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Mercruiser Sterndrives: R, MR, Alpha One & Alpha One SS 19831990 Service Manual 90‑12934‑2
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Number 6
90-12934--2 1097
Sterndrive Units
R·MR·Alpha One·
Alpha One SS
SERVICE
MANUAL

90-12934--2 1097 0
90-12934--2
90-12934--2

90-12934--2 1097 i
Models Covered in This Manual
Transom Assemblies
Model Serial Number
MC 120R/140B/185R/488R/898R/228R/260R 6216687 – 6849289
MC 120MR/140MR/170MR/185MR/190MR/200MR/
230MR/260MR/300 Tempest MR
6849290 – 0A471374
MC Alpha One/Alpha One SS 0A471375 and Above
Sterndrive Units
MC 120R-260R
Original Ratio Ratio If Serviced With 13:21 Gears Serial Number
1.98:1 1.94:1 6237861-6854392
1.84:1 1.81:1 6225577-6862701
1.65:1 1.62:1 6268065-6810537
1.50:1 1.47:1 6229158-6847029
1.32:1 1.29:1 6231571-6663947
MC 120 MR - 300 TEMPEST MR
Original Ratio Ratio If Serviced With 13:28 Gears Serial Number
1.98:1 1.94:1 6854393-0A476746
1.84:1 1.81:1 6862702-0A470164
1.65:1 1.62:1 6810538-0A475786
1.50:1 1.47:1 6847030-0A479506
MC ALPHA ONE / ALPHA ONE SS
Original Ratio
Ratio If Serviced With
13:28 Gears
R.H. Serial Number L.H. Serial Number
1.98:1 1.98:1 0A476747 and Above 0C881740 and Above
1.84:1 1.84:1 0A470165 and Above 0C884040 and Above
1.65:1 1.65:1 0A475787 and Above 0C881790 and Above
1.50:1 1.50:1 0A479507 and Above 0C852908 and Above
1.32:1 1.32:1 0A613927 and Above
Alpha One SS
1.50:1/1.32:1
1.50/1.32:1 0A545004 and Above

90-12934--2 1097 ii
Sea Ray Sterndrive Units and Transom Assembly (Oyster White)
Original Ratio Ratio If Serviced With 13:21 Gears Serial Number
1.98:1 0B803866-0C348031
1.84:1 0B800854-0C343526
1.50:1 0B796473-0C340666
1.32:1 0B804807-0C340030
Transom Assembly 0B815811-0C340333
When making repairs to Sea Ray (Oyster White) sterndrives, follow procedures as outlined for corresponding
MerCruiser models as shown in the cross-reference chart below.
Sea Ray MerCruiser
3.0L 3.0L Alpha One
4.3L2 4.3L Alpha One
4.3L4 4.3LX Alpha One
5.7L 5.7L Alpha One
SRX 5.7L 350 Magnum Alpha One
SRX 7.4L 454 Magnum Alpha One

90-12934--2 1097 iii
Notice
Throughout this publication, “Warnings” and “Cau-
tions” (accompanied by the International HAZARD
symbol
!
) are used to alert the mechanic to special
instructions concerning a particular service or opera-
tion that may be hazardous if performed incorrectly
or carelessly. — OBSERVE THEM CAREFULLY!
These “Safety Alerts” alone cannot eliminate the
hazards that they signal. Strict compliance to these
special instructions when performing the service,
plus “common sense” operation, are major accident
prevention measures.
DANGER
!
DANGER - Immediate hazards which will result in
severe personal injury or death.
!
WARNING
WARNING - Hazards or unsafe practices which
COULD result in severe personal injury or death.
! CAUTION
CAUTION - Hazards or unsafe practices which
could result in minor personal injury or product
or property damage.
IMPORTANT: Indicates information or instruc-
tions that are necessary for proper operation
and/or maintenance.
Notice To Users of This Manual
We could not possibly know of and advise the service
trade of all conceivable procedures by which a
service might be performed and of all possible
hazards and/or result of each method. We have not
undertaken any such wise evaluation. Therefore,
anyone who uses a service procedure and/or tool
which is not recommended by the manufacturer, first
must completely satisfy himself that neither his nor
the product’s safety will be endangered by the
service procedure selected.
All information, illustrations, and specifications con-
tained in this manual are based on the latest product
information available at time of publication.
As required, revisions to this manual will be sent to
all dealers contracted by us to sell and/or service
these products.

90-12934--2 1097 iv
Replacement Parts
!
WARNING
Electrical system components on gasoline
engines and MerCruiser Stern Drives are
designed and manufactured to comply with U.S.
Coast Guard Rules and Regulations to minimize
risks of fire or explosion.
Use of replacement electrical system compo-
nents, which do not comply to these rules and
regulations, could result in a fire or explosion
hazard and should be avoided.
When servicing electrical systems, it is extremely
important that all components are properly installed
and tightened. If not, any electrical component
opening would permit sparks to ignite fuel vapors
from fuel system leaks, if they exist or develop.
!
WARNING
Electrical system components on diesel engines
are not external ignition protected. DO NOT
STORE OR UTILIZE GASOLINE ON BOATS
EQUIPPED WITH THESE ENGINES, UNLESS
PROVISIONS HAVE BEEN MADE TO EXCLUDE
GASOLINE VAPORS FROM ENGINE COMPART-
MENT (ref: 33 CFR). Failure to comply could result
in fire, explosion and/or severe personal injury.

Important
Information
Removal, Installation
and Adjustments
Sterndrive Unit
Transom Assembly
Power Trim
Steering Systems
Corrosion Protection
1
2
3
4
5
6
7
90-12934--2 1097 v
Service Manual Outline
Section 1 - Important Information
A - Important Information
B - Maintenance
C - Troubleshooting
Section 2 - Removal, Installation and Adjustments
A - All Models
Section 3 - Sterndrive Unit
A - Drive Shaft Housing
B - Gear Housing - MR/Alpha One/Alpha One SS
C - Gear Housing - I-R
D - Gear Housing - Alpha One Counter Rotation
Section 4 - Transom Assembly
A - Service Procedures Requiring Minor
Disassembly
B - Service Procedures Requiring Major
Disassembly
Section 5 - Power Trim
A - Oildyne Power Trim Pump (Plastic Reservoir)
B - Oildyne Power Trim Pump (Aluminum Reservoir)
C - Prestolite Power Trim Pump
D - Trim Cylinders
E - Dual Power Trim Control
F - Auto Trim II
Section 6 - Steering Systems
A - Power Steering
B - Manual Steering
Section 7 - Corrosion Protection
A - All Models

GENERAL INFORMATION
IMPORTANT INFORMATION
A
1
23253

90-12934--2 1097
Table of Contents
Page
How To Use This Manual 1A-1 . . . . . . . . . . . . . . . . . .
Page Numbering 1A-1 . . . . . . . . . . . . . . . . . . . . . .
Introduction 1A-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Special Product Information 1A-1 . . . . . . . . . . . . . . .
Directional References 1A-1 . . . . . . . . . . . . . . . . . . . .
Propeller Rotation 1A-1 . . . . . . . . . . . . . . . . . . . . . . . .
Serial Number Locations 1A-2 . . . . . . . . . . . . . . . . . .
Hi-Performance Boating 1A-2 . . . . . . . . . . . . . . . . . . .
Sterndrive Unit 10-Hour Break-In Period 1A-2 . . . .
Decal Application 1A-2 . . . . . . . . . . . . . . . . . . . . . . . . .
Decal Removal 1A-2 . . . . . . . . . . . . . . . . . . . . . . . .
Temperature 1A-2 . . . . . . . . . . . . . . . . . . . . . . . .
Surface Preparation 1A-2 . . . . . . . . . . . . . . . . .
Decal Application 1A-2 . . . . . . . . . . . . . . . . . . . . . .
Painting Procedures 1A-3 . . . . . . . . . . . . . . . . . . . . . .
Cleaning & Painting Aluminum Propellers
& Gear Housings 1A-3 . . . . . . . . . . . . . . . . . . . . . .
Propellers 1A-3 . . . . . . . . . . . . . . . . . . . . . . . . . .
Gear Housings 1A-3 . . . . . . . . . . . . . . . . . . . . .

IMPORTANT INFORMATION - 1A-1 90-12934--2 1097
How To Use This Manual
This Manual is divided into sections which represent
major components and systems.
Some sections are further divided into parts which
more fully describe the component.
Sections and parts are listed on page.
Page Numbering
Two number groups appear at the bottom of each
page. Following is an example and description.
Introduction
This comprehensive overhaul and repair manual is
designed as a service guide for the MerCruiser
models previously listed. It provides specific
information, including procedures for disassembly,
inspection, assembly and adjustment, to enable
dealers and service mechanics to repair these
products.
Before attempting repairs, it is suggested that proce-
dure first be read through to gain knowledge of the
methods and tools used and the cautions and
warnings required for safety.
Special Product Information
During production of these models, special product
improvements and changes have been made to
increase product reliability and performance. Such
changes to a sterndrive assembly component(s) are
covered in the “Special Information” portion of the
appropriate sterndrive assembly section. (Refer to
the section “Index”.) Serial number breaks are
provided, where applicable, for ease of identification.
Directional References
Front of boat is bow; rear is stern. Starboard side is
right side; port side is left side. In this service manual,
all directional references are given as they appear
when viewing boat from stern, looking toward bow.
Propeller Rotation
Propeller rotation for sterndrive can be right hand or
left hand rotation as viewed from the aft end of the
propeller.
22266
22267
Right Hand Rotation Left Hand Rotation
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Full Search Facility
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$35.99
$46.99
This is a factory service manual covering Mercruiser Sterndrives: R, MR, Alpha One & Alpha One SS from 1983 to 1990 with model numbers 90-12934-2. It encompasses all aspects of service and repair for your Mercruiser engine or Stern Drive unit. The manual is fully indexed and bookmarked for ease of use, making it useful for both professional mechanics and DIY enthusiasts.