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Mercury 40 FourStroke (3 cylinder, carbureted) service guide

Oil, gear oil, spark plugs and the service schedule for the Mercury 40 FourStroke (3 cylinder, carbureted) (40 hp, four-stroke), from the factory service manual, plus how we service it in Hawaiʻi.

Models covered: Mercury 40 FourStroke, 3 cylinder, carbureted, serial 1C018826 and above. The 30/40 EFI 3 cylinder (1C453840 and above) has its own sheet.

Quick answers

How much oil does a Mercury 40 FourStroke take?
3.0 L (3.0 US qt) with filter.
What engine oil does a Mercury 40 FourStroke use?
FC-W synthetic blend 25W-40; FC-W 10W-30 below 4 °C (40 °F).
What gear oil does a Mercury 40 FourStroke use, and how much?
High Performance Gear Lubricant.
What spark plug does a Mercury 40 FourStroke use?
NGK DPR6EA-9. Gap: 0.9 mm (0.035 in).
How often should a Mercury 40 FourStroke be serviced?
The factory schedule lists: Every 100 hours or once a year; Every 300 hours or 3 years; Every 400 hours. 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 FourStroke?
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 FourStroke?
Forward 775 to 825 r/min; neutral 875 to 925 r/min.
What is the propeller nut torque on a Mercury 40 FourStroke?
75 N·m (55 lb·ft); bend the retainer tabs.

Maintenance specs

Engine oilFC-W synthetic blend 25W-40; FC-W 10W-30 below 4 °C (40 °F)
Engine oil quantity3.0 L (3.0 US qt) with filter
Oil filter3/4 to 1 turn after the gasket touches
Drain plug (driveshaft housing)24 N·m (18 lb·ft)
Gear lubricantHigh Performance Gear Lubricant
Gearcase drain plug and vent screwDrain plug 6.5 N·m (58 lb·in); vent screw 6.8 N·m (60 lb·in)
Gearcase leak test69 to 83 kPa (10 to 12 psi) for 5 minutes
Spark plugNGK DPR6EA-9
Spark plug gap0.9 mm (0.035 in)
Idle speedForward 775 to 825 r/min; neutral 875 to 925 r/min
ThermostatStarts to open 48 to 51 °C (118 to 123 °F); lift at least 3 mm (0.12 in) above 63 °C (145 °F)
Water pressure800 r/min 7 to 21 kPa (1 to 3 psi); 6000 r/min 83 to 172 kPa (12 to 25 psi); warm
Transom bracket anode bolts (M6 x 25)6 N·m (54 lb·in)
Propeller nut75 N·m (55 lb·ft); bend the retainer tabs
Valve clearance, coldIN 0.15 to 0.25 mm (0.006 to 0.010 in); EX 0.25 to 0.35 mm (0.010 to 0.014 in)

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. Every 100 hours or once a year

    Replace: engine oil, oil filter, oil drain plug gasket, spark plugs, gear case oil, gear case plug gaskets

    Check: fuel filter, thermostat, anodes

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

    Replace: engine oil, oil filter, oil drain plug gasket, gear case oil, gear case plug gaskets, water pump impeller

    Check: spark plugs, fuel filter, thermostat, anodes

  3. Every 300 hours or 3 years

    Replace: engine oil, oil filter, oil drain plug gasket, gear case oil, gear case plug gaskets, water pump impeller

    Check: spark plugs, fuel filter, thermostat, anodes

  4. Every 400 hours

    Replace: engine oil, oil filter, oil drain plug gasket, gear case oil, gear case plug gaskets, 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: Mercury 40 FourStroke 3 Cyl. service manual, 90-899974. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.

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