Evinrude 33 HP Ski-Twin 20″ Long Shaft (1965) Outboard Motor Specifications & Maintenance Guide
The 1965 Evinrude 33 HP Ski-Twin Model 33503 is a long-shaft, manual-start two-stroke outboard built for deeper transoms and commercial utility hulls. Powered by Evinrude’s 40.5 cubic-inch twin-cylinder powerhead, it produces 33 certified horsepower at 4,500 RPM. Equipped with a 20-inch transom extension and weighing 131 pounds dry, Model 33503 incorporates a heavy-duty Simplex pull starter alongside full Forward-Neutral-Reverse gear shifting for versatile watercraft applications.
Preserving smooth engine operation across its 4,000–5,000 WOT range requires strict compliance with factory calibration limits. Key service tasks—such as keeping ignition point gaps at 0.020″, torquing 14mm spark plugs to 20–20.5 ft-lbs, verifying float levels flush with the carburetor bowl, and ensuring correct crankshaft journal clearances—prevent unwanted power loss and safeguard lower unit components.
Maintenance Recap & Tune-Up Checklist
- Ignition System Calibration: Inspect magneto breaker points set to 0.020″, check condenser values between 0.25 and 0.29 mfd, and gap Champion J4J or equivalent 14mm spark plugs to 0.030″, torquing to 20–20.5 ft-lbs.
- Carburetor & Choke Tuning: Set single-barrel float height flush with the casting rim, confirm inlet needle seating tolerances (0.062″–0.065″), and inspect manual/electric choke linkage for full travel.
- Gearcase & Drive Pin Care: Check gear oil in the 12:21 lower unit and inspect the 10-3/8″ x 12-1/2″ 3-blade propeller. Always replace shear pins with authentic 1/4″ x 1-15/32″ stainless steel pins (Part No. 304575).
- Cooling & Journal Inspections: Monitor the Centri-matic water pump output and verify crankshaft tolerances during teardown (1.2495″–1.2500″ top journal; 0.9995″–1.0000″ center/bottom journals; 1.1823″–1.1828″ crank pin).
Technical Specifications
| Specification Parameter | Details / Value |
|---|---|
| Model Number | 33503 |
| Model Year | 1965 |
| Horsepower (O.B.C.-Certified) | 33 HP @ 4500 RPM |
| Full Throttle Operating Range | 4000 to 5000 RPM |
| Engine Type | 2 Cylinder, 2-Cycle |
| Bore and Stroke | 3-1/16″ Bore x 2-3/4″ Stroke |
| Piston Displacement | 40.5 cu. in. |
| Starter Type | Simplex self-winding (Ski-Twin) |
| Transom Length | 20″ (5″ longer than standard) |
| Outboard Weight (Without tank) | 131 lbs. |
| Cooling System | Centri-matic (Combination positive displacement & centrifugal) |
| Ignition Type | Flywheel magneto |
| Spark Plug Type | AC-M42K, Champion J4J, Auto-Lite A21X (14mm) |
| Spark Plug Gap | .030 inch |
| Spark Plug Torque | 20 – 20-1/2 foot-pounds |
| Breaker Point Gap | .020 inch |
| Condenser Capacity | .25 – .29 mfd. |
| Carburetion | Single barrel, float feed, high/low-speed adjustments, manual/electric choke |
| Float Level Setting | Flush with rim of casting |
| Inlet Needle Seat | .062 – .065 inch |
| Fuel Capacity | 6 gallons, suction type tank |
| Fuel Tank Weight | 12 pounds net |
| Speed Control | Knob on steering bracket or remote (Synchronized throttle & spark) |
| Gear Shift Control | Forward, Neutral, Reverse |
| Propeller Gear Ratio | 12:21 |
| Propeller Drive Pin | Part No. 304575, 1/4″ x 1-15/32″ stainless steel |
| Propeller | 10-3/8″ diameter x 12-1/2″ pitch, 3 blade |
| Piston Ring Set – Standard | Part Number 378436 |
| Piston Ring Set – .020″ Oversize | Part Number 378437 |
| Piston Ring Set – .040″ Oversize | Part Number 378438 |
| Diameter of Ring (Standard) | 3.0625 in. |
| Width of Ring | .0925 – .0935 in. |
| Compression When Compressed | 6 to 9 lbs. |
| Piston Less Rings – Standard | Part Number 380039 |
| Piston Less Rings – .020″ Oversize | Part Number 380557 |
| Piston Less Rings – .040″ Oversize | Part Number 380558 |
| Crankshaft Top Journal | 1.2495 – 1.2500 in. |
| Crankshaft Center Journal | .9995 – 1.0000 in. |
| Crankshaft Bottom Journal | .9995 – 1.0000 in. |
| Connecting Rod Crank Pin | 1.1823 – 1.1828 in. |
Disclaimer & Information Notice: Specifications, fuel ratios, and maintenance guidelines listed on VintageMotorSpecs.com are compiled for historical reference and educational purposes only. Always verify data against an official OEM service manual prior to ordering parts or executing repairs. VintageMotorSpecs.com assumes no liability for repair errors, equipment damage, or personal injury resulting from the use of this data.
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