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newSchoolWorks.com › VoTechWorks › Transportation › Marine Engine / Outboard › Inboard and Sterndrive Systems › Lesson 6
TRN-MARI21-03 · Lesson 6 · 17 clock hours · lab

Sterndrive Disassembly and Gear Case Overhaul

Objective. Students will disassemble a sterndrive upper and lower gear case, measure bearings and gears to specification, and reassemble with correct preload and torque.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Sterndrive Gear Case Overhaul: Overview

A sterndrive unit (also called an inboard/outboard, or I/O) transfers engine power from an inboard-mounted engine through a transom plate, then down through an upper gear case and into a lower gear case where bevel gears redirect torque 90 degrees to spin the propeller shaft. When a sterndrive develops noise, vibration, or water intrusion, a complete gear case overhaul is often required. This lab teaches the disciplined disassembly, measurement, and reassembly sequence that produces a reliable, leak-free result.

Safety Before You Touch a Tool

Sterndrive overhaul involves heavy castings, sharp gear teeth, and pressed bearings under tension. Follow these rules every time:

Disassembly Sequence and Documentation

Service manuals sequence disassembly for a reason: internal snap rings must come out before housings can separate, and shims must be kept in their exact location for reference during reassembly. Work from the propeller shaft end inward:

1. Remove the propeller, propeller shaft nut, and thrust hub.

2. Drain gear lube and note its condition. Milky lube indicates water intrusion; metallic particles indicate gear or bearing wear.

3. Remove the gear case from the drive shaft housing by unthreading the mounting bolts in the sequence shown in the manual.

4. Disassemble shift mechanism components, retaining springs and detent balls in a labeled container.

5. Press or pull bearing carriers, then remove the propeller shaft and forward/reverse bevel gears.

6. Remove the pinion gear from the drive shaft.

7. Lay every part on a numbered bench mat as you go. Photograph each stage. This visual record becomes your reassembly guide and documents the condition of parts before cleaning.

Measuring for Wear and Setting Shims

Clean all metal parts in parts-washing solvent before measuring. Measurement is the heart of gear case overhaul.

Bearing races and cups: Inspect visually for spalling, pitting, brinelling (flat spots from shock loading), and heat discoloration. Then use a telescoping gauge and outside micrometer, or a bore micrometer, to measure bearing bore diameters. Compare to service manual specifications. Record actual measurements and the service limit on your spec sheet.

Gear backlash: Backlash is the small amount of free rotational movement between mating bevel gears. Too little causes noise and heat; too much causes rough engagement and wear. Backlash is adjusted by selecting the correct shim thickness behind the pinion gear bearing. The service manual lists the target backlash range (typically measured in thousandths of an inch) and a shim selection table. Measure existing shim thickness with a micrometer before discarding shims.

Preload: Tapered roller bearings require a specific axial load (preload) to function correctly. Preload is set by shim thickness or bearing spacer selection. When reassembling, rotating torque (measured with an inch-pound torque wrench or a beam-type wrench on the shaft) must fall within the manual's specification before the gear case is sealed.

Feeler gauges: Use feeler gauges to measure clearances such as shift fork-to-gear groove clearance, and to verify shim stack thickness when individual shims are being combined.

Reassembly Best Practices

Connecting to Your Credentials

ABYC Marine Engine standards address proper service procedures for marine drive systems. Yamaha and Mercury factory certification programs test technicians on exactly the measurement and shim-selection skills practiced in this lab. Accurate documentation—your spec sheet and photo record—is evidence of professional work quality that both credential programs emphasize.

Key terms8

BacklashThe small rotational clearance between mating bevel gears, measured in thousandths of an inch; adjusted by shim thickness to keep gears quiet and durable.
PreloadA controlled axial load applied to tapered roller bearings during assembly to eliminate internal clearance and extend bearing life; verified by measuring rotating torque on the shaft.
Shim packA stack of precisely sized thin metal washers used to set the position of a gear or bearing to achieve correct backlash and preload.
Bevel gearA cone-shaped gear that meshes with another bevel gear to redirect rotational force 90 degrees; forward and reverse bevel gears in the lower unit engage the pinion to drive the propeller shaft.
Pinion gearThe drive bevel gear mounted on the lower end of the vertical drive shaft that meshes with the forward and reverse bevel gears on the propeller shaft.
SpallingSurface fatigue on a bearing race or gear tooth that causes small flakes or pits to break away; a sign the component must be replaced.
Lip sealA shaft seal with a flexible rubber lip that contacts a rotating shaft to retain lubricant and exclude water; the lip must face the fluid it is retaining.
BrinellingIndentations pressed into a bearing race by static overload or shock loading; named after the Brinell hardness test, these flat spots cause vibration and accelerated wear.

Worked examples4

1. The service manual specifies pinion gear backlash of 0.010–0.020 inch. You measure the existing backlash at 0.028 inch using a dial indicator. The existing shim behind the pinion bearing is 0.060 inch thick. How thick must the new shim be to bring backlash to the midpoint of specification (0.015 inch)?
  1. Identify the current backlash: 0.028 inch.
  2. Identify the target backlash: midpoint of 0.010–0.020 inch = 0.015 inch.
  3. Calculate the required reduction in backlash: 0.028 − 0.015 = 0.013 inch.
  4. Increasing the shim thickness moves the pinion closer to the ring gear, reducing backlash. Therefore, add 0.013 inch to the existing shim thickness.
  5. New shim thickness = 0.060 + 0.013 = 0.073 inch.
  6. Select the nearest available shim equal to or closest to 0.073 inch from the manufacturer's shim kit, then recheck backlash after assembly.
Answer: The new shim should be approximately 0.073 inch thick.
2. You are measuring a bearing bore with a telescoping gauge and outside micrometer. You take three readings: 1.5008 inch, 1.5010 inch, and 1.5009 inch. The service manual specifies a maximum bore diameter of 1.5015 inch. Is the bore within spec, and what is the average measured diameter?
  1. Add the three readings: 1.5008 + 1.5010 + 1.5009 = 4.5027 inch.
  2. Divide by the number of readings to find the average: 4.5027 ÷ 3 = 1.5009 inch.
  3. Compare the average to the service limit: 1.5009 inch < 1.5015 inch.
  4. The bore is within the service limit. Record 1.5009 inch on the spec sheet and note 'Within spec.'
Answer: The average bore diameter is 1.5009 inch, which is within the 1.5015-inch service limit.
3. The service manual specifies propeller shaft bearing preload as a rotating torque of 2–5 inch-pounds after the bearing nut is torqued. You measure rotating torque with a beam-type inch-pound torque wrench and read 1 inch-pound. What is the correct action?
  1. Compare measured rotating torque (1 in-lb) to the specified range (2–5 in-lb).
  2. 1 in-lb is below the minimum of 2 in-lb, meaning there is insufficient preload — the bearing has too much internal clearance.
  3. Disassemble the bearing stack and reduce the shim thickness (or bearing spacer thickness) to increase preload.
  4. Reassemble with a thinner shim, re-torque the bearing nut, and re-measure rotating torque.
  5. Repeat adjustments until rotating torque falls between 2 and 5 in-lb, then document the final measurement on the spec sheet.
Answer: Preload is insufficient; reduce shim thickness to increase rotating torque into the 2–5 in-lb range, then verify and document.
4. A shim pack consists of three shims: 0.025 inch, 0.015 inch, and 0.008 inch. The technician needs to increase the total pack thickness by 0.010 inch. What is the current total thickness, and what is the required new total?
  1. Add the current shims: 0.025 + 0.015 + 0.008 = 0.048 inch.
  2. Add the required increase: 0.048 + 0.010 = 0.058 inch.
  3. Select shims from the kit that total 0.058 inch; for example, add a 0.010-inch shim to the existing pack to reach 0.058 inch.
Answer: Current total is 0.048 inch; new required total is 0.058 inch.

The math in this lesson2

Learners total shim pack thicknesses and select shims to add 0.010 inch.
7.EE.3N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Disassemble the lower gear case following the service manual sequence; lay parts on a numbered bench mat and photograph each stage to create a visual disassembly record. · Measure pinion gear backlash shims, check bearing races for wear using a micrometer and feeler gauges, and record all measurements on a spec sheet comparing against service limits. · Reassemble the gear case using the correct shim pack and new seals; use a bearing driver set to seat bearings squarely, torque all fasteners to spec, and perform a completed-assembly check with the instructor.
Materials: Sterndrive unit for overhaul, bearing driver set, inch-pound and foot-pound torque wrenches, micrometer, feeler gauges, bench mat, service manual, spec sheet, new seal and gasket kit
Check: All measurements recorded and within spec or flagged as out-of-spec with correct disposition; reassembly torque values verified by instructor; no missing or reversed seals (inspector checklist, 100% compliance).

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Safety and Shop Practices
1. Before beginning bench disassembly of a sterndrive lower gear case, what is the FIRST safety action a technician should take?
Sterndrive Systems Diagnosis
2. Milky or cream-colored gear lube drained from a sterndrive lower unit most likely indicates:
Sterndrive Reassembly and Sealing
3. When installing a lip seal in a sterndrive gear case, the seal lip should be oriented:
Gear Case Measurement and Adjustment
4. Pinion gear backlash is adjusted in a sterndrive lower unit by:
Gear Case Measurement and Adjustment
5. A technician measures pinion backlash at 0.006 inch. The service specification is 0.010–0.020 inch. Which action is correct?
Bearing Installation and Preload
6. When using a bearing driver set to install a bearing race in an aluminum housing, force should be applied to:
Bearing Installation and Preload
7. After assembling a propeller shaft bearing stack and torquing the retaining nut, rotating torque is measured at 6 inch-pounds. The service manual specifies 2–5 inch-pounds. What does this indicate and what is the correct response?
Component Inspection and Measurement
8. Which condition found during inspection of a bearing race requires immediate replacement of that bearing?
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