🔎 Mercury 55 HP Reference Library — Exploded parts diagrams (39 diagrams), specifications, common issues, and maintenance checklist.
The Mercury 55 HP is a 2-Stroke, 3-Cylinder outboard motor. Suited for bay boats, pontoons, and center-console fishing boats requiring reliable mid-range power.
Quick Reference Specifications
| Specification |
Value |
| Engine Type |
2-Stroke, 3-Cylinder |
| Horsepower |
55 HP |
| Displacement |
967 cc |
| Bore x Stroke |
76 mm x 71 mm |
| Spark Plug (Type & Gap) |
NGK BUHW-2 |
| Engine Oil Capacity |
Pre-mix or Oil Injection |
| Recommended Engine Oil |
Quicksilver Premium 2-Cycle TC-W3 |
| Gear Case Oil Capacity |
380-440 mL (12.8-14.9 fl oz) |
| Idle Speed (in gear) |
650 ± 50 RPM (in gear) |
| Full Throttle RPM Range |
5000-5500 |
| Fuel |
Unleaded Gasoline, 87 Octane min. |
Always verify specifications against your owner's manual for your specific serial number range.
Common Issues & Recommended Parts
Below are common maintenance scenarios for the Mercury 55 HP:
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Water Pump / Overheating: If the tell-tale stream is weak or absent, the water pump impeller likely needs replacement. Inspect annually or every 100 hours. → Recommended: Water Pump Impeller Kit
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Engine Oil / Maintenance Light: 4-stroke engines require regular oil and filter changes. Use marine-grade oil rated for your engine. Change every 100 hours or annually. → Recommended: Oil Filter, Engine Oil
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Fuel Injector Issues (EFI models): Rough idle or misfires may indicate clogged fuel injectors. Use fuel stabilizer and clean injectors periodically. → Recommended: Fuel Injector, Fuel Filter
-
Gear Case Oil Leak: Check lower unit seals if gear oil appears milky (water contamination). Replace seals and gaskets during lower unit service. → Recommended: Lower Unit Seal Kit, Drain Plug Gaskets
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Anode Corrosion: Inspect sacrificial anodes regularly, especially in saltwater. Replace when 50% depleted to protect against galvanic corrosion. → Recommended: Zinc/Aluminum Anode Kit
100-Hour / Annual Service Checklist
Regular maintenance keeps your Mercury 55 HP running reliably season after season. Most of these tasks can be performed with basic tools and the factory service manual.
- Replace water pump impeller and inspect wear plate
- Change engine oil and oil filter (check drain plug washer)
- Replace spark plugs and check ignition timing
- Drain and refill lower unit gear oil; inspect for water contamination
- Inspect fuel lines, primer bulb, and fuel filter for deterioration
- Check and replace sacrificial anodes (zinc or aluminum)
- Inspect propeller for damage; check shaft seal and cotter pin
- Check valve clearance (refer to service manual for specifications)
- Lubricate all grease points (steering, tilt tube, shift linkage)
- Inspect control cables and throttle/shift linkage for smooth operation
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Pro Tip: Always flush the engine with fresh water after saltwater use — even a quick 10-minute flush significantly extends component life.
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Pro Tip: Use fuel stabilizer in the tank before winter storage, and fog the cylinders to prevent internal corrosion during extended periods of non-use.
