MerCruiser · Sterndrive

MerCruiser Alpha One

The Alpha One is MerCruiser’s single-propeller sterndrive leg, fitted behind most 3.0 to 5.7 litre petrol engines in runabouts, bowriders and small cruisers. Generation I (MR / Alpha One, drive serial numbers up to 0D469858) was built from about 1983 to 1990. Generation II followed for 1991 from serial 0D469859 with a different water pump, bellows and anode layout and a gear lube monitor bottle; it is still in Mercury’s range. The heavier Bravo drives cover the bigger engines.

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

Manuals & documents

The links go to MerCruiser’s own documents — some as archived copies of pages the maker has since taken down. We host no manuals or drawings.

Operation manual carbureted engines, Alpha/Bravo models (2018, 90-8M0149738) Operation manual servicelit.mercurymarine.com · PDF · EN
Operation manual 4.3/5.0 MPI and SeaCore models with ECT (2016, 90-8M0113859) Operation manual servicelit.mercurymarine.com · PDF · EN
Operation manual 4.5L/6.2L MPI (2018, 90-8M0145503) Operation manual servicelit.mercurymarine.com · PDF · EN
Owner’s manual carbureted engines, Alpha/Bravo models (2003, 90-860171032) Operation manual servicelit.mercurymarine.com · PDF · EN
Owner’s manual petrol engines, Alpha models (1996, 90-860168970) Operation manual servicelit.mercurymarine.com · PDF · EN
Owner’s manual 4.3L/4.3LX Alpha (Alpha V6), 1993/94: first Gen II years, with drive parts list (90-821071951) Operation manual servicelit.mercurymarine.com · PDF · EN
Owner’s manual Alpha One V6/V8 models, 1989: Gen I era, scanned (90-12546891) Operation manual servicelit.mercurymarine.com · PDF · EN
Mercury Marine literature portal: owner’s manuals by serial number or model (MerCruiser, group ALPHA); also lists the printed service manuals #6 R/MR/Alpha One (12934-2) and #14 Alpha One Gen II (8M0065951) Manual portal mnappsp.mercurymarine.com · EN
Brunswick publications store: printed MerCruiser service manuals Manual portal publications.brunswick.com · EN
Mercury Alpha One drive product page (archived July 2025) Archived maker page web.archive.org · EN

Wear parts & part numbers

Part	OEM number	Also fits	Note
Raw water pump impeller	`47-43026T2`^1,2,3,4,5replaces 47-430262	—	Gen II only, drive s/n 0D469859 and up (also Vazer). Mercury’s 1993 owner’s manual lists the older number; the Quicksilver application chart gives 47-43026T2 for Gen II.
Impeller kit (with gasket)	`47-8M0100526`^4,6,7replaces 47-43026Q06	—	Gen II only, s/n 0D469859 and up: impeller, key, face plate, gaskets, O-rings and seals, without the upper pump housing. Also sold as 8M0100526 without the prefix.
Water pump repair kit	`817275Q05`^4,9,10replaces 817275Q4replaces 817275A4	Sierra `18-3147`⁸	Gen II, s/n 0D469859 and up (also Vazer): complete upper pump with housing, impeller, key, gaskets, O-ring, plates and water guide fittings. Mercury-branded version: 817275K05.
Raw water pump impeller	`47-89984T4`^4,11,12	—	Gen I only: MR / Alpha One up to s/n 0D469858 (the Quicksilver chart starts the MR at 6854393). Short-vane impeller, the same as on the older MC-I and many Mercury outboards.
Impeller kit (with gasket)	`47-89984Q5`^4,13,14,11replaces 89984T6	—	Gen I: MerCruiser I, R, MR and Alpha with the short-vane impeller, up to s/n 0D469858. Crowley’s Gen I diagram shows the Mercury-branded 89984T5 in the same place.
Water pump repair kit	`46-96148A8`^4,15,11	—	Gen I: MerCruiser I, R, MR and Alpha, s/n 2491586 to 0D469858. Upper kit with plastic pump housing, impeller, face plate, gaskets, key, face seal, seal carrier and water tube coupling.
Water pump base	`46-44292A3`^4,16,11	—	Gen I MR / Alpha One only: the lower half (base) of the drive’s water pump. With the upper kit 46-96148A8 it makes a complete pump.
Service kit	`8M0147073`^17,18	—	Gen II, 1991 on, standard and counter rotation: Mercury’s 300-hour kit with drive gasket/O-ring kit, aluminium anode kit 888756K01, drain plug seals, transom (bellows) kit 30-803099T1, shift cable 865436A02, water pump kit 47-8M0100526 and drain/fill screws.
Service kit	`8M0147072`^19,20	—	Gen I Alpha One, 1984–1990: the same scope for the older drive, with drive gaskets, aluminium anodes, drain seals, transom (bellows) kit, shift cable, impeller kit and fill/drain screws.
Transom seal kit (bellows)	`30-803099T1`^22,23,24	Sierra `18-8218`²¹	Transom seal (bellows) kit for Gen II, 1991 on, s/n 0D469859 and up, with exhaust bellows: U-joint bellows 816431A1, exhaust bellows 18654A1, gimbal bearing 30-862540A3 (now 30-879194A01), shift cable bellows 74639A2, water hose 32-4172533 (now 32-8M0062309) and drive gasket kit 27-94996Q2.
Transom seal kit (bellows)	`30-803097T1`^25,26	—	Transom seal (bellows) kit for Gen I: R, MR and Alpha One 1983–1990, s/n 6225577 to 0D469858. Gen I drives with an exhaust tube take 30-803098T1 instead.
U-joint bellows	`816431A1`^28,29,30	Sierra `18-2747`²⁷	U-joint (drive shaft) bellows for Gen II, 1991 on, s/n 0D469859 and up (also Vazer), clamps included.
U-joint bellows	`60932A4`^26,31replaces 60932A1	Sierra `18-2751`^31,32	U-joint bellows for Gen I: MC-I, R, MR and Alpha One up to 1990 (s/n up to 0D469858).
Exhaust bellows	`8M0124227`^26,28,33replaces 18654A1	—	The same part on Gen I and Gen II (also MC-I, R, Bravo, Blackhawk).
Shift cable bellows	`74639A2`^34,35,36	—	Shift cable bellows for Gen I (s/n 6225577 to 0D469858) and Gen II (0D469859 and up). Quicksilver kit version: 74639Q02.
Gimbal bearing	`30-879194A01`^37,38,24replaces 30-862540A3	—	Gimbal bearing for Gen II from 1998 (Crowley’s diagram for s/n 0L100040 and up), also Bravo and Vazer transoms; the sealed 30-879194A01 replaces 30-862540A3. Included in transom kit 30-803099T1.
Gimbal bearing	`30-60794A4`^35,36,39	—	Gimbal bearing on Gen I (up to s/n 0D469858) and early Gen II (0D469859–0F679999). One source: Crowley’s diagrams, where it is marked no longer available; a successor number could not be confirmed.
Shift cable	`8M0176525`^40,41replaces 865436A03	—	Lower shift cable kit for Gen I and Gen II (MC-I, R, MR, Alpha One, Alpha One Gen II): cable, O-rings, gaskets, connectors and bellows. The Gen II 300-hour kit carries it as 865436A02; old Gen I diagrams show 19543T1/19543A8, no longer available.
Gasket set	`27-94996T2`^42,26,24,1	—	Drive (bell housing) mounting gaskets for R, MR, Alpha One and Gen II from s/n 6225577; Quicksilver version 27-94996Q2. Mercury’s 1993 manual lists the Gen II drive gasket 27-64818A3 and O-ring 27-35982 of that time.
Lower unit seal kit	`26-816575A3`^5,43,44	—	Gear housing seal kit for Gen II: s/n 0D469859–0F679999 (1991–1995) and 0L100040 and up (1998 on).
Drive shaft housing seal kit	`26-88397A1`^45,46	—	Drive shaft housing (upper) seal kit for Gen II. The two sources disagree on the range: Crowley shows it from s/n 0L100040, Wholesale Marine only for 2004 and newer drives. Check against your serial number.
Trim cylinder seal kit	`87399A3`^48,49,50	Sierra `18-2429`⁴⁷	Trim cylinder overhaul kit for Gen II cylinders up to s/n 2A480243. Gen I diagrams show 87399A2 instead, and the O-ring-only kit 87400A2 is listed for R, MR, Alpha One, Gen II and Bravo cylinders (both from one source, Crowley).
Anode (lower unit)	`888756Q03`^52,53replaces 888756A1replaces 888756Q01	Sierra `18-6158A`⁵¹	Aluminium anode kit for Gen II, 1991 on; fresh, brackish or salt water. Six pieces: trim tab, gimbal anode, drive shaft housing plate, two trim cylinder anodes and the bearing carrier ring. Mercury-branded version: 888756K01.
Anode (lower unit)	`888755Q03`^54,55	—	Magnesium anode kit for Gen II, 1991 on, fresh water only. Same positions as the aluminium kit.
Anode (lower unit)	`888755Q04`⁵⁶	—	Magnesium anode kit for Gen I MR and Alpha One, 1984–1990 (from s/n 6225577), fresh water only: trim tab, gimbal nut anodes and drive shaft plate. One source; the same dealer also lists an aluminium Gen I kit, 888756Q04, with less detail.
Ring anode	`806105Q1`^1,57,58replaces 806105A1	—	Bearing carrier anode, the ring in front of the propeller, for Gen II (and Vazer); current Quicksilver aluminium part. Mercury’s 1993 manual lists 806105A1, now no longer available.
Anode (lower unit)	`806189T1`^1,59,60replaces 806189A1	—	Trim cylinder anodes for Gen II, one per cylinder, so two per drive. Current aluminium version of 806189A1 from Mercury’s 1993 manual.
Anode (lower unit)	`821631A1`^1,36,38	—	Anodic block under the gimbal housing, Gen II (boats with MerCathode have the electrode there instead). No longer available on its own; it comes with the anode kits above.
Anode (lower unit)	`34127T2`^61,62	—	Zinc trim tab for Gen I (R, MR and Alpha One, up to s/n 0D469858); on these drives the trim tab is the main sacrificial anode. One source (Crowley).
Propeller nut	`11-52707A1`^1,62,63	—	Propeller nut (.750-16) for Gen I and Gen II. Also sold as nut and tab washer kit 11-52707Q1.
Propeller tab washer	`14-816629`^1,58,62	—	Gen I and Gen II; replace it when a tab is cracked. The splined washer behind the propeller is 12-31211A2 (1993 manual).
Drain plug	`10-14834`¹	—	Gear lube drain/fill screw for Gen II per Mercury’s 1993 parts list, with sealing washer 19183; the vent screw is 10-79953 with O-ring 12-19183.
Gear oil	`92-858064K01`^64,65,66,1replaces 92-802854A1replaces 92-816026A1	—	Mercury High Performance Gear Lubricant for Gen I and Gen II drives; number as printed in the 2016 and 2018 manuals (2003 manual: 92-802854A1, 1993 manual: 92-816026A1).
U-joint & coupler grease	`8M0071842`⁶⁵	—	Mercury’s grease for the gimbal bearing (greaseable type), drive shaft splines, U-joint O-rings and the engine coupler (2018 manual).
Propeller shaft grease	`8M0071838`⁶⁵	—	Anti-corrosion grease for the propeller shaft; the same manual also allows Special Lubricant 101 (802859Q1) or 2-4-C with PTFE (92-802859A1).
Flushing attachment	`91-44357Q2`^64,65,67	—	Flushing attachment (earmuffs) for drives with side water pickups; drives with dual (low) pickups also need flush seal kit 91-881150K1 to block the front inlets.

Numbers checked against the sources below on 13 October 2026. Always confirm against your serial number and exact type before ordering.

Service intervals

Task	Interval
Oil level check	Before each day’s use, at the gear lube monitor (1990s manuals: weekly)⁶⁴
Gear oil change	Every 100 h or yearly, first time after 20 h⁶⁵
Water pump	Every 300 h or 3 years: take the drive’s seawater pump apart, check it and replace worn parts; also whenever cooling flow seems low⁶⁴
Anode	Inspect weekly; replace at 50 % erosion⁶⁴
Propeller	Every 2 months or 50 h (fresh water only: every 4 months) take the propeller off, grease the shaft and re-torque the nut; check the nut after the first use, then every 20 h^64,65
Bellows & U-joints	Inspect U-joints, splines, bellows and clamps every 300 h or 3 years (1990s manuals: bellows and clamps yearly)⁶⁴
Gimbal bearing	Check for roughness every 300 h or 3 years; a greaseable bearing gets 8–10 pumps of grease (1990s schedule: every 100 h or yearly)⁶⁴
Re-torque gimbal ring	Every 100 h or yearly⁶⁴
Engine coupler splines	Grease every 300 h or 3 years, every 150 h if the boat idles for long periods⁶⁵
Engine alignment	Check engine alignment every 300 h or 3 years⁶⁴
Cooling system	Flush after every use in salt, brackish or polluted water⁶⁴

From Mercury’s operation manuals for MerCruiser petrol engines with Alpha drives (2016 and 2018 editions); hours or time, whichever comes first. Where the owner’s manuals of the 1990s differ, their value is in brackets. The scanned Gen I chart in the 1989 manual could not be read reliably, so it is not given separately. The gimbal ring re-torque (every 100 h or yearly) is listed under tightening torques.

Specifications

Versions	Gen I (MR / Alpha One): drive s/n up to 0D469858. Gen II: s/n 0D469859 and up (model year 1991 on). Water pump, bellows and anode parts differ between the two. The drive serial number is on the port side of the drive; the transom serial is on the transom assembly decal and stamped on the gimbal housing^4,67
Years built	Gen I about 1983–1990, Gen II from 1991 to today²⁵
Gear ratio	Standard and counter rotation. Gen I (1988–1990 catalogue): 1.32 / 1.50 / 1.65 / 1.84 / 1.98:1. Gen II 1991–1995: 1.32 (until 1992) / 1.50 / 1.65 / 1.84 / 1.98 / 2.40:1 (from 1992). Later Gen II: 1.47 / 1.62 / 1.81 / 1.94 / 2.00 / 2.40:1⁶⁸
Gear oil capacity	Gen II: about 1892 ml (64 US fl oz) including the gear lube monitor. Gen I without monitor (1989 manual): about 946 ml (32 US fl oz)^65,69
Gear oil grade	Mercury/Quicksilver High Performance Gear Lubricant⁶⁵
Tightening torques	Propeller nut: at least 75 Nm (55 lb-ft), check again after the first use. Gimbal ring clamping screws (gimbal ring to steering shaft): 67–74 Nm (50–55 lb-ft), re-torque every 100 h or yearly^65,1,64
Engine options	Petrol 4.3L V6, 5.0L and 5.7L V8, 350 MAG MPI and 4.5L MPI (200/250 hp) packages, among others⁶⁸

Good to know

Find out the generation before you order: the serial number is on the port side of the drive. Gen II starts at 0D469859; water pump, bellows, anodes and gimbal bearing are different parts on Gen I.^4,67
Look at the gear lube monitor bottle before each start. Water at the bottom, milky oil, or needing more than about 60 ml to top it up usually means a leaking seal: have it checked before the gears suffer.⁶⁵
To change the drive oil, trim the drive fully out and drain it, then bring the propeller shaft level and pump new oil in through the bottom drain hole until it comes out of the vent hole without bubbles. A Gen II drive takes about 1.9 l including the monitor, an older Gen I without monitor only about 0.95 l.^65,69
Store the boat with the drive fully down. A U-joint bellows left folded for months can take a set and split when the drive is lowered again, and then water gets into the boat.⁶⁵
New bellows mean pulling the drive. Mercury’s Gen II transom kit 30-803099T1 already contains the gimbal bearing, the U-joint, exhaust and shift cable bellows and the drive gaskets, so doing them all at once saves a second job. Check engine alignment when the drive goes back on.²⁴
Never paint the anodes, and replace them once they are half gone. Aluminium kits suit any water, magnesium kits only fresh water.⁶⁵

On the marketplace

Browse the category on boatparts.app

Looking for a part for the MerCruiser Alpha One? Post a wanted ad in the app — it is free, it sits on the live map, and boaters nearby who have the part aboard can answer.

Got spares you no longer need? List them in a minute. No commission, ever.

Get the app

Sources

MerCruiser is a trademark of its owner; boatparts.app is not affiliated with it. Part numbers are quoted for identification only. Spotted an error? Write to mail@boatparts.app.