Yamaha 20D / 25N two-stroke service guide
Oil, gear oil, spark plugs and the service schedule for the Yamaha 20D / 25N two-stroke (20 to 25 hp, two-stroke), from the factory service manual, plus how we service it in Hawaiʻi.
Models covered: 20D and 25N two-stroke twins: 2 cylinders in line, 395 cm³ (24.11 cu in), 67.0 x 56.0 mm, 2 carburetors. 20 hp, approved model code 6L3: 20DM (USA and Canada 20MH), 20DMO (Canada 20MH2), 20DEM (Canada 20EH), 20DEMO (Canada 20EH2), 20DEO, 20DERO. 25 hp, approved model code 6L2: 25NM (Canada 25MH), 25NMO (USA 25MH, Canada 25MH2), 25NEMO (USA and Canada 25EH), 25NE, 25NEO, 25NERO (USA and Canada 25ER). Models with an O in the name (MO, EMO, EO, ERO) are oil injected with a 0.7 L oil tank; 20DM, 20DEM, 25NM and 25NE run on premixed fuel.
Quick answers
- What two-stroke oil does a Yamaha 20D / 25N two-stroke use?
- TC-W3.
- What gear oil does a Yamaha 20D / 25N two-stroke use, and how much?
- GEAR CASE LUBE (USA) or hypoid gear oil, SAE #90. Quantity: 370 cm³ (12.5 US oz, 13.0 Imp oz); fill through the drain hole until it overflows at the level hole.
- What spark plug does a Yamaha 20D / 25N two-stroke use?
- NGK BR7HS-10 (B7HS-10), qty 2. Gap: 0.9 to 1.0 mm (0.035 to 0.039 in).
- How often should a Yamaha 20D / 25N two-stroke be serviced?
- The factory schedule lists: Initial 10 hours (Break in); Initial 50 hours (3 months); Every 100 hours (6 months); Every 200 hours (1 year). In Hawaiʻi's salt water we do a full yearly service every 200 hours or 1 year, whichever comes first, and a new engine needs its first (break-in) service.
- When should the water pump impeller be replaced on a Yamaha 20D / 25N two-stroke?
- The factory schedule gives no replacement interval. In Hawaiʻi's warm salt water we replace it every 200 hours or 1 year, whichever comes first, because a worn impeller is the most common cause of overheating.
- What is the idle speed of a Yamaha 20D / 25N two-stroke?
- 750 ± 50 r/min (Canada 25 hp: 780 ± 30 r/min); pilot screw 20 hp 2-1/2 ± 3/4 turns out, 25 hp 2 ± 3/4 turns out; neutral speed limit 3,800 ± 300 r/min.
- What is the propeller nut torque on a Yamaha 20D / 25N two-stroke?
- 35 N·m (3.5 m·kg, 25 ft·lb).
Maintenance specs
| Two-stroke engine oil | TC-W3 |
|---|---|
| Gear oil | GEAR CASE LUBE (USA) or hypoid gear oil, SAE #90 |
| Gear oil quantity | 370 cm³ (12.5 US oz, 13.0 Imp oz); fill through the drain hole until it overflows at the level hole |
| Gear oil drain and level plugs | Refit the oil level plug, then the oil drain plug |
| Lower unit holding pressure | 100 kPa (1.0 kg/cm², 14.2 psi) at the oil-level hole, must hold 10 seconds; do not over-pressurize. Pressure tester YB-03595 / 90890-06762 |
| Spark plug | NGK BR7HS-10 (B7HS-10), qty 2 |
| Spark plug gap | 0.9 to 1.0 mm (0.035 to 0.039 in) |
| Spark plug torque (M14) | 25 N·m (2.5 m·kg, 18 ft·lb); without a torque wrench, 1/4 to 1/2 turn past finger-tight |
| Idle speed | 750 ± 50 r/min (Canada 25 hp: 780 ± 30 r/min); pilot screw 20 hp 2-1/2 ± 3/4 turns out, 25 hp 2 ± 3/4 turns out; neutral speed limit 3,800 ± 300 r/min |
| Thermostat | Opens at 48 to 52 °C (118.4 to 125.6 °F); fully open at 60 °C (140 °F); valve lift 3 mm (0.12 in) |
| Anodes | One on the lower unit (inspect: clean off scale, oil and grease; replace if excessively consumed; do not oil, grease or paint) and one in the cylinder head under a cover (bolt 6 x 16 mm). |
| Propeller nut (M14) | 35 N·m (3.5 m·kg, 25 ft·lb). |
Service schedule
The factory maintenance intervals, with what is replaced and what is checked at each. Rows marked Hawaiʻi are our shop's yearly service for salt water use.
Initial 10 hours (Break in)
Replace: gear case oil, gear case plug gaskets, propeller cotter pin
Check: spark plugs, fuel filter
Initial 50 hours (3 months)
Replace: gear case plug gaskets, propeller cotter pin
Check: spark plugs, fuel filter, anodes
Every 100 hours (6 months)
Replace: gear case oil, gear case plug gaskets, propeller cotter pin
Check: spark plugs, fuel filter, thermostat, anodes
Every 200 hours (1 year)
Replace: gear case oil, gear case plug gaskets, propeller cotter pin, water pump impeller
Check: spark plugs, fuel filter, thermostat, anodes
We replace the water pump impeller at every service from 200 hours up, which can be sooner than the factory schedule, because of Hawaiʻi’s warm salt water.
Source: Yamaha 20D, 25N service manual. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.