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Mercury 25 BigFoot FourStroke (1998 on, carbureted) service guide

Oil, gear oil, spark plugs and the service schedule for the Mercury 25 BigFoot FourStroke (1998 on, carbureted) (25 hp, four-stroke), from the factory service manual, plus how we service it in Hawaiʻi.

Models covered: Mercury 25 Bigfoot (4-Stroke), starting model year 1998, starting serial number 0G590000. 2 cylinder, 4 stroke, overhead camshaft, 30.4 cu in (498 cc), bore 65 mm x stroke 75 mm, one carburetor, 25 hp (18.6 kW) at 5750 rpm, full throttle range 5500 to 6000 rpm. Bigfoot gear housing, 2.42:1 gear ratio.

Quick answers

How much oil does a Mercury 25 BigFoot FourStroke take?
3 U.S. quarts (3.0 liters); Fill to the dipstick.
What engine oil does a Mercury 25 BigFoot FourStroke use?
4-Cycle Marine Oil, SAE 10W-30 for all temperatures; SAE 25W-40 may be used above 40 °F (4 °C). If not available, a premium 4-cycle engine oil certified API SH, SG, SF, CF-4, CE, CD or CDII.
What gear oil does a Mercury 25 BigFoot FourStroke use, and how much?
Quicksilver Premium Gear Lubricant. Quantity: Approximately 440 ml (14.9 fl oz); fill through the fill/drain hole until it appears at the vent hole.
What spark plug does a Mercury 25 BigFoot FourStroke use?
NGK DPR6EA-9, qty 2. Gap: 0.035 in (1.0 mm).
How often should a Mercury 25 BigFoot FourStroke be serviced?
The factory schedule lists: 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 25 BigFoot 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 25 BigFoot FourStroke?
825 ± 25 rpm out of gear; 775 ± 25 rpm in forward gear.
What is the propeller nut torque on a Mercury 25 BigFoot FourStroke?
55 lb-ft (75 N·m); bend the retainer tabs up against the flats of the nut.

Maintenance specs

Engine oil4-Cycle Marine Oil, SAE 10W-30 for all temperatures; SAE 25W-40 may be used above 40 °F (4 °C). If not available, a premium 4-cycle engine oil certified API SH, SG, SF, CF-4, CE, CD or CDII
Engine oil quantity3 U.S. quarts (3.0 liters); Fill to the dipstick
Oil filterp/n 35-822626A2, wrench 91-802653. Oil the gasket (no grease), screw on until the gasket contacts the base, then tighten 3/4 to 1 turn.
Engine oil drain plug17.5 lb-ft (23.5 N·m); lubricate the seal on the drain plug with oil. The oil can also be pumped out through the dipstick tube
Gear lubricantQuicksilver Premium Gear Lubricant
Gear lubricant quantityApproximately 440 ml (14.9 fl oz); fill through the fill/drain hole until it appears at the vent hole
Gear case plugsVent plug and fill/drain plug (magnetic), 2 plugs, each with a sealing washer: 60 lb-in (7 N·m); Install new gaskets on both plugs
Gear housing pressure test10 to 12 psi (69 to 83 kPa) at the vent hole, must hold 5 minutes; rotate the drive shaft and prop shaft and move the shift rod while pressurized
Spark plugNGK DPR6EA-9, qty 2
Spark plug gap0.035 in (1.0 mm)
Spark plug torque150 lb-in (17 N·m), or finger tight then 1/4 turn
Idle speed825 ± 25 rpm out of gear; 775 ± 25 rpm in forward gear
ThermostatOpens at 136 to 143 °F (58 to 62 °C); full open at 158 °F (70 °C); minimum valve lift 0.12 in (3 mm). Exhaust and thermostat cover screws (M6 x 35) 106 lb-in (12 N·m)
AnodesFour anodes: the trim tab on the gear case, one on the bottom of the transom bracket, and two in the engine block (each with a rubber seal and cover). Engine block anode cover screw 70 lb-in (8 N·m); clamp bracket anode screw (M6 x 25) 60 lb-in (6.8 N·m); trim tab screw (M8 x 20) 16 lb-ft (21.5 N·m). Replace the trim tab if 50% or more corroded
Propeller nut55 lb-ft (75 N·m); bend the retainer tabs up against the flats of the nut
Power trim fluidQuicksilver Power Trim & Steering Fluid; if not available, automotive (ATF) automatic transmission fluid. Tilt full up on the tilt support lock; the level should be even with the bottom of the fill hole
Valve clearance, coldIntake 0.006 to 0.010 in (0.15 to 0.25 mm); exhaust 0.010 to 0.014 in (0.25 to 0.35 mm). Valve adjusting nut 120 lb-in (13.5 N·m)

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, gear case oil, gear case plug gaskets

    Check: spark plugs, 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

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 25 BigFoot FourStroke service manual, starting model year 1998, 90-854785R2. 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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