1958 Mercury 22 HP Mark 28 Outboard Specs & Part Numbers
Technical Guide & Service Manual: 1958 Mercury 22 HP Mark 28
The 1958 Mercury Mark 28 (and its electric-start counterpart, the Mark 28E) represents a high-water mark of mid-century marine engineering. Rated at 22 horsepower, this twin-cylinder, two-stroke outboard motor was marketed by Kiekhaefer Mercury as the ultimate versatile family motor—powerful enough for water skiing, yet quiet and smooth enough for slow trolling. Featuring classic styling and advanced engineering like the “automatic transmission” single-lever control, the Mark 28 remains a highly sought-after collectible and a reliable runner when properly maintained.
Overview & Technical Specifications
The heart of the Mark 28 is a 22-cubic-inch inline twin-cylinder powerhead. Known for its high-revving power band and lightweight design, this motor utilized Kiekhaefer’s signature internal reed valves and alternate-firing design to deliver smooth power delivery across the RPM range.
| Specification | Value / Measurement |
|---|---|
| Horsepower Rating | 22 HP @ 5800 RPM |
| Engine Type | 2-Cylinder, Inline, Alternate Firing, 2-Stroke |
| Displacement | 22.0 Cubic Inches (approx. 360 cc) |
| Bore & Stroke | 2.406″ x 2.125″ (61.1 mm x 54.0 mm) |
| Fuel/Oil Ratio (Original) | 16:1 (1/2 pint of oil per 1 gallon of regular gasoline) |
| Fuel/Oil Ratio (Modern) | 24:1 using high-quality TC-W3 2-stroke outboard oil |
| Weight | Approx. 85 lbs (Manual) / 95 lbs (Electric Start) |
| Ignition System | Phelon Magneto (Dual Points & Condensers) |
| Carburetor | Tillotson AJ Series float-feed carburetor |
| Fuel System Type | Dual-line pressurized fuel tank system (original configuration) |
Common Maintenance & Tune-up Specs
Keeping a vintage Mark 28 running smoothly requires careful attention to the ignition system, fuel delivery, and cooling. Below are the critical tune-up specifications required for servicing this motor.
Ignition System
- Spark Plug Type: Champion L86C (or modern equivalent NGK B6HS)
- Spark Plug Gap: 0.025 inches (0.64 mm)
- Breaker Point Gap: 0.020 inches (0.51 mm)
- Timing: Set points to break exactly at top dead center (TDC) or according to the timing marks on the flywheel and magneto plate.
Carburetor Adjustments
The Tillotson carburetor features high-speed and low-speed adjustable jets. To set them from a completely seated position (do not overtighten when seating):
- High-Speed Jet (Main): Open 1 to 1-1/4 turns as a starting point. Adjust under load at wide-open throttle (WOT) until the engine runs smoothly without bogging (lean) or four-stroking (rich).
- Low-Speed Jet (Idle): Open 1-1/2 turns as a starting point. Adjust with the motor warm, in gear, at its lowest reliable idle speed. Turn clockwise to lean, counterclockwise to richen.
Cooling & Impeller Info
The Mark 28 uses a rubber impeller water pump located inside the lower unit gear housing. Never run the motor dry; even a few seconds without water can destroy the rubber vanes.
- Impeller Replacement: Recommended every 2 seasons or if the motor has been sitting idle for years.
- Water Pump Housing: Inspect the stainless steel cup inside the aluminum housing for scoring or wear. Replace the cup if grooved.
- Gear Lubricant: Use a high-quality SAE 90 outboard gear lube. Fill from the bottom drain screw until lubricant flows out of the top vent screw.
Notable Features & History
Introduced in 1958, the Mercury Mark 28 was part of Carl Kiekhaefer’s push to dominate the mid-range family boating market. It filled the gap between the smaller utility fishing motors and the larger, heavier four-cylinder dock-busters.
The “Automatic Transmission” (Single-Lever Control)
One of the most defining and technically unique features of the Mark 28 was its shifting mechanism. Unlike competitors that used a separate shift lever on the side of the cowling, the Mark 28 utilized a single-lever remote control system (or a twisting tiller handle) that handled both shifting and throttling in one continuous motion.
Internally, the engine utilized a unique slipping-clutch assembly and a reverse-lock mechanism in the midsection. When the operator pushed the throttle forward, the lower unit shifted into forward gear automatically before the carburetor opened up. Pulling the handle back past the idle detent automatically engaged reverse gear. While revolutionary for its ease of use, it required precise adjustment of the shift cables and linkages to prevent gear dog wear.
Super Silent Power design
The Mark 28 featured Mercury’s “Super Silent” design. The entire powerhead was suspended on soft rubber shear-mounts inside an enclosed, sealed cowl. This isolated engine vibrations from the boat transom and redirected intake and exhaust noise. Exhaust was routed down through the driveshaft housing and discharged deep underwater through the propeller hub—a design pioneered by Mercury that is still the industry standard today.
Aesthetic Appeal
1958 was a legendary year for outboard styling, and the Mark 28 is no exception. It featured striking two-tone color options such as “Sandtan and Burgundy” or “Marlin Blue and Sandtan,” complete with cast aluminum chrome trim, polished faceplates, and a wraparound cowl that opened easily via tension latches for service access.
Pro-Tips for Modern Operators
If you are restoring or running a 1958 Mercury Mark 28 today, keep these critical tips in mind:
- Avoid Modern Ethanol Fuel: The original fuel lines, carburetor gaskets, and fuel pump diaphragms on these vintage motors were not designed to withstand ethanol. Use 91+ octane ethanol-free gasoline (recreational fuel) to prevent deterioration of the fuel system components.
- The Dual-Line Pressure Tank System: The original configuration relied on a pressurized fuel tank (where the motor pumps air into the tank to force fuel up to the engine). If you are using an original 2-line tank, ensure the tank gaskets are perfectly sealed. Many owners choose to convert these motors to a modern single-line vacuum pulse fuel pump system (using a Mikuni or later Mercury fuel pump) for safety and convenience.
- Mind the Bearings: The Mark 28 utilizes a mix of sleeve, needle, and ball bearings. Because of the brass sleeve bearings in certain points of the powerhead, do not run this motor at a 50:1 fuel mixture. Stick to a 24:1 mixture using high-quality TC-W3 oil to ensure adequate lubrication and prevent catastrophic internal engine wear.