Mercury / Mariner 40 / 45 / 50 and 50 Bigfoot FourStroke (4 cylinder 935 cc, carbureted, serial 0G231123 and above) service guide

Quick answers

How much oil does a Mercury / Mariner 40 / 45 / 50 and 50 Bigfoot FourStroke take?

Either 3 US quarts or 3 liters (3.0 L, 3 US qt). it does not give separate with-filter and without-filter amounts.

What engine oil does a Mercury / Mariner 40 / 45 / 50 and 50 Bigfoot FourStroke use?

Quicksilver 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 meeting API SH, SG, SF, CF-4, CE, CD or CDII.

What gear oil does a Mercury / Mariner 40 / 45 / 50 and 50 Bigfoot FourStroke use, and how much?

Quicksilver Gear Lube-Premium Blend. Quantity: Standard gear case: approximately 440 ml (14.9 US fl oz). Bigfoot 2.3:1 gear case: approximately 655 ml (22.5 US fl oz).

What spark plug does a Mercury / Mariner 40 / 45 / 50 and 50 Bigfoot FourStroke use?

NGK DPR6EA-9, qty 4. Gap: (1.0 mm).

How often should a Mercury / Mariner 40 / 45 / 50 and 50 Bigfoot FourStroke be serviced?

The factory schedule lists: Every 100 hours of use or once yearly, whichever occurs first; Every 300 hours of use or three 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 / Mariner 40 / 45 / 50 and 50 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 / Mariner 40 / 45 / 50 and 50 Bigfoot FourStroke?

Out of gear: 45 hp 950 ± 25 rpm; 40/50 hp 825 ± 25 rpm. In forward gear: 45 hp 850 ± 25 rpm; 40/50 hp 725 ± 25 rpm. WOT range 5500 to 6000 rpm.

What is the propeller nut torque on a Mercury / Mariner 40 / 45 / 50 and 50 Bigfoot FourStroke?

55 lb-ft (75 Nm); bend the retainer tabs against the nut.

Maintenance specs

Engine oil	Quicksilver 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 meeting API SH, SG, SF, CF-4, CE, CD or CDII
Engine oil quantity	Either 3 US quarts or 3 liters (3.0 L, 3 US qt). it does not give separate with-filter and without-filter amounts
Oil filter	155 lb-in / 12.9 lb-ft (17.5 Nm) in the parts list; the maintenance procedure says screw on until the gasket contacts the base, then 3/4 to 1 turn. Oil filter wrench 91-802653
Engine oil drain plug	210 lb-in / 17.5 lb-ft (23.5 Nm); lubricate the seal on the drain plug with oil
Gear lube	Quicksilver Gear Lube-Premium Blend
Gear lube quantity	Standard gear case: approximately 440 ml (14.9 US fl oz). Bigfoot 2.3:1 gear case: approximately 655 ml (22.5 US fl oz)
Gear lube plugs	Standard 1.83:1 gear case: 1 vent screw (.375-16 x.25) and 1 magnetic drain (fill/drain) screw, each with a sealing washer; the parts list gives 60 lb-in (7.0 Nm), the filling procedure gives 55 lb-in (6.2 Nm). Bigfoot gear case: 3 screws (vent, fill/drain magnetic, oil level vent), 3 sealing washers, 60 lb-in (6.8 Nm). Replace sealing washers if damaged
Gear case leak test pressure	10 to 12 psi (69 to 83 kPa), must hold for 5 minutes; test before adding gear lube
Spark plug	NGK DPR6EA-9, qty 4
Spark plug gap	(1.0 mm)
Idle speed	Out of gear: 45 hp 950 ± 25 rpm; 40/50 hp 825 ± 25 rpm. In forward gear: 45 hp 850 ± 25 rpm; 40/50 hp 725 ± 25 rpm. WOT range 5500 to 6000 rpm
Thermostat	Opens at 136 to 143 °F (58 to 62 °C); full open 158 °F (70 °C); valve lift 0.12 in (3 mm). Exhaust and thermostat cover bolts 53 lb-in (6 Nm) then 106 lb-in (12 Nm)
Anodes	Engine block anodes (four): cover bolts 70 lb-in (8 Nm), install each with its rubber seal and cover. Trim tab bolt: standard gear case 15.8 lb-ft (21.4 Nm); Bigfoot 22 lb-ft (29.8 Nm). Bigfoot side anode nut 60 lb-in (6.8 Nm).
Propeller nut	55 lb-ft (75 Nm); bend the retainer tabs against the nut
Power trim fluid	Quicksilver Power Trim & Steering Fluid, or automotive ATF if not available. Tilt full up on the tilt support lock; level even with the bottom of the fill hole
Valve clearance, cold	Intake 0.15 to 0.25 mm (0.006 to 0.010 in); exhaust 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 of use or once yearly, whichever occurs first

   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 of use or three 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/Mariner 40/45/50/50 Bigfoot service manual. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.