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Mercury 6 / 8 / 9.8 / 9.9 two-stroke (169 cc, international) service guide

Oil, gear oil, spark plugs and the service schedule for the Mercury 6 / 8 / 9.8 / 9.9 two-stroke (169 cc, international) (6 to 9.9 hp, two-stroke), from the factory service manual, plus how we service it in Hawaiʻi.

Models covered: 6 hp (4.4 kW), 8 hp (5.9 kW), 9.8 hp (7.2 kW) and 9.9 hp (7.3 kW) two-stroke twins, serial 0N055110 and above (cover): 2 cylinders, 169 cc (10.3 cu in), 50 x 43 mm, premixed gasoline, thermostat controlled. Models: 6, 8, 9.8, 9.9 MH; 8, 9.8, 9.9 EH (electric start, tiller); 8, 9.8, 9.9 E (electric start, remote). Clamp brackets change at serial 0N096909 and below / 0N096910 and above (M10 clamp screw, then 0.5 in.).

Quick answers

What two-stroke oil does a Mercury 6 / 8 / 9.8 / 9.9 two-stroke use?
Mercury/Quicksilver or recommended 2-stroke engine oil (TC-W3); do not mix brands.
What is the fuel to oil ratio for a Mercury 6 / 8 / 9.8 / 9.9 two-stroke?
50:1. Break-in: 25:1 in the first tank of fuel, break-in period 10 hours. Unleaded regular, pump posted 87 octane (91 RON).
What gear oil does a Mercury 6 / 8 / 9.8 / 9.9 two-stroke use, and how much?
Mercury/Quicksilver gear oil API GL-5, SAE #80-90 (Premium Gear Lubricant). Quantity: Approximately 320 cc (10.8 fl oz); fill at the fill hole until it flows from the vent hole with no air bubbles, housing vertical.
What spark plug does a Mercury 6 / 8 / 9.8 / 9.9 two-stroke use?
NGK BPR7HS-10 or Champion RL82YC10, qty 2. Gap: 0.9 to 1.0 mm (0.035 to 0.039 in.).
How often should a Mercury 6 / 8 / 9.8 / 9.9 two-stroke be serviced?
The factory schedule lists: Every 50 hours (freshwater; every 25 hours in saltwater); 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 6 / 8 / 9.8 / 9.9 two-stroke?
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 6 / 8 / 9.8 / 9.9 two-stroke?
950 RPM in neutral, 750 RPM in forward gear.
What is the propeller nut torque on a Mercury 6 / 8 / 9.8 / 9.9 two-stroke?
Grease the shaft with Extreme Grease or 2-4-C with PTFE.

Maintenance specs

Two-stroke engine oilMercury/Quicksilver or recommended 2-stroke engine oil (TC-W3); do not mix brands
Fuel to oil ratio50:1. Break-in: 25:1 in the first tank of fuel, break-in period 10 hours. Unleaded regular, pump posted 87 octane (91 RON)
Gear oilMercury/Quicksilver gear oil API GL-5, SAE #80-90 (Premium Gear Lubricant)
Gear oil quantityApproximately 320 cc (10.8 fl oz); fill at the fill hole until it flows from the vent hole with no air bubbles, housing vertical
Gear oil fill and vent screwsLower fill (drain) screw and upper vent screw, the two oil plugs, new gaskets: 6.7 N·m (59.3 lb-in). Vent screw first, then quickly the fill screw. A third plug on the housing is the water plug
Spark plugNGK BPR7HS-10 or Champion RL82YC10, qty 2
Spark plug gap0.9 to 1.0 mm (0.035 to 0.039 in.)
Spark plug torque27 N·m (20 lb-ft), or finger tight and then 1/4 turn
Idle speed950 RPM in neutral, 750 RPM in forward gear
ThermostatOpens at 52 °C (125.6 °F). The cylinder head must be removed to take the thermostat out
AnodesAnode/trim tab under the anti-ventilation plate, screw 5.6 N·m (50 lb-in), and one on the mount bracket; replace when eroded more than 2/3, never grease or paint; retighten the anode bolt at each inspection. An anode screw is also on the exhaust cover
Propeller nutGrease the shaft with Extreme Grease or 2-4-C with PTFE
Power trim and tiltNone: manual trim, 6 positions

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 50 hours (freshwater; every 25 hours in saltwater)

    Replace: gear case plug gaskets

  2. Every 100 hours or once a year

    Replace: gear case oil, gear case plug gaskets, propeller cotter pin

    Check: thermostat, anodes

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

    Replace: gear case oil, gear case plug gaskets, propeller cotter pin, water pump impeller

    Check: thermostat, anodes

  4. Every 300 hours or 3 years

    Replace: spark plugs, gear case oil, gear case plug gaskets, propeller cotter pin, water pump impeller

    Check: 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 6/8/9.8/9.9 HP 2-Stroke Outboard service manual, 90-8M0131801, November 2017. 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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