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Replacing your R, MR and Alpha One Upper Gear Set


About Your Upper Driveshaft Housing Gears.
Upper gears can fail from one or more things.
Lack of lube, improper installation, water intrusion or severe impact are a few
of the most common reasons for gear failure.
Rebuilding an upper is tricky and it must be done right if the drive is to last.
If the factory spec gives you a rolling torque of 6-10 INCH pounds and you
build the drive with 11 INCH pounds, it will fail.
Even the tension that a feeler gauge has when measuring a space clearance is critical.
Take the time to build your drive right and it will last.

The following instructions are only for you to get a general idea of what is required to build an upper.

You must have a factory shop manual to perform this repair.
Refer to all factory manual images and procedures before performing your repairs.

For a complete listing of all Upper Gears and Upper Gear Kits, click HERE.

Points of Fact
1. Rebuilding your driveshaft housing is not easy.
2. Special tools are required to properly build an upper driveshaft housing.
3. All measurements, clearances, rolling torques and torques must be perfect.
4. Taking "shortcuts" will result in gear failure.
5. Everything must be perfectly clean.
6. Metal bits from your upper have contaminated your lower too.
7. Uppers can fail from a bad lower gearcase component.
8. If your upper needs to be rebuilt, your lower should also be rebuilt.
9. You can not just "flush" a lower clean of metal bits.
10. Gears are guarantied for manufacture defects, NOT incorrect installation.
11. You can not build an upper by "feel".
12. Do not mix-and-match used gears and/or bearings and races.
13. New gears deserve new bearings.
14. Completed assemblies must be pressure and vacuum tested.
15. It is best to have an experienced Certified Mercruiser tech rebuild your drive.
16. NEW is always best.
17. Expect to have to assemble, measure, disassemble, reshim and reassemble
measure and disassemble a few times.
18. Building a drive takes patience.
19. If you have a "bad feeling" about a procedure that you have done,
chances are good you have done it wrong.

The special tools from Mercury for building the Driveshaft Housing are:
NOTE: Click for price. Prices are subject to change.
Mercruiser changes prices on a regular basis.
91-38756 Adaptor
91-38918T Bearing Cup Driver
91-33493T Driver Cup
91-43591T Oil Seal Driver
91-60526T Shim Tool
91-60523 Shim Tool
91-34569A1 Slide Hammer Kit
91-66274 Torque Wrench (inch)
91-17256 Retainer Wrench
91-37241 Puller Plate
90-12934--2 Service Manual #6 for R, MR and Alpha Drives

For a complete listing of all Upper Gears and Upper Gear Kits, click HERE.

In addition to the tools listed above you will need:
A hydraulic press (you can not hammer on a bearing).
An old propshaft (used to hammer in the lower race).
An old driveshaft with a pinion nut on the end (used to obtain a rolling torque).
Old bearing cones (you can cut the cages off your old bearings).
An adequate section of pipe (about a 2" ID, used to press the bearing on to the gear).
A pressure tester (can be fabricated with a bike pump and gearlube pump).
A vacuum tester (can be a modified brake system bleeder pump).
A dead-blow hammer.
Compressed air and blow gun.
A piston ring compression tool (to align the double roller bearing set and obtain pre-load).
A housing holding fixture of some sort. (We use a Bob Kerr fixture).
A large accurate torque wrench (20 to 150 lbs with 1/2 drive).
A 12 point 3/8" socket.
A 3/4" socket.
A long slotted screwdriver.
Misc hand tools.
A can of Mercury Perfect Seal or Permatex Liquid Aviation Gasket Sealant.
A tube of red loc-tight threadlocker.
An oil can with clean Mercury Gear Lube.
A Good bench vise.
A .025" long feeler gauge.
An accurate micrometer.

Disassembly and Inspection

Place the upper housing in your fixture.
Remove the four top cover 12 point X 3/8" screws and pop off the top cap.

The entire ujoint shaft, double bearing and gear will pull out as an assembly.
Use your new Bearing Retainer Wrench and unthread the front retainer (castle) nut.
(this is standard thread, LH-Loosen).
You will usually have to hit the wrench's handle in the off direction with a dead blow hammer.
Don't use a torque wrench or a breaker bar to remove the retainer nut.

Once you have completely unthreaded the retainer nut, you should be able to pull out
the entire ujoint, bearing and gear assembly.
There is a good chance that it will be stuck in place.
Care must be taken when coaxing the shaft assembly out of the housing.
You can easily crack the front nose of the housing or damage an oring sealing surface.
Use care not to hammer on the thin areas of the ujoint knuckles.
Gently wrap on the ujoint knuckle with a dead blow hammer while pulling and turning the assembly out.
If it's stubborn, extra measures will have to be taken.
Take a heat lamp and heat the nose of the driveshaft housing.
This will expand the housing and help to loosen the assembly.
Constant gentle tapping while pulling, turning and heating will eventually work.
Don't loose the shims when you remove the assembly from the housing.
Once the assembly has been removed from the housing, you can start to remove the pinion nut and gear.
This is best done by clamping the Bearing Retainer Wrench in the vise with the small end up.
Slip the ujoint shaft assembly down over the wrenches handle by slipping the handle through the center yoke.
This will hole the whole assembly while you remove the pinion nut with a 3/4" socket.
Once you have remove the pinion nut, the gear (with the bearing) will slide right off the end.

REMEMBER!
It is good to take note and remember how the gear, bearings, thrust washer,
seal carrier and threaded retainer all come off of the shaft.
Notice how the oring fits between the thrust washer and the seal carrier.
Notice how the thrust washer has a bevel and notice which direction that bevel was facing.
Notice which direction the seal is facing and how it's pressed into the seal carrier.
Notice how the pinion nut washer says "NUT" on it. That's the side that touches the nut.
Notice how when you removed the assembly from the case that there were shims and a thick split shim.

If you are replacing your upper gears, there is no reason to waste time trying to
press the old bearings off of the gear. Set the gear to the side and move on.

Inspect Your Yoke
Take a clean rag and wipe off the yoke. Look closely at the area where the oil seal was riding.
The seal will usually wear a groove into the yoke. A very slight smooth groove is OK.
If, however, there are rusted pit holes, pits or a deep groove (one that can be felt with your finger nail) then
the yoke will have to be replaced.
Lean towards the "safe" side when it comes to the condition of a part(s).
Installing a worn, pitted or grooved yoke is not a good thing.
It will result in an oil leak and eventual gear failure.
If the yoke is grooved, just replace it. A new yoke is a lot less than having to
pay for, and install another new gear set.

Inspect Your Cross Bearings
If the yoke looks good, remove the ujoint shaft assembly from the holder and work
the cross bearings in all directions to make sure there is no looseness, slop, play, binding
or grinding of any of the cross bearings. This is best done by holding the entire shaft assembly
in both hands and moving the joints in all possible directions.
New cross bearings will have a smooth yet firm feel to them.
Used cross bearings should be smooth but not firm.

Back to the Housing
Set the ujoint shaft assembly to the side for now and let's go back to the housing.
You can now lift the vertical gear assembly right out of the housing.
The only part of this assembly that you will be using is the 4" long vertical stub shaft.
If you flip the assembly over and look up into the gear and see the splines, you will
"think" that is all part of the shaft. It's not. The splines are actually a
part of the gear itself. This is good! When you replace the gear you also replace the splines.

The stub shaft will have to be pressed off the gear.
This is done with an adequate driver rod (such as a 5" socket extension or hard solid rod). Support the assembly in a press upside down between two press blocks.
Place the driver rod into the female splined hole in the bottom of the gear
and press the shaft out of the gear.
This sometimes requires a lot of pressure.
The shaft has a tight fit in the gear and sometimes extra measures have to be taken
to get the shaft out of the gear. Sometimes the driver rod bends and you need to try
something stronger. Sometimes it helps to hammer the top of the presses arbor shaft
to "shock" the assembly. Sometimes it just won't come out.
If the stub shaft is stuck in the gear, just buy a new one!

Next, press the top bearing off of the shaft.
Keep the top bearing. We like to cut off the cage and use the center cone later on.
Inspect the stub shaft. Replace the shaft if you see any cracks, scores or chipping.
Take a close look at the step near the large end.
See the o-ring?
This is a very thin o-ring that is NOT included in the Seal Kits.
It seals the gear to the stub shaft. If you forget this o-ring your drive will fail
the pressure test and leak oil.
The o-ring is part 25-55801. Order a new o-ring.
Inspect the Driveshaft Housing

Before the housing is cleaned you should knock out the seals and remove the
lower bearing race.
Use care in knocking out the oil seals that you don't score the housing.
With housing right-side-up, take a long screwdriver from above and knock out the
smaller bottom seal.
Flip the housing upside down and with the long screwdriver knock out the larger upper seal.

Now take your slide hammer (with the two jaws facing outward) and position the teeth
of the jaws under the bearing race.
NOTE: The housing has two notches under the race to make room for the jaw's lip.
NOTE: In our shop... we have filed the lip of the jaws thinner to get under the race better.
Use the slide hammer and slam the race up and out of the housing.
There will be shims under this race.
Remove and retain all old shims.

For a complete and thorough inspection, the housing will have to be cleaned.
Most shops have a parts washer. Some customers use diesel fuel or mineral spirits.
You can even use soap and water with a high pressure hose nozzle.
Make sure you adequately clean the small oil passage.
Double check that you have removed all the shims. They can stick in place.
Clean, clean and clean again. The housing must be CLEAN.
Use compressed air top blow out the oil passage and dry the unit.

Take a flashlight and look at every nook and cranny.
Make sure the damaged gear hasn't perforated the housing.
A housing with a hole is junk. Welding is usually impossible.
Clean away any old sealant or glue.

Check the Water Pocket Cover

The copper water tube fits up into the "Water Pocket".
The water pocket is held in place with four stainless bolts.
There is a gasket between the water pocket cover and the housing.
If the gasket is leaking, the engine will run hot.
The gasket tends to leak after a few years, especially in salt water units.
Take a rubber hose and clamp it to the copper tube.
Hold your hand tightly over the front water passage hole to plug the
passage and blow through the hose. There should be no leakage.
If you suspect a leak, pour a little soapy water around the water pocket and blow.
If you see bubbles around the gasket area, you will have to remove the water pocket
and replace the gasket.
WARNING: This can be a problem.
The stainless bolts are likely to snap off in the housing.
If the bolts snap off, you will need a new housing.
Even one broken bolt will render the housing useless.
They can not be drilled out and retapped.
I SUPPOSE that if you had a machine shop that MAYBE you could find a way to
drill the bolts out but we have never seen it done successfully.
Bottom line, don't remove the water pocket bolts unless you REALLY need to and
if you do have to remove the bolts, use heat, care and patience.

Clamp the top cover upside down in the vise and use the slide hammer to
pull the little top bearing race out of the top cover.
Keep all of the old shims which were located under the race.

Clean and Reinspect All Your Parts

Use a parts washer and wash all of the other parts.
Use a wire wheel or a glass bead blaster to remove any rust from the ujoint
shaft assembly. Don't blast the seal area on the yoke.
Knock the large oil seal from the front seal carrier.
Take a single edge razor blade and scrape the green sealant from the carrier ID.
Do not score the seal carriers ID.
Discard the old pinion nut and washer.
Always use a new pinion nut and washer.
Remove the old thin o-ring from the stub shaft and carefully clean.
Before you put the stub shaft down, install the new o-ring.
Nothing sucks more than to have the drive completely assembled only
to remember that you forgot the thin o-ring.

Lay Everything Out

Once all the parts are clean and dry, lay everything out on the bench
in nice neat order.
Unpack all the new parts.
Neatly spread out and organize all your bearings and shims in logical order.
Double check that you have all your parts and make sure you have the
correct bearings and gears.
Count the teeth on your old gears and compare to your new gears.
Have your Perfect Seal handy as well as your Loc-tight and drive lube.
Make sure your tools are clean and neatly placed out on the bench.
Take some time and read through the manual and familiarize yourself with
the measurements and procedures.
Take your micrometer and measure a few shims to get familiar with the different
thicknesses and how they feel.
Getting clean and organized is just as important as anything else when building drives.

Preparing the Ujoint Driveshaft Assembly

No Yoking Around.
Replace your yoke if needed.
Make sure you get the correct yoke, there are two styles.
Never reuse a cross-bearing.
Never hammer on a cross-bearing cap with a hard hammer.
Use care to make sure that all the needle bearings are in place before assembly.
Always make sure your retainer C-clips are fully seating into the caps groove.
We use a special cross-bearing tool by Blue Point.
Double check that the cross-bearings feel smooth and firm after assembly.
New style cross-bearings do not have grease zirt fittings.
If the cross-bearing has orange rubber seals, it has no zirt fitting.
Orange sealed cross-bearings are PermaLube and need no additional grease.

Your New Double Bearing Set
Your new double bearing set will come with detailed installation instructions.
It is important to remove and discard the new small diameter spacer from between
the two bearings.
This small diameter spacer ring is only used on older drives if you were replacing
only the bearings and reusing the older gears.
Take a note as to how the bearings are seated with the backs of each race against each other.
Your will now be pressing the first bearing on to the gear.
This is where you will need some sort of pipe. Place the gear teeth down in the vise.
Take a bearing cone (the part with the needles) and, using a section of pipe, press the cone
all the way down onto the gear. It MUST be ALL the way down on the gear.
Place a race over the cone and then place the large thick spacer washer on that race.
Next, place the second race on the assembly. The races will be back-to-back with the spacer in between them.
Now for the tricky part. You need to carefully press the second cone onto the gear
making sure to leave a very small space.
Do not press the second cone on all the way!
You need to leave it as so you can just see a small space between the bearings.
If you over-press the cone into place you will have to press off the bearings and start over.
When your done... you will have a small space and the bearings will spin easy and loose.
This "space" will be taken up later when you are tightening the pinion nut.
Pressing the Large Oil Seal into the Carrier
Find the large front oil seal.
Notice it has a green substance on its shell.
This is a sealant.
Use Seal Driver Tool 91-36577 to press the new seal into the aluminum seal carrier.
Care must be taken to have the press perfectly aligned or the seal will cock off to one side.
If the seal cocks off to one side, remove it and install a new one.
Take your time and press it in slowly and evenly.
Coat the rubber lip seal with clean drive lube when finished.

The Ujoint Shaft Assembly
Place your Retainer Wrench in your vise again with the small side up.
Place the ujoint shaft assembly over the wrench to act as a holding fixture.
NOTE: The wrench must pass through the area closest to the long splined shaft end.
Squirt some clean drive lube onto the bearings.
Place all the parts in the proper order and position onto the yoke.
Put a drop of lock-tight on the yokes threads.
Place the pinion washer with the "nut" side toward the nut and thread the nut on by hand.
Take a 3/4" wrench and gently snug up the pinion nut BUT do not tighten.
Joggle the complete assembly around a little to shift all the parts into place.
Take a wide piston ring compression tool (SK and Blue Point make a good one) and place it over the
double roller bearing set and tighten it up. Turn the bearings while tightening the compressor tool.
The compressor tool will assist is aligning the bearings when you start to tighten the pinion nut
to get your proper rolling torque.
Now, remove the complete assembly from the wrench and remove the wench from the vise.
Reposition the wrench in the vise as so it is laying flat.
Drop the splined end of the shaft assembly down through the retainer wrench as so the castle
retainer nut fits into the wrench.
This will allow you to turn the assembly to check for the preload.
The instructions that came with the double roller bearing set will have a diagram.
The instructions will also specify the rolling torque specification.
The rolling torque must be 6-10 INCH pounds.
Tighten the pinion nut little by little while checking the rolling torque between
tightenings to make sure you don't go over the 6-10 inch pounds of rolling torque.
I like to turn the nut in 1/4 turn increments.
Take the time to do this right.
Do not accept a rolling torque that is not 6-10 inch pounds.
If you go over the 10 inch pounds.... you will have to disassemble the entire assembly
and press off the bearings from the gear and start all over.
A bearing with 11 inch pounds of rolling torque will only last a couple of hours.
A bearing with too little rolling torque will only last a few hours.
If you get frustrated, walk away and try again another day.
Once you are sure you have the exact rolling torque, set the assembly to the side
on a clean bench.

Measure Your Old Shims
Use your micrometer and measure your old lower bearing shims.
Write down the thickness of each shim.
Replace each old shim with a new one of the same thickness.
Old shims are not good to reuse. Get new shims.
Shim packs contain a variety of thicknesses. Sometimes you need two shim packs.
Shims usually come in .001", .002", .003", .005", .010", .020".

Installing the Lower Bearing Race
Once you are done measuring, recording and replacing the shims, take the new shims
and place down into the housing.
NOTE: If you have lost the shims... start with a pack of .015".
Coat the housing and the bearing race with clean drive lube.
We usually freeze our races for easier installation.
Hammering the race into place is tricky. Make sure the race doesn't cock to one side.
Use a deadblow hammer for installing races.
Use an old propshaft and driver head 91-33493 to hammer the new race into place.
If the race binds or cocks off to one side... STOP. Don't keep hammering!
You will ruin the case if you force the race in crooked. Instead, pull the race out and
de-burr the housing making sure there is no build up of aluminum in the bore.
Re-lube the bore and race and try again being careful to keep the assembly straight.

There is a unique difference in sound when the race is fully seated.
It will be a clear "tink" sound when hammered fully into place.

This race must be fully seated into the housing before proceeding.
If the race is not fully seated, you will have gear failure.

Installing the Bearing Race in the Top Cover
Replace the original upper race shims with the equivalent in new.
NOTE: If you have lost the shims... start with a pack of .015".
Oil the bore and the race. We prefer to freeze the race before installation. Use Bearing Cup Driver 91-38918 and Driver Rod 91-37323 to hammer the race into place.
NOTE: If the race just drops in too easy, replace the entire cap.
NOTE: If you cock the race to one side it will spread the bore and ruin the cap.
NOTE: Make sure the race is fully seated.

Pressing the Stub Shaft Into the Lower Gear
Support the gear between two press blocks right-side-up.
Freeze the stub shaft when possible.
Make sure the tin o-ring is in place on the stub shaft.
Lightly coat the end of the shaft and the ring with drive lube.
Press the stub shaft into the gear until fully seated.
This is a tight fit and it requires extreme pressure from the press.
Keep the assembly in the press to install the small top bearing.

Pressing the Small Top Bearing Onto the Stub Shaft
Lightly coat the top end of the stub shaft with clean drive lube.
Use the old cone that you cut out of the old top bearing to press the new bearing into place.
Make sure the bearing is fully seated on the stub shaft.

Installing the Lower Bearing Onto the Lower Gear
Freeze your lower gear if feasible. This will make it easier to press the bearing in place.
Place the gear and shaft assembly upside down in the press.
Use care not to bend or damage the installed top bearing.
Use the bearing cone from the old bearing and an adequate pipe to press the new bearing
onto the lower gear. Make sure the bearing is fully seated.

You will now do a "trial run" assembly to get the lower gear assembly height and rolling torque.
This procedure is done BEFORE installing the two lower seals.

Obtaining Proper Gear Height and Rolling Torque
We must first partially assemble drive to measure and obtain the proper rolling torque.
Squirt a little clean gear lube on both the top and bottom bearings.
Lower the gear and shaft assembly into the housing.
Install the top cover WITH its o-ring in place.
Torque the top cover screws to 20 ft. lbs.
Flip the housing assembly up-side-down.
Slide an old driveshaft (with a pinion nut installed) into the splines of the gear.
This shaft will act as a tool to help measure the rolling torque.
Spin the shaft around and back and forth a little to make sure the assembly turns freely
and that it's not bound too tight. If the assembly won't turn, remove the a shim from the race
in the top cap and try again.
Use your Inch Pound Torque Wrench to turn the shaft in a constant smooth circular motion while
reading the instrument.
The goal is to obtain a rolling torque of 6 to 10 inch pounds for new bearings or 2.5 to 4 inch pounds for used bearings At the same time, you will take into consideration the height of the gear when or if changes have to be made.

Chances are good that your reading is too high or too low.
It is very rare to have the correct rolling torque on your first try.
The rolling torque is changed by adding or subtracting shims from either the upper or lower bearing race.
Which shim you add or remove depends on your gear height.
If your rolling torque is OK, go on and measure your gear height.
The gear height is measured with shim tool 91-60526T and a long .025" feeler gauge.

Oil the inside of the gear housing where the shim tool will be inserted.
Oil the outer perimeter of the shim tool.
The position that you insert the tool depends on the gear ratio of the drive.
The tool has three holes. X, Y and Z.
Use the letter that corresponds to your drive ratio.

Shim Tool 91-60526T
Gear Ratio Tool Position
1.32:1 or 1.30:1
Z
1.50:1 or 1.47:1
Z
1.65:1 or 1.62:1
X
1.84:1 or 1.81:1
Y
1.98:1 or 2.00:1
Y


The tool must slide in all the way and seat against the shoulder in the housing.
The tool must also clear the face of the gear. If the gear is much too high, you won't
be able to get the shim tool slid in all the way.

Once the tool is fully inserted, rock it back and forth a few times.
Take a .025" feeler gauge and slide it between the face of the gear and the flat spot on the tool.
You might have to rock the tool a little until its flat spot is parallel to the face of the gear.

About Feeler Gauges
Contrary to popular belief, a feeler gauge is not supposed to slide smoothly.
The feeler gauge, when the clearance is exact, will be very stiff and it will grab when being
pulled in and out.
If your feeler gauge slips in and out like a glove, you have too much clearance.
If you just can't get the gauge to slip in at all, you don't have enough clearance.
When the clearance is perfect, it will be difficult to slide the gauge in and it will get stuck
and seem very difficult to pull it back out again. It will have a "jammed" feel to it.
If you build the drive with an easy-slip-feel to the gauge, it will not last.

Back to the Shimming Procedure
You need to consider your previous rolling torque measurement and gear height when changing shims.
FOR EXAMPLE: If your rolling torque was too low (loose) and your gear height was too low (too much clearance),
you would want to add shims under the lower race. This will raise the gear and increase the rolling torque.
Use a thicker gauge until you find the gauge that feels tight.
Let's say that a .030" gauge has a nice tight feel, that means the gear needs to come up .005".
You would disassemble the upper, pull out the lower race and add a .005" shim.
Reassemble the drive and recheck the rolling torque and gear height.
Now let's say that the .025" feeler gauge fits perfect and tight. Your height is correct!
You recheck the rolling torque again to find it's too high.
You would then remove the top cap and remove a shim(s) from under the top bearing race.
Reassemble the unit and recheck the height and rolling torque again.
It is not uncommon to have to do this a few times before it's perfect.
This is where most technicians fail. They get impatient and take short cuts.
ANOTHER EXAMPLE: If your rolling torque was too high (tight) and your gear height was too low (too much clearance),
then you have much too many shims under the top race and not enough under the lower race.
Let's say you can fit a .035" feeler gauge just right. That means your gear is .010" too low
Adding a .010" shim under the lower race might help with the height but it will make the rolling torque even tighter.
You would then remove AT LEAST .010" from the top race and then a little more to loosen the rolling torque.

Install the Lower Oil Seals
Now that you have a perfect gear height and rolling torque, remove the shaft assembly and
wipe clean any oil that has dripped where the lower seals press into place.
You will use Oil Seal Driver Head 91-43591 with Driver Rod 91-37323 to install the seals.
The Driver Head is reversible on the driver rod. One side of the tool does the small seal and
the other side of the tool does the larger seal.
Install the tool in the way for installing the larger seal.
Wet the seal rubber lip with a little gear lube.
Slip the seal onto the tool with the seals spring towards the handle (up).
Take some Lock-tight and coat the outside of the seal.
With the housing right-side-up, hammer the seal into place. Make sure it is fully seated.

Remove the tool and reverse the driver head on the driver rod.
Flip the housing up-side-down.
Pick up the small oil seal.
Wet the rubber seal lip with drive lube and slide it on the tool with the spring away from the handle.
Coat the outside of the seal with Lock-tight and hammer it into place.
Make sure it is fully seated. The tool installs the seals at the correct depths.

Flip the housing right-side-up and gently lower the shaft/gear assembly into the housing
being careful to keep it centered as so not to catch the lip of the large seal.
Install the large diameter shims into the nose of the housing.
Start with .025" of shims if you are not sure what you had (does not include the split shim).
Install the thick split shim after the thinner shims.
Make sure the shims are against the shoulder in the housing.
The split shim helps hold the thinner shims in position.

Prepare the ujoint shaft/gear assembly for installation.
Slip the oring into place over the gear and the two bearings until it is in the proper position.
Wet the ring and the threads of the retainer nut with Perfect Seal.
Wet the inside of the housing with drive lube.
Wet the outer races of the two bearings with drive lube.
With both hands, carefully slide the entire assembly into the housing.
Do not force the assembly into the housing.
If the assembly jams, lightly rock the ujoints back and forth to "nudge" the assembly in.
As the assembly slides into place, you will have to turn the shaft to mesh the gears.

IMPORTANT: Older 1.65:1 gears (24-24 teeth) have "timed" gears.
You will find marks scribed into the gears which act as alignment marks.
Line up the marks when sliding the shaft/gear assembly in place.

When the shaft/gear assembly is slid in far enough, you can start to thread the retainer nut in.
Screw the retainer nut in as far as possible by hand while occassionally turning the shaft assembly.
Keep checking for tightness and/or binding when tightening.
As you tighten the retainer nut, the gears come closer together.
Use your Bearing Retainer Wrench 91-17256 and a good torque wrench and tighten the nut to 200 ft. pounds.
Keep checking to make sure the shaft does not bind or that the gears do not collide and jam.

After the retainer nut is torqued to 200 ft. lbs, turn the shaft assembly several times.
Lower Shim Tool 91-60523T into the top of the housing making sure to use the correct side of the
shim tool against the gear. Use the following chart to determine the tool position.
Shim Tool 91-60523T
Gear Ratio Tool Position
1.32:1 or 1.30:1 Z
1.50:1 or 1.47:1 Z
1.65:1 or 1.62:1 Y
1.84:1 or 1.81:1 Y
1.98:1 or 2.00:1 Y


Use a .025" feeler gauge to check your clarence.
If the clarence is too tight, you will need to add shims.
If the clarence is too loose, you will need to subtract shims.
Add or remove shims as neccessary to achieve the proper drag with your .025" feeler.
When the clarence is perfect, leave the shaft assembly torqued in place.
Turn the shaft several times in each direction while carefully feeling for any abnormalities.
The assembly should feel smooth yet firm. There should be no binding, crunching, grinding or rubbing.

Make sure your oring is installed on the top cover.
Coat the top cap o-ring with perfect seal and position the cap in place.
Coat the threads of the four screws with Perfect Seal and torque to 20 ft. lbs each in a cross pattern.

After the top cap is in place, turn the driveshaft again several times to recheck for problems.

Thread your pressure tester into the vent plug hole.
Hold your thumb over the oil passage hole and pump the unit to 15 psi.
You will have to apply force to your thumb to keep air from leaking out.
With the unit pressurized, turn the ujoint shaft in both directions several times.
Watch your pressure tester gauge for loss of pressure.
Listen for any air leaks and hissing.
If you find any leaks the drive will have to be disassembled and the seals and surfaces checked.
Do not put a leaking drive into service.
Sterndrive-Information
Change your Alpha Gear Lube
Change your Bravo Gear Lube
Why does my drive lube have white streaks?
Why does my drive lube have gray mush?
Replace Your Alpha One Water Pump
Replace your Alpha Gen II Water Pump
Rebuild your Bravo Raw Water Pump

Troubleshoot & Replace your Alpha Shift Cable
Troubleshoot & Replace your Bravo Shift Cable

Replace your Alpha One Bellows & Gimbal Bearing
Replace your Alpha Gen II Bellows & Gimbal bearing
Replace your Bravo Bellows & Gimbal Bearing
Why does my drive "Growl" when I turn?
Water is leaking into my boat from the transom

Replacing your Trim Sender Switches
My Trim doesn't work but my Trailer Switch does
My Trim Gauge is Pegged

Rebuild your Alpha One Upper Driveshaft Housing Assembly
Rebuild your Alpha One Lower Gear Housing Assembly

Check your Engine Alignment
Check for Excessive Steering Play
Troubleshoot an Overheating Problem
Should I remove my thermostat?
Why do I overheat when going fast?
Troubleshoot a No-Crank Condition
Troubleshoot a No-Start Condition
My Engine Backfires and/or Surges and has Lack of Power
What is an Anti-Syphon Valve?
My Engine blows Steam from the Exhaust
Why do I hear a "clakity" sound while at idle?
Exhaust Manifolds...IMPORTANT Information
How do I Winterize my engine?

What is Galvanic Corrosion?
I have a Bravo III Galvanic Corrosion Problem
What is a Mercathode?
All about Zinc Anodes
How do I check my carburetor?
What carburetor do I have? A Rochester or a Mercarb?
I have water in my fuel
My fuel smells like varnish
My fuel is old
My engine has been sitting for several months without being run.

I have a Mercruiser 470, 485, 488, 190, 3.7L Engine
I have coolant dripping from the front of my engine
Can I install a regular Alternator instead of the Stator-Rotor?

Converting an OMC to a Mercruiser


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