1986 MerCruiser MCM 120-260 Stern Drive Service Manual No. 4 (P/N 90-86137 286)

This 1986 MerCruiser factory Service Manual No. 4 from Brunswick Corporation (cover reference 90-86137 286) covers I-Drive stern drive units and transom assemblies for MCM 120, 140, 165, 470, 485, 898, 228 and 260. It documents component repair and adjustment within the serial-number coverage listed above.

• Drive unit overhaul: Drive shaft housing and gear housing disassembly, gear shimming, bearing inspection, universal-joint cross bearings, propeller shafts and water pump impellers.

• Transom assemblies: Gimbal bearing and gimbal ring service, bell housing work, universal-joint bellows replacement, exhaust elbows and water shutters.

• Shift and trim adjustments: Shift cables, reverse lock valve adjustment, shift cutout switches, trim limit switches and trim position senders.

• Power trim: Hydraulic pressure testing, pump and cylinder repair, filling and bleeding, electrical tests and single- and dual-solenoid wiring diagrams.

• Power steering: Pump, control valve and booster cylinder diagnosis, seal replacement, cable guide tube service and control valve balancing.

• Maintenance and troubleshooting: Lubrication schedules, corrosion protection, anodic plates, storage procedures and symptom-based troubleshooting charts.

Written for Mercury dealer mechanics and company service personnel, this workshop manual assumes familiarity with the product, appropriate service training and the specified tools.

Drive Unit and Transom Assembly Procedures

Exploded assembly views accompany drive shaft housing and gear housing disassembly, inspection, shimming and reassembly. Model-specific tool positions and gear identification charts support selection of the applicable shimming setup.

Transom instructions include gimbal ring replacement with the engine and transom plate installed, together with access-hole templates. Steering and shift cable requirements, directional references and adjustment illustrations identify the relevant mounting relationships.

Engine alignment uses coupler alignment tool C-91-48247. Engine removal, installation and engine-specific fuel or ignition diagnosis are referred to the separate engine service manual.

The manual's safety and pressure-relief precautions apply to hydraulic and gear work. Exposed intake and exhaust openings must be protected from foreign material during service.

Power Trim and Steering Diagnosis

The power trim chapter distinguishes metal- and plastic-reservoir pumps, external and integral control valves, and cylinders with internal or external return lines. Pressure-test procedures must match the pump style, including installations combining an older pump with newer cylinders.

Hydraulic circuit diagrams and separate trim-cylinder and pump tests help distinguish an internally leaking cylinder from poor pump output or a trail-out valve fault. Reverse lock valve checks, solenoid tests and trim-motor inspection are also covered.

Service Bulletin 85-10 describes Prestolite trim pump valve bodies as uneconomical to repair, with no replacement parts offered; authorized stations can repair the electric motor.

Power steering procedures cover system, pump and booster-cylinder pressure tests, control-valve overhaul and balancing, and pump inspection and reassembly. Filling and bleeding instructions address air in the steering system.

Troubleshooting Charts and Service Bulletins

Reference material includes lubrication and maintenance schedules, torque and special-service-tool tables, steering-cable travel diagrams and cavitation-burn illustrations. Troubleshooting charts address excessive engine RPM, cavitation, gear housing noise, steering and power trim.

Service bulletins add seawater pickup pump flow diagnosis, propeller/thrust-hub compatibility, shift-cable retrofit tooling and drive shaft housing gear identification. They also update trim-cylinder replacement and grounding requirements.

The gimbal bearing and cartridge must remain a matched assembly. The gimbal bearing bulletin explains how incorrect installation can cause U-joint-like knocking or lubrication failure.

Selected Gear Backlash and Fastener Specifications

These examples represent a small selection of the manual's gear-adjustment and assembly specifications.

For gear housing backlash checks at the “I” mark on indicator rod C-91-53459, the forward-gear specification is 0.010–0.012 in. with the 20/33-tooth pinion/forward-gear set. Reverse-gear backlash is 0.040–0.060 in. using the specified reverse-gear test setup.

The bell housing-to-drive shaft housing stud nuts are specified at 50 ft. lbs.; the bearing carrier retainer-to-gear housing nut is specified at 150 ft. lbs.

Bookmarked PDF and Included Pages

Search, navigation and diagram detail: The scanned manual text is not searchable, but PDF bookmarks link to model coverage and the service sections. Small wiring labels and terminal numbers are readable when enlarged; the dual-trim wiring diagram is scanned sideways.

Supplied as one printable PDF with 230 pages: 6 front-matter pages, 192 pages covering Sections 1–6, and 32 pages in the Service Bulletins section. These totals already include five blank or nearly blank pages and four repeated pages.