Mercury 40 / 50 two-stroke (3 cylinder, 697 cc) service guide

Quick answers

What two-stroke oil does a Mercury 40 / 50 two-stroke use?

Mercury or Quicksilver oil or recommended oil (TC-W3); any NMMA certified TC-W3 2-cycle outboard oil for the oil tank and for premix; do not mix brands or types. Oil tank on oil injected models approximately 2.0 L (0.53 US gal).

What is the fuel to oil ratio for a Mercury 40 / 50 two-stroke?

Premix models, and oil injected models run with the system disabled: 50:1 (2% oil). Oil injected models: the pump varies the ratio between 120:1 (0.8% oil) and 50:1 with the throttle. New engines, reconditioned powerheads and engines coming out of storage: 25:1 (4% oil) in the first tank, for the first 10 hours; on oil injected models, in addition to oil in the tank, a 50:1 mixture is required during break-in.

What gear oil does a Mercury 40 / 50 two-stroke use, and how much?

Mercury or Quicksilver gear oil or API GL-5, SAE #80 to #90 (Premium Gear Lubricant). Quantity: 40/50: approximately 500 mL (0.53 US qt; 16.9 fl oz). W50 models: approximately 700 mL. Fill at the lower fill hole until it appears at the upper plug hole.

What spark plug does a Mercury 40 / 50 two-stroke use?

40: NGK B7HS-10 / BR7HS-10 or Champion L-82C / RL-82C. 50 and W50: NGK B8HS-10 / BR8HS-10 or Champion L-78C / RL-78C. Qty 3. Gap: 1.0 mm (0.039 in.) in the model specifications; 0.9 to 1.0 mm (0.035 to 0.039 in.) in the ignition specifications and the maintenance section.

How often should a Mercury 40 / 50 two-stroke be serviced?

The factory schedule lists: Initial 10 hours (after engine break-in); Every 50 hours (3 months); Every 100 hours or once a year; Every 300 hours or 3 years. 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 Mercury 40 / 50 two-stroke?

The factory schedule says to replace it at 300 hours. 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 Mercury 40 / 50 two-stroke?

900 RPM in neutral, 750 RPM in gear (trolling).

What is the propeller nut torque on a Mercury 40 / 50 two-stroke?

75 N·m (55 lb-ft) in the maintenance section, with the stopper (nut retainer) and cotter pin.

Maintenance specs

Two-stroke engine oil	Mercury or Quicksilver oil or recommended oil (TC-W3); any NMMA certified TC-W3 2-cycle outboard oil for the oil tank and for premix; do not mix brands or types. Oil tank on oil injected models approximately 2.0 L (0.53 US gal)
Fuel to oil ratio	Premix models, and oil injected models run with the system disabled: 50:1 (2% oil). Oil injected models: the pump varies the ratio between 120:1 (0.8% oil) and 50:1 with the throttle. New engines, reconditioned powerheads and engines coming out of storage: 25:1 (4% oil) in the first tank, for the first 10 hours; on oil injected models, in addition to oil in the tank, a 50:1 mixture is required during break-in
Gear oil	Mercury or Quicksilver gear oil or API GL-5, SAE #80 to #90 (Premium Gear Lubricant)
Gear oil quantity	40/50: approximately 500 mL (0.53 US qt; 16.9 fl oz). W50 models: approximately 700 mL. Fill at the lower fill hole until it appears at the upper plug hole
Gear oil fill and vent plugs	Lower fill/drain plug and upper vent plug, each with a sealing washer (2 plugs): vent screw 6.8 N·m (60 lb-in); Install the vent plug first, then the fill plug
Lower unit holding pressure	69 to 83 kPa (10 to 12 psi) at the vent hole, must hold for five minutes while the shafts are turned and the shift shaft moved
Spark plug	40: NGK B7HS-10 / BR7HS-10 or Champion L-82C / RL-82C. 50 and W50: NGK B8HS-10 / BR8HS-10 or Champion L-78C / RL-78C. Qty 3
Spark plug gap	1.0 mm (0.039 in.) in the model specifications; 0.9 to 1.0 mm (0.035 to 0.039 in.) in the ignition specifications and the maintenance section
Spark plug torque	27 N·m (19.9 lb-ft), or finger tight and then tighten
Idle speed	900 RPM in neutral, 750 RPM in gear (trolling)
Thermostat	Opens at 60 °C (140 °F)
Anodes	One on the bottom of the transom bracket assembly and the trim tab on the gearcase: never grease or paint; tighten the anode attaching screw at each inspection. Gear housing screw (M6 x 16) and trim tab bolt (M6 x 20) 5.5 N·m (48.7 lb-in)
Propeller nut	75 N·m (55 lb-ft) in the maintenance section, with the stopper (nut retainer) and cotter pin
Power trim and tilt fluid (PT models)	Mercury or Quicksilver Power Trim and Steering Fluid; if not available, automotive automatic transmission fluid (ATF). Capacity 0.55 L (0.14 US gal). Check with the engine vertical: the level reaches the bottom of the plug hole; do not remove the plug with the engine tilted down

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.

1. Initial 10 hours (after engine break-in)

   Replace: gear case plug gaskets

   Check: fuel filter

2. Every 50 hours (3 months)

   Replace: gear case plug gaskets

   Check: fuel filter

3. Every 100 hours or once a year

   Replace: gear case oil, gear case plug gaskets, propeller cotter pin

   Check: fuel filter, thermostat, anodes

4. Every 200 hours or 1 year (our Hawai‘i yearly service) Hawai‘i

   Replace: gear case oil, gear case plug gaskets, propeller cotter pin, water pump impeller

   Check: fuel filter, thermostat, anodes

5. Every 300 hours or 3 years

   Replace: spark plugs, gear case oil, gear case plug gaskets, propeller cotter pin, water pump impeller

   Check: 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: Mercury 40/50 TwoStroke Outboards service manual, 90-8M0228641, May 2024. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.