice performance. It was a key player in the “outboard horsepower wars” of the 1950s.
Key Design Elements
- Dyna-Float Suspension: To combat the high-frequency vibrations inherent in two-stroke vertical twin engines, Mercury utilized shear-type rubber mounts. This “Dyna-Float” suspension isolated the motor’s powerhead vibration from the boat’s transom, providing a remarkably smooth ride.
- Super Silent Cowling: The 1956 model featured full-wrap aluminum cowls lined with sound-absorbing blankets, reducing airborne engine noise compared to older, open-bottom designs.
- Integrated Controls: The Mark 25 offered compatibility with Mercury’s proprietary single-lever remote throttle and shift systems, making it highly versatile for family runabouts and sporty utility boats.
- Classic 1950s Aesthetics: Characterized by its sleek, rounded, aerodynamic cowl and two-tone color schemes (such as Marlin Blue and Desert Sand, or Sunset Orange and Sand Tan), the Mark 25 looked fast even when parked at the dock.
Whether you are performing a mechanical restoration or maintaining a survivor engine, the 1956 Mercury Mark 25 stands as a testament to classic American craftsmanship, offering a nostalgic and thrilling boating experience when properly tuned.