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Mercury 2.5 / 3.5 FourStroke service guide

Oil, gear oil, spark plugs and the service schedule for the Mercury 2.5 / 3.5 FourStroke (2.5 to 3.5 hp, four-stroke), from the factory service manual, plus how we service it in Hawaiʻi.

Models covered: Mercury 2.5 and 3.5 hp FourStroke, 1 cylinder, carbureted, serial 0R448033 and above.

Quick answers

How much oil does a Mercury 2.5 / 3.5 FourStroke take?
About 300 ml (10 fl oz). No oil filter.
What engine oil does a Mercury 2.5 / 3.5 FourStroke use?
10W-30 all temperatures; 25W-40 above 4 °C (40 °F).
What gear oil does a Mercury 2.5 / 3.5 FourStroke use, and how much?
Premium or High Performance Gear Lubricant. Quantity: About 180 ml. Fill until it comes out the vent.
What spark plug does a Mercury 2.5 / 3.5 FourStroke use?
NGK DPR6E.
How often should a Mercury 2.5 / 3.5 FourStroke be serviced?
The factory schedule lists: Every 100 hours or once a 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 Mercury 2.5 / 3.5 FourStroke?
The factory schedule says to replace it at 100 hours, and we follow that. In any case we never let it go longer than a year in Hawaiʻi's salt water.
What is the idle speed of a Mercury 2.5 / 3.5 FourStroke?
Neutral 1,400 and forward 1,300 r/min, ± 100 or ± 50.
What is the propeller nut torque on a Mercury 2.5 / 3.5 FourStroke?
No nut: drive (shear) pin with a new cotter pin.

Maintenance specs

Engine oil10W-30 all temperatures; 25W-40 above 4 °C (40 °F)
Engine oil quantityAbout 300 ml (10 fl oz). No oil filter
Oil drain bolt20 N·m (177 lb·in)
Gear lubricantPremium or High Performance Gear Lubricant
Gearcase capacityAbout 180 ml. Fill until it comes out the vent
Gearcase vent and fill/drain plugs6.8 N·m (60 lb·in), new sealing washers
Spark plugNGK DPR6E
Spark plug torque27 N·m (20 lb·ft), or finger tight then 1/4 turn
Idle speedNeutral 1,400 and forward 1,300 r/min, ± 100 or ± 50
ThermostatStarts to open 50 to 54 °C (122 to 129 °F); fully open 60 to 64 °C (140 to 147 °F); lift 2 mm (0.079 in)
Water pressureIdle (1,300 r/min) 3 to 7 kPa (0.4 to 1.0 psi); WOT (5,000 r/min) 40 to 75 kPa (5.8 to 10.9 psi)
Anode bolt5.6 N·m (50 lb·in); replace at 50% consumed
PropellerNo nut: drive (shear) pin with a new cotter pin
Valve clearance, coldIN 0.06 to 0.14 mm (0.0024 to 0.0055 in); EX 0.11 to 0.19 mm (0.0043 to 0.0075 in); set at TDC compression; rocker nut 11 N·m (97 lb·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 drain plug gasket, spark plugs, gear case oil, gear case plug gaskets, water pump impeller

    Check: anodes

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

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

    Check: spark plugs, anodes

Source: Mercury 2.5/3.5 FourStroke service manual, 90-8M0065421. 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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