Mercury 2.5 / 3.5 FourStroke (1 cylinder 85.5 cc, serial 0R131842 to 0R318095) service guide

Quick answers

How much oil does a Mercury 2.5 / 3.5 FourStroke take?

Approximately 300 ml (10 fl oz). No oil filter. Level is read at the oil level inspection window with the outboard upright; from the lower mark add 100 ml (3 oz).

What engine oil does a Mercury 2.5 / 3.5 FourStroke use?

Mercury or Quicksilver SAE 10W-30 4-Stroke Outboard Oil for all temperatures; SAE 25W-40 may be used above 4 °C (40 °F). Oil must meet API SH, SG, SF, CF-4, CE, CD or CDII.

What gear oil does a Mercury 2.5 / 3.5 FourStroke use, and how much?

Premium Gear Lubricant. Quantity: Approximately 180 ml; Fill until lubricant appears at the vent hole.

What spark plug does a Mercury 2.5 / 3.5 FourStroke use?

NGK DPR6E, qty 1.

How often should a Mercury 2.5 / 3.5 FourStroke be serviced?

The factory schedule lists: Every 100 hours of use or once yearly, whichever occurs first. 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?

1400 rpm in neutral and 1300 rpm in forward gear; WOT range 4500 to 5500 rpm (2.5) and 5000 to 6000 rpm (3.5).

What is the propeller nut torque on a Mercury 2.5 / 3.5 FourStroke?

No propeller nut: the propeller slides onto a drive (shear) pin and is held by a cotter pin through the propeller; bend the cotter pin ends.

Maintenance specs

Engine oil	Mercury or Quicksilver SAE 10W-30 4-Stroke Outboard Oil for all temperatures; SAE 25W-40 may be used above 4 °C (40 °F). Oil must meet API SH, SG, SF, CF-4, CE, CD or CDII
Engine oil quantity	Approximately 300 ml (10 fl oz). No oil filter. Level is read at the oil level inspection window with the outboard upright; from the lower mark add 100 ml (3 oz)
Engine oil drain bolt	20 Nm (177 lb-in), with washer; lubricate the seal on the drain plug with oil
Gear lube	Premium Gear Lubricant
Gear lube quantity	Approximately 180 ml; Fill until lubricant appears at the vent hole
Gear lube fill/drain and vent plugs	1 fill/drain plug ("OIL") and 1 vent plug ("OIL LEVEL"), 2 plugs and 2 gaskets, 6.8 Nm (60 lb-in). Replace the sealing washers with new washers
Spark plug	NGK DPR6E, qty 1
Spark plug torque	27 Nm (20 lb-ft); or finger tight then 1/4 turn
Idle speed	1400 rpm in neutral and 1300 rpm in forward gear; WOT range 4500 to 5500 rpm (2.5) and 5000 to 6000 rpm (3.5)
Thermostat	Starts to open at 50 to 54 °C (121 to 129 °F); full open 60 to 64 °C (146 to 154 °F)
Anodes	One anode on the gear case, on the bottom of the anti-ventilation plate; anode bolt (M6 x 16) 5.6 Nm (50 lb-in). Replace it if 50% or more is consumed
Propeller	No propeller nut: the propeller slides onto a drive (shear) pin and is held by a cotter pin through the propeller; bend the cotter pin ends
Valve to rocker arm clearance, cold	Intake 0.06 to 0.14 mm (0.0024 to 0.0055 in); exhaust 0.11 to 0.19 mm (0.0043 to 0.0075 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 of use or once yearly, whichever occurs first

   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, 90-899925. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.