Mercury · Outboard

Mercury 4 / 5 / 6 hp FourStroke

Mercury’s 4, 5 and 6 hp FourStroke, in Europe also sold as Mariner, are one engine: a 123 cc single-cylinder four-stroke with carburettor, made in Japan by Tohatsu and sold in parallel as the Tohatsu MFS4, MFS5 and MFS6. The three ratings share the service parts on this page, and at about 26 kg with a 1 l built-in tank and a shallow-water position they are common on tenders and as auxiliaries on small sailboats. The first generation came in 2000 with the shift lever on the side; from mid-2011 the lever sits at the front and the tiller is longer, and the 5 hp Sail Power adds a long shaft, a high-thrust propeller and a 4 A charging coil. Not covered: the 5 hp Propane version and the 6 hp EFI of 2025, a different 209 cc engine on the 8/9.9 hp EFI platform.

Manuals & documents
Wear parts & part numbers
Service intervals
Specifications
Good to know
Videos & how-tos
On the marketplace

Manuals & documents

The links go straight to Mercury. We host no manuals or drawings — the manufacturer’s copy is the current one.

Operation, maintenance and installation manual 4/5/6 FourStroke (2019, 8M0165090) Operation manual servicelit.mercurymarine.com · PDF · EN
Operation, maintenance and warranty manual 4/5/6 FourStroke, front-shift generation (2011/2012, 90-8M0055686) Operation manual download.brunswick-marine.com · PDF · EN
Operation, maintenance and installation manual 4/5/6 FourStroke (2017, 8M0136713, German) Operation manual download.brunswick-marine.com · PDF · DE
Mercury owner’s manual lookup by serial number Manual portal mercurymarine.com · EN
Brunswick publications: printed parts, owner’s and service manuals for Mercury four-strokes up to 30 hp Manual portal publications.brunswick.com · EN
Owner’s manual Tohatsu MFS4C / MFS5C / MFS6C (EU), the same engine Operation manual tohatsu.com · PDF · EN · FR
Owner’s manual Tohatsu MFS4D / MFS5D / MFS6D (EU, 2026), the current version of the same engine Operation manual tohatsu.com · PDF · EN · DE · ES
Tohatsu maintenance kits: kit 3GR-87500-1 for the MFS4/5/6D with its part numbers Parts catalogue tohatsu.com · EN
Tohatsu propeller chart (choose 4, 5 or 6 HP) Manual portal tohatsu.com · EN

Wear parts & part numbers

Part	OEM number	Also fits	Note
Service kit	`8M0120836`^2,3,4,5,6	Tohatsu `3GR-87500-1`¹	Mercury’s 300-hour kit for the 4/5/6 hp FourStroke from serial 0R411838: fuel filter, oil drain seal, two gear plug seals, thermostat with gasket, spark plug, water pump impeller kit and one anode; oil and gear lube are not included. Some dealers call it “EFI”, but these engines are carburetted. 3GR-87500-1 is Tohatsu’s own kit for the MFS4D/5D/6D twin.
Service kit	`8M0181315`^7,8	—	Smaller Quicksilver kit for the carburetted 4/5/6 hp: spark plug, water pump repair kit, drain plug seals and cotter pin.
Raw water pump impeller	`8M0204676`^10,11,12replaces 47-16154-3	Tohatsu `369-65021-1`¹Sierra `18-3098`⁹	Sold as 47-8M0204676; dealers list it for the 4–6 hp four-strokes from serial 0R411838. It replaces 47-16154-3, the impeller on the 2006 parts diagram of the first generation. 369-65021-1 is the impeller in Tohatsu’s own kit for the twin; the dealer lists the Sierra impeller for 4/5/6 hp four-strokes from 1999.
Water pump repair kit	`8M0155395`^13,14,15,12replaces 16159A03replaces 16159A1	—	Impeller, key, gaskets, wear plate, hardware and the plastic pump housing, for the carburetted 4/5/6 hp from serial 0R411838; worth it when the housing is scored. It supersedes 16159A03; the 2006 diagram of the first generation lists 16159A1, and the housing alone is 826053T there.
Spark plug	`8M0234113`^{16,18,19,20,21}replaces 8M0204682replaces 803507	NGK `DCPR6E`^16,17Tohatsu `9701-1-1103`¹	NGK DCPR6E, gap 0.9 mm (Tohatsu: 0.8–0.9 mm), one per engine. 8M0234113 is the current Quicksilver number; 8M0204682 and, on the 2006 diagram, 803507 are earlier numbers for the same plug.
Fuel filter (engine)	`8M0157133`^22,23,19,24replaces 35-16248	Tohatsu `369-02230-1`¹	In-line filter under the cowl; dealers list it for the 4/5/6 hp four-strokes from 1999. The 2006 diagram shows the earlier 16248. 369-02230-1 is the filter in Tohatsu’s kit for the twin.
Thermostat	`8M0228409`^25,26,21replaces 804540	—	120 °F thermostat for Mercury 2.5–6 hp carburetted engines; it replaces 804540 from the 2006 diagram. Sold without gasket; the 300-hour kit has both.
Anode (lower unit)	`875208`^27,28,29,16	Tohatsu `3H6-60218-0`¹	Square aluminium anode bolted to the gearcase (2006 propeller shaft diagram); sold as 97-875208, dealers list it for Mercury/Mariner 4–9.9 hp. Mercury’s manual names this one gearcase anode. 3H6-60218-0 is the anode in Tohatsu’s kit for the twin.
Anode (engine)	`823912`^30,31,28	—	Small round aluminium anode (97-823912) that dealers list for the carburetted 4/5/6 hp from model year 1999. Where it sits is not shown in an official diagram, and Mercury’s manual only describes the gearcase anode, so compare it with the part on your engine before ordering.
Gear oil	`802844K01`^16,19,32,33	—	Mercury Premium gear lube 80W-90 in a 237 ml (8 oz) tube, enough for one change of about 195 ml; the 946 ml bottle is 858058K01. The manual allows Premium or High Performance gear lube. Fit new seal washers on the plugs.
Propeller	`812949A02`^37,38,27	Tohatsu `3R1B64514-2`^34,35Quicksilver `QA3182R`³⁶	Black Max aluminium, 3 blades, 7.8 × 7 in: per Tohatsu’s chart the standard size on the 4 hp and the heavy-load choice on the 5 and 6 hp. The 2006 diagram lists it as 812949002.
Propeller	`812950A02`^40,41,27	Tohatsu `3R1B64516-2`^34,35Quicksilver `QA3184R`³⁹	Black Max aluminium, 3 blades, 7.8 × 8 in: per Tohatsu’s chart the standard size on the 5 and 6 hp. The 2006 diagram lists it as 812950002. For light boats Tohatsu also lists a 7.9 × 9 in (369B64518-1).
Propeller	`812951A02`^42,43,44,27	Tohatsu `399B64512-1`^34,35	Black Max high-thrust, 3 large blades, 8.4 × 6 in, for sailboats, heavy tenders and pushing; the Sail Power version comes with a high-thrust prop. The 2006 diagram lists it as 812951A1 (8.38 × 6 in). Tohatsu’s chart gives its own 8.3 × 6 in high-thrust prop as standard on the SP models.
Starter rope	`12066K10`^45,46,47	—	Braided nylon starter rope that dealers list for the Mercury/Mariner 2.5–6 hp FourStroke from 1995; the 2006 diagram shows the rope on the recoil starter as 12066Q9.
Flushing attachment	`12612Q2`^48,16	—	Screws into the flushing port on the side of the driveshaft housing in place of the plug and gasket. One source: a dealer lists it for the 4/5/6 hp FourStroke. Mercury’s manual: flush for 3–5 minutes with the engine not running.
Emergency stop switch & lanyard	`823054Q`^49,50	—	Coiled lanyard with fork clip for the stop switch of the 2.5–6 hp four-strokes; the engine will not start without it. The current number comes from one source, a dealer; the 2006 diagram lists the lanyard as 823054.
Propeller shaft grease	`92-802859A1`¹⁶	—	Mercury 2-4-C with PTFE, named in the manual for the propeller shaft, swivel bracket, copilot, transom clamp screws, tiller bushing and shift detent; Extreme Grease 8M0071842 is the alternative.

Numbers checked against the sources below on 12 October 2026. Always confirm against your serial number and exact type before ordering.

Service intervals

Task	Interval
Oil level check	Before every outing: oil level (motor upright), lanyard stop switch, fuel system for leaks, transom clamps, steering and propeller; after starting, the tell-tale stream¹⁶
Fresh-water wash	After every use: rinse the outside with fresh water; after salt, brackish, muddy or polluted water also flush the cooling system for 3–5 min through the flushing port, engine not running¹⁶
Engine oil change	Every 100 h or yearly, more often with a lot of idling, trolling or short trips; no oil filter. Tohatsu’s manual for the same engine adds a first change after 20 h or 1 month^16,17
Gear oil change	Every 100 h or yearly¹⁶
Anode	Inspect every 100 h or yearly, more often in salt water; replace before it is fully eroded, never paint it¹⁶
Lubrication	Every 100 h or yearly: copilot, swivel bracket, transom clamp screws and propeller shaft splines; tiller bushing, shift detent and driveshaft splines by a dealer¹⁶
Fuel filter	Check the in-line filter regularly; replace the filters on the suction side every 100 h or yearly (dealer)¹⁶
Thermostat	Inspect every 100 h or yearly when used in salt or brackish water¹⁶
Spark plug	Replace every 300 h or 3 years; anti-seize on the threads every 100 h or yearly. 2011 manual: replace after the first 100 h or first year, then inspect every 100 h or yearly^16,51
Raw water impeller	Replace every 300 h or 3 years (dealer) per the 2019 manual; the 2011 Mercury manual says every 100 h or yearly, and Tohatsu’s manual for the same engine every 12 months^16,51,17
Valve clearance	Check and adjust every 100 h or yearly (dealer) per the 2011 manual; the 2019 schedule has no entry. Cold: intake 0.06–0.14 mm, exhaust 0.11–0.19 mm^51,16
Carburettor	Once a year: add Quickleen fuel system cleaner to the fuel tank¹⁶
Storage / winterising	Before a lay-up of two months or more: drain fuel that contains ethanol, or fill up with stabilised fuel and run for 10 min; change engine oil and gear lube; store upright or tiller side down, fully lowered¹⁶

Hours or years, whichever comes first. The schedule follows Mercury’s 2019 operation manual for the 4/5/6 FourStroke (8M0165090); “dealer” marks jobs Mercury leaves to a dealer, and the valve clearance comes from the 2011 manual (90-8M0055686). Mind the impeller: the 2019 manual allows 3 years or 300 h, while the 2011 manual and Tohatsu’s manual for the same engine say every year. Running-in per the 2019 manual: first hour up to 2000 rpm (about half throttle), second hour up to 3000 rpm with one minute at full throttle every ten minutes, then for eight hours no more than five minutes at a time at full throttle.

Specifications

Cylinders	1 cylinder, four-stroke, one intake and one exhaust valve; carburettor with choke, recoil start^52,16
Displacement	123 cc (7.5 cu in)^16,17
Bore × stroke	59 × 45 mm^16,17
Power	2.9 kW (4 hp), 3.7 kW (5 hp), 4.4 kW (6 hp) at the propeller shaft^16,17
Full-throttle range	4 and 5 hp: 4500–5500 rpm; 6 hp: 5000–6000 rpm^16,17
Idle speed	1100 rpm in forward gear, 1300 rpm in neutral (Tohatsu’s 2026 manual: 1150 rpm in gear)^16,52
Ignition	Flywheel magneto, CD ignition⁵²
Cooling	Raw water with thermostat; impeller pump in the gearcase; a steady tell-tale stream shows the pump is working^52,16
Lubrication	Wet sump with trochoid pump; warning light for low oil pressure^52,16
Engine oil capacity	approx. 450 ml (15 fl oz), no oil filter; at the add mark, 100 ml brings it back to full^16,17
Engine oil grade	NMMA FC-W SAE 10W-30 (mineral or synthetic blend) for all temperatures, SAE 25W-40 FC-W as an option; full synthetic oil not recommended. Tohatsu: API SH, SJ or SL, SAE 10W-30/40^16,17
Gear oil capacity	approx. 195 ml (6.6 fl oz)^16,17
Gear oil grade	Mercury/Quicksilver Premium or High Performance gear lube; Tohatsu: API GL-5, SAE 80-90^16,17
Gearbox	Forward, neutral, reverse; shift lever at the front from mid-2011, on the side before^16,53
Gear ratio	2.15:1 (13:28)^16,17
Tilt	Manual, 6 trim positions and a shallow-water position; tilting up only in forward gear^17,16
Shaft length	15 in (short) and 20 in (long); some 5 hp versions also 25 in (extra long)¹⁷
Fuel	Unleaded petrol, at least RON 91 (USA/Canada: 87 (R+M)/2); up to 10 % ethanol¹⁶
Fuel tank	Built-in tank approx. 1 l (Tohatsu: 1.15 l) with fuel valve; remote tank optional (Tohatsu: 12 l)^54,17,16
Dry weight	approx. 26 kg (57 lb) for the lightest model; 5 hp Sail Power 27 kg (59 lb)⁵⁴
Alternator	Standard models: none (a 12 V, 5 A charging coil is an option); Sail Power: 4 A (50 W) lighting and charging coil as standard^54,17
Spark plug	NGK DCPR6E, gap 0.9 mm (Tohatsu: 0.8–0.9 mm); 27 N·m (20 lb-ft) or finger-tight plus 1/4 turn^16,17
Valve clearance (cold)	Intake 0.06–0.14 mm, exhaust 0.11–0.19 mm^16,17
Propeller	3-blade aluminium: 7.8 × 7 in standard on the 4 hp, 7.8 × 8 in on the 5 and 6 hp, 8.4 × 6 in high-thrust; held by thrust washer, nut and cotter pin^34,16,27
Years built	First generation from 2000 (shift lever on the side); front-shift generation from mid-2011 (Tohatsu MFS4C/5C/6C, today MFS4D/5D/6D), in production^53,52,54

Good to know

Carry, store and transport the motor only upright or lying on its tiller side. In any other position oil drains out of the crankcase and can damage the engine.¹⁶
Mercury’s current manual asks for a new impeller only every 3 years or 300 h, but the 2011 Mercury manual and Tohatsu’s manual for the same engine say every year. On a sailboat auxiliary in salt water, change it every season or two, carry a spare kit and check the tell-tale stream after every start.^17,51
There are only about 450 ml of oil and no filter: check the level with the motor upright, never tilted, and don’t overfill; at the add mark, 100 ml brings it back up. Long idling, trolling and short trips dilute the oil with fuel and water, so change it sooner if the level rises.¹⁶
Before taking the motor off the boat, close the fuel valve and let it run until it stops, which empties the carburettor. For a lay-up of two months or more, use stabilised fuel or drain fuel that contains ethanol, as it can form acids in storage.¹⁶
Tilting fully up and the shallow-water position only work in forward gear. On a mooring, tilt the motor so the gearcase is out of the water, except in frost, when it stays down; never paint or spray the anode.¹⁶
For a sailboat or a heavy tender, the 8.4 × 6 in high-thrust prop fits all three ratings. After any prop change, check that full-throttle rpm stays within range: 4500–5500 on the 4 and 5 hp, 5000–6000 on the 6 hp.^17,42,34

Videos & how-tos

Links to YouTube, nothing embedded. The maker’s own channel comes first; the rest are channels we found clear and correct for this model.

Changing the water pump impeller on a Mercury/Tohatsu 4/5/6 hp four-stroke (shown on a Mercury 6) Video Skinny River · EN · 25 min
Oil, spark plug and fuel filter change and greasing on a Tohatsu 4/5/6 hp, the same engine Video Cruising Association · EN · 13 min
Gear oil change and propeller shaft greasing on a Tohatsu 4/5/6 hp, the same engine Video Cruising Association · EN · 11 min
Servicing a Mercury F4M: spark plug, engine oil, gear oil, impeller and greasing (German) Video rekurt100 · DE · 10 min

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Sources

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