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Inner Workings of Gearcases

Shifting Capabilities

Most outboard gearcases have forward, neutral, and reverse shifting capabilities. However, some smaller outboards have either forward and neutral shifting, or direct drive.
Outboard models with direct drive only operate in forward gear. 360 degree steering provides reverse operation.

Gearcase Types

Large outboard gearcases are available in standard and counter rotation. Using a counter rotation model on a twin engine boat reduces steering torque and increases the ability to track straight.
You can identify a counter rotation gearcase by the letters C R stamped on the end of the propeller shaft.

We will use a larger horsepower standard rotation gearcase to discuss principles of operation. However, most of these principles apply to all Evinrude and Johnson models.

The primary function of a gearcase is to transfer power form the crankshaft to the propeller. The rotating motion of the crankshaft is transferred down the driveshaft to the pinion gear.

The pinion gear is splined to the driveshaft and rotates constantly with the crankshaft, driving a set of gears which revolve freely in opposite directions on the propeller shaft.

Powertrain Support

The driveshaft is supported in the gearcase by three bearings.

Three Bearings in Gearcase

  1. The bearing in the driveshaft bearing housing

  2. and the pinion bearing pressed into the gearcase housing, hold the driveshaft in position

  3. The thrust bearing is located below the driveshaft bearing housing and supports the vertical thrust of the pinion.


  1. The propeller shaft is supported on the front by the forward gear and shift housing assembly.

  2. And on the rear by the propeller shaft bearing housing

  3. The propeller shaft bearing housing also supports the reverse gear

Sealing Areas

Driveshaft (left) Propeller Shaft (right)

The driveshaft and propeller shaft bearing housings are very important sealing areas, and contain o-rings and seals to prevent water from leaking into, or oil from leaking out of, the gearcase.

The clutch dog is splined to the propeller shaft between the forward and reverse gears, and slides on the propeller shaft to engage either gear

Shifting Operation

Both gears and clutch dog have drive lugs that enable them to engage one another and turn as a unit. The drive lugs must have square corners because drive lugs can lead to the clutch dog jumping out of gear.

If the drive lugs are rounded, replace the damaged parts and try to determine the root cause of the problem.
Possible causes of rounded drive lugs include : improper shifting, incorrect adjustment of the shift cable, and incorrect shift rod height.

In standard rotation gearcases, the gear closest to the front of the gearcase housing is the forward gear. When the clutch dog engages this gear, the propeller turns clockwise and pushes the boat forward. The gear closest to the propeller is the reverse gear. When the clutch dog engages this gear, the propeller turns counter clockwise and the boat moves backward. In counter-rotation gearcases, the position of the gears is reversed.

Forward Gear (left) Clutch Dog (middle) Reverse Gear (right)

The shift housing converts vertical motion of the shift rod into horizontal motion of the clutch dog, causing it to engage either the forward or reverse gear.
Let´s review the shifting operation before moving on.

Shift Housing

Pulling up on the shift rod causes the clutch dog to slide forward on the propeller shaft until its lugs engage forward gear.
Power is then transmitted form the pinion gear to the forward gear, then to the clutch dog, and finally to the propeller shaft and propeller.
Pushing down on the shift rod causes the clutch dog to slide rearward on the propeller shaft until its lugs engage reverse gear. Because the reverse gear is rotating in the opposite direction, the propeller shaft and propeller will also turn in the opposite direction, causing the boat to move rearward.

When shifting into or out of gear, it is important that you move the remote control handle quickly. If not quick and positive with your shifting motion, the clutch dog will ¨ratchet¨ while attempting to engage the gear.

In addition to their load handling and shifting responsibilities, the gearcases on Evinrude and Johnson Outboards perform several other functions. The water pump is attached to the top of gearcase housing, and circulates water through the outboard to cool the powerhead.

Trim Tab

When correctly adjusted, the trim tab offsets propeller torque, reducing the effort required to steer the boat.

Trim Tab

New boats often require adjustments to the trim tab. Changing propeller pitch or type may also require changes to the trim tab setting.
The adjustment procedure is described in the Operator's Guide, and must be performed under actual operating conditions on the water.

When service a customer´s gearcase, make a mark on the trim tab and gearcase before you remove it. So you may return it to the same position after servicing.

Water Pick-Up Flow

Most trim tabs include a water pick-up to cool the propeller hub. Water enters the pick-up and travels through a passage in the gearcase to the exhaust cavity.
From there, it flows out of the gearcase and over the propeller hub with the exhaust gases.

When servicing a gearcase you must follow the correct procedures. A procedure used on one type of gearcase may permanently damage another type.
Make sure you are using the correct manual.

Maintenance Tips

40 horsepower and larger engines require precise location of the driveshaft pinion to ensure durability under extreme torque loads.
The driveshaft pinion is precisely meshed with the forward and reverse gears by using one or more shims between the driveshaft bearing housing and the thrust washer.
You must set the gear mesh whenever the drive shaft, gear set, thrust washer, thrust bearing, or drive shaft bearing housing are replaced.

These components and the gearcase housings are machined to extremely close tolerances. Because of this, only the depth of the driveshaft must be controlled to achieve proper mesh between the gears.
A Driveshaft Shim Fixture is available to shim the driveshafts on 40 horsepower and larger two-stroke engines.
The driveshaft shim fixture must be used to select the correct number of shims to position the driveshaft pinion accurately in the gearcase.

When assembling the gearcase, the shims must always be located between the driveshaft bearing housing and the thrust washer.
Special removal and installation tools are available for servicing gearcase seals and bearings.

If you fail to use the recommended seal or bearing removal tool, you may damage the housing. In many instances, proper installation of a part can only be assured by using the correct tool.
The service manual specifies what tools to use for specific applications.

Another important service tool, the shift rod height gauge, sets the initial height of the shift rod. If overlooked, the gearcase will not completely engage both forward and reverse gears.
Partial engagement of either gear will cause premature gearcase failure. Always verify that the shift rod is adjusted to the correct height before reinstalling a gearcase.

When you drain the gearcase, inspect the lubricant. If water has entered the gearcase it will flow out first, unless it has been mixed with the oil. Oil mixed with water has a milky appearance.
Oil that is black and has a burned odor indicates possible bearing failure, or oil that has not been changed within the recommended service interval.

Inspect the magnetic plug for metal particles.

Small fine shavings: normal wear
Large particles or excessive shavings : problem

Outboard gearcases must be filled from the bottom using a pump or pressurized container. Fill the gearcase with recommended lubricant unit oil right before it comes out of the top lubricant level hole.

All Evinrude and Johnson gearcases are connected to the crankshaft by a set of splines located on top of the driveshaft.
The splines provide a positive connection to the crankshaft and allow easy removal of the gearcase or powerhead. These splines will see extreme loads that create high temperatures capable of breaking down an ordinary grease.

Before you install the gearcase, you should clean and recoat the driveshaft splines liberally with Moly Lube. Molly lube has high load carrying capabilities and can stand up under high temperatures.
It is the only grease recommended for outboard driveshaft splines.

Before installing a propeller shaft, the shaft should be coated with Triple Guard Grease. Triple Guard is a waterproof, anti-corrosion grease that performs very well in this application.
Once a season, the propeller should be removed, the propeller shaft cleaned, and Triple Guard Grease applied.

Any time a gearcase has been reassembled, or if the gearcase displays evidence of leakage, its seals should be checked under both pressure and vacuum. The pressure tester determines if oil can leak out of the gearcase.
The vacuum tester determines if water can leak into the gearcase. In some cases, adapters may be required to match the threads in the gearcase housing.

Most four-strokes require an adapter. For other adapter, consult the appropriate service manual or the Evinrude/Johnson Genuine Parts Catalog. Most outboards are equipped with at least one sacrificial anode to protect the outboard from galvanic corrosion.
Some larger outboards have several anodes located on the midsection and gearcase. Because an anode is a less-noble alloy than the aluminum alloy of the gearcase housing, the anode erodes away, protecting the outboard. Erosion or disintegration of the anodes indicates the anodes are working.
Anodes should be inspected periodically. Any anode that has been reduced to two-thirds of its original size must be replaced.For the Anodes to function properly, they must be clean and have a good electrical bond to the outboard. Never paint them.


Larger gearcases have forward, neutral, and reverse shifting capabilities. Smaller gearcases
have either forward and neutral shifting or direct drive.

Main components of the gearcase are:

• Driveshaft – transfers power from the crankshaft to the gearcase.
• Forward and reverse drive gears – determine the direction of rotation of the propeller
• Pinion gear – transfers power to the forward and reverse gears.
• Clutch dog – engages the drive gears and transfers power to the propeller shaft.
• Shift rod – determines the location of the clutch dog.
• Propeller shaft – transfers power to the propeller.

Larger gearcases are available in standard and counter rotation. Counter rotation gearcases
can be identified by the letters CR stamped on the end of the propeller shaft.

On standard rotation gearcases, the forward gear is the closest to the front of the gearcase. On
counter rotation gearcases, the reverse gear is the closest to the front of the gearcase.

Anodes should be replaced when they are reduced to two-thirds original size.

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