Gearcase Related Info

All material on this website, and these sections, is ©Runner Outboards LLC and is intellectual property.  You may freely distribute this information as long as it is NOT edited, and credit is given to the author.

Disclaimer: The information provided should never replace common sense or the recommendations of the OEM.  I do not assume responsibility for the use or misuse of this information.  The information provided is based on my experience working as a full time mechanic, on hundreds of motors over time, reading a lot of manuals, education, and consulting other experienced mechanics along with a number of retired service reps I am friends with.
If I can offer any advice from experience, it would be NOT to try and fix your own motor if you don't have a good understanding of what you're doing.  You need to have the right special tools, reference materials, and most importantly, UNDERSTANDING of what is wrong and how to properly fix this issue.  Most people do more harm then good if just messing around blindly.  The reason why I can do these repairs is I've put in thousands of hours reading, fixing, and practicing.  I learn something new everyday.  I have also gone out and acquired the necessary, CORRECT tools and reference manuals to work on the motors.  These are very important to promote correct operation of the motor.  The idea is to have a reliable motor, not just one that 'kinda runs.'

Index - Click below to Jump to That Section

Internal Combustion Essentials
The Basics, What You Need To Know About Motors

Cooling System
Gearcase Components
Ignition System
Fuel System
Mechanical Components
Trailer 101

Some basics of a gearcase I filmed.  I go through this conversation over, and over, and over each year with customers.

What happens when you don't maintain a gearcase

Maintenance of a gearcase isn't all that complicated.  OEM recommendations are to change your oil once a season, or every 50 hours, whatever happens first.  The problem is, how do you know how many hours you've logged?  The simple way, is to buy a tachometer/hour meter off of ebay.  These serve 2 great purposes, 1) to keep a log of how many hours, and 2) to properly set up your boat with the correct propeller (see the guide page for more info). 

If you don't change your oil regularly, that's like running your car without changing the transmission fluid.  Outboard gearcases take a lot more abuse than your automobile does (most cars go 40-50,000 miles, or roughly 4-5 years, before changes...I do it annually).  The other problem is if your gearcase has water intrusion (which there is always a little bit, even when it's perfect), or if your seal(s) have failed, you are on a short amount of borrowed time.  This is why you should have your motor serviced at least annually to monitor it's health. 

Below is a picture of gearcase oil that has been contaminated with water.  If it looks 'milky,' you have water getting in and shouldn't run the motor again until it's been addressed.  This can happen after just 1 outing for a few minutes.  All depends on how bad the leak is.  Regardless, don't use the motor until it's fixed properly!  The oil should look relatively clear, but may be different colors depending on the brand; brown, green, blue.  If it smells like rotten eggs then the oil is very old, burned, and expired.

You should certainly change the oil before temperatures drop below freezing.  If there is water in there, the gearcase can split (see pictures).  The pics I have are of a badly split gearcase, where someone with no clue to what they were doing decided to try and fix it by putting a glue/epoxy on the inside.  This skeg is too damaged to salvage, and the owner decided to run the motor, and in this case, run it filled with water, no oil, and when I acquired the motor, had to do a complete overhaul and replace everything including the driveshaft with exception of the upper gearcase housing.

crack1  crack2  driveshaft splines

If a gearcase is left without new oil and water sitting inside for an extended period, the other picture is what happens to the internals.  Lots of rust, gunk, and a complete loss.  In other words - CHANGE YOUR OIL REGULARLY.  If the internals aren't corroded, then careful cleaning may be able to salvage them.  But more often than not there is pitting on the metals and it is a completel loss.

1 2

Here is a wallowed out berring, which also happens when run with old or bad oil, or worst yet, straight water due to a crack or faulty seal.


Maintaining Your Propeller

The propeller is like your tires, it's where the rubber hits the road.  Make sure the blades are all very closely curved to each other (in other words, not out of alignment, bent).  The leading curve is the most important one, but the entire blade should be evaluated.  If you see any nicks on the blades, those should be removed using a metal file.  Shave it down to a smooth, tapered edge.  You can repaint the blade using a good automotive paint (for durability).  If you have any major chips or bends, you may need to replace it or have a marina try to fix it.  But these days, unless you're dealing with an $800.00 propeller (say, for big boats), it may make sense to go and buy a new one.  Most props aren't much more than $150.00.  If your prop is spinning and vibrating wrong due to bent blades, that can cause problems with your entire motor, specifically the gearcase, driveshaft, seals, and potentially the crankshaft and powerhead.

You should pull your prop every so often to make sure there isn't fishing line wrapped around it.  Even if you don't fish, check the propshaft for line.  If you don't, this stuff will wind around behind the prop in a matter of seconds and cut into your prop shaft seals, leading to water intrustion, and gearcase failure (quickly, too).  Put some grease on your prop shaft to make sure the prop doesn't get seized to the propshaft, which I see happen. 

The picture I have here is what I found on a motor where the gearcase had a bad seal.  The beginning of that repair process involves pulling the prop.  This is what I found.  It doesn't take long to figure out why the gearcase had a bad seal after seeing this. There was about 20 feet of line wrapped around this prop, and it was leader and regular line, along with 5 hooks.  Believe it or not, all of this was hidden out of sight and could not be seen with the prop on the gearcase.  Picture #2 is what happens if this is left for awhile, and the line heats up enough (yes, even under water) to melt.

fishing line  2

If you allow this to stay on your prop shaft long enough, believe it or not, rubber and plastic can in fact cut through metal when enough physics are applied.  Here is a prop shaft that has divots on it from the seal and fishing line applying friction long enough.  The picture doesn't do it justice; you can take your thumb nail and catch it on the various "rings" of divots this shaft had.  This happens more frequently on driveshafts, but that is uncommon too.

prop shaft

Your prop nut (if equiped, newer motors have splined hubs, prop nut, and cotter pin) should be the correct one.  Often times the 'redneck fix' is to locktite a hardware store nut to the prop shaft.  WRONG.  It usually takes a blow torch and a long breaker bar to pull the nut off, and potentially drilling out the cotter pin orifice or even replacing the prop shaft all together if damaged.  Taking shortcuts may seem sensible in the interim, but causes bigger problems down the road and more repair bills.  (picture 2)

  prop nut

How to Shift Correctly

This is a conversation I have with every operator that I sell a motor to, or just generally deal with.  The #1 reason why your gearcase fails is incorrect shifting.  If you watch the video on this page, it explains the problem more.  Here is a picture of a worn clutch dog.  You should here a 'clunk' when you shift, and do it as fast as possible, with the motor at the lowest RPM you can make it go (low idle, for instance).

When the clutch dog is worn, the motor may jump out of gear, or not stay in gear at all.  This may also happen due to a worn forward gear, or if your shift linkages are not calibrated properly (which happens a lot with DIY-ers who service their own impellers).  The first picture is of a good clutch dog.  Pictures 2-4 are bad (3 different ones...fairly common situation I run into in repairs).  Notice the leading edge is rounded off, where a good clutch has little to no wear on the leading edge.  Some wear is normal, and there is nothing you can do about stopping this.  They all go bad eventually and this is normal replacement item over time. 

Remember many of these things last decades before failure.  But if you're consistently shifting wrong, this will significantly accelerate this issue, and if you are using a motor with remote controls where your shift box is set up wrong, you can ruin your gearcase in a matter of a few days or weeks.  All the more reason to have things set up by a good mechanic.

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If you don't shift correctly sometimes the shift cradle or lever can become damaged.   These are the least costliest parts to replace, but will cause the gearcase to jump out of gear at any range, even idle.  A collision with an object can cause this too.  Remember there are a lot of forces being generated by a working motor.  A sudden jolt to these parts get's transferred to something; there's really no way to predict what can happen.

The top row shows (left to right) 1) a worn lever, 2) what it looks like zoomed out, and 3) a pitted lever from a rotted out gearcase left to sit with water and no oil (failed seals, water gets in from condensation and the motor sitting for years).

The bottom row shows the cradle that is damaged (likely a collision) and zoomed out what it looks like.  In a perfect world, either of these situations would be the source of a failed gearcase.  In reality, it's almost always the clutch dog or forward gear.  The clutch dog is between $70-200, the forward gear generally is $500.00+.  Not because it costs more to make, but because it is a common wear item that manufacturers can kill us all on.

1  2  3  4  5

Here is another pic of a more modern shifter yoke where the hook that grabs the cradle which actuate a clutch dog has broken off.  The motor was stuck in forward gear; pulling the gearcase apart revealed a simple problem.  Replace this part and voila, the gearcase is restored to proper working order.


Stuck Driveshafts

stuck driveshaftHere we have a situation where you go to remove the gearcase for water pump service, only to find the gearcase won't come out of the motor after having taken all the gearcase fasteners out, and disconnecting the appropriate shift linkages.  This generally happens because the last person who did this forgot to put grease on the driveshaft splines and/or put an O-ring at the top of the driveshaft if the motor calls for it (some don't).  Again - this is amateur hour at it's finest.

Now your driveshaft has seized up into the crankshaft of the motor.  The next option is to force the gearcase off, causing the driveshaft to pull up through the impeller housing and breaking a small portion of the impeller housing due to the impeller key forcing it's way up through the impeller housing.  On newer motors where the housing has a water seal, this will require a full replacement of the imeller housing.  But  this is the least of your problems.  Now you have to figure out how to get the driveshaft out of the crank, if even possible.  I have had about a 50% success rate at doing this.  If you can't get the driveshaft out, whatever way you try, then you have to do a full teardown of the motor and replace the crank.  Lots of headaches.

For the average person, you might as well accept the fact that you will need to buy a new motor at this point.  Cranks cost a ton of money, and there will be many, many hours of labor involved and other parts (i.e. gasket, fastners, etc) that now need replacement.  The reason is because when you pull the motor apart completely, chances are the gaskets that hold it together will not be reusable. 

You *might* get lucky by hanging the motor upside down from a vice and pouring a lot of penetrating oil down the driveshaft to the bottom of the powerhead.  From here you put a large piece of thick flat stock metal across either the exhaust housing or the bottom of the powerhead, then strike it with a 8lb sledge.  This works sometimes and when done correctly will not damage the motor (you may damage the lower crank seal, still requiring a full teardown of the motor to replace the seal).  Depending on the motor, this may not be possible, particularly for very small or very large motors.  Either you're going to break the exhaust housing or powerhead striking it, or the motor is too big to hang upside-down to attempt this process. 

Just another reason to leave repairs to someone who knows what they're doing.

I had a customer donate a 15hp motor one time (this happens frequently with people who decide to take on jobs well outside of their knowledge base, usually destroying the motor, and donating it when they buy a replacement).  The motor showed up with the driveshaft cut in half.  Keep in mind this is 3/4" stainless steel rod.  (picture 1 below)

He went on to explain that the motor wouldn't shift, and when he went to pull the gearcase off the driveshaft was stuck in the crank.  Rather than follow the procedure he should have, he made a bad problem catastrophic by cutting the driveshaft in half.  I was able to remove the driveshaft after pulling the powerhead and using the process I described previously.

Water Relief Holes and Why They're Important

Continuing on from the previous title (stuck driveshafts) the gearcase was a different story.  The reason why the shifter wasn't working is because apparently he had used the motor in shallow, sandy waters quite a bit.  Most gearcases have water relief holes to allow water to drain out of the various passages and cupped internal sections.  This helps prevent filty build up, carbon and exhaust ooze to wash away, and otherwise discourage burst housings due to water freezing when left out to cold climates (picture 2).

ds1  water relief hole 

When these passages get clogged, this allows sand and other debris to accumulate and start to cause issues.  If you are exposed to salt, this can quickly become a problem.  If you are not in salt, it will still cause problems if neglected.  This can also happen quickly too.  The shift rod was stuck because there was about 2-3" of solid packed sand that had become so hardended that it had the consistency of cement.  After working on it for about a half hour with a pick and, believe it or not, a wood boring drill bit, I was able to free the shift rod up. 

Getting to the water relief hole proved impossible (which is common), so I ended up drilling another 1/4" hole above the original configuration.  Later revisions of this same motor had larger holes drilled (about 3/16" - 1/2") to essentially eliminate this issue.  The sand was so densely packed that the chances of any significant water getting into there and causing issues seemed very unlikely.  The pictures below show the internal gearcase before and after I cleared it out.  I was able to restore this gearcase to proper working condition.

clog 1  clog 2

When I pulled the 'nub' of the driveshaft from the crank, it revealed that the crank orifice was also full of hard packed sand, with no evidence of having grease.  This is why the driveshaft was stuck.  I also had to use a pick, compressed air, and repeatedly driving the driveshaft 'nub' into the splines of the crank orifice to essentially purge all contaminants until it could be installed and removed normally.  I actually would up using the removal trick 8 times in a row, each time getting easier and easier, until eventually the driveshaft could be removed with just the force of my arm pulling on it (normal).  Adding plenty of grease and making sure both the female and male end splines where clean brought things back to normal.

Once again, poor servicework by either a low-skilled mechanic or unknowing operator lead to needless headaches and problems for this motor.  Fortunately, I was able to restore it to proper working condition and it lives on to put smiles on it's new owners face.  This is a happy ending, but for everytime this has worked, I can think of at least the same number if not more times where I have to replace the crank and driveshaft completely to restore the motor, or just send it to the scrap yard.  All due to laziness/incompetency in carrying out a repair properly.

Here is a picture of what happens when you let this stuff build up with salt and sand, then let it freeze.  This is from another 15hp gearcase, and it actually blew out completely.  Let's just say the first words out of my mouth when I saw this weren't exactly "I have to take a picture of this one."  You can fill in the blank.


Stuck/Frozen Shifters

If you have ever tried to shift a motor while it wasn't running, you may have noticed that it won't shift.  This is normal for a motor that is off.  What is happening is the clutch dog lugs are lined up with the lugs on the forward or reverse gear, and when you pull/push the shift lever to slide the clutch dog along the prop shaft, it can't interlock with the respective gear because of being out of alignment. 

This is why you shouldn't try to force shift your motor when it isn't running; you can in fact break things fairly quickly if you are really persistent.  If you MUST shift your motor into gear while it isn't running, you should turn the flywheel clockwise (to avoid potential water pump damage) OR you should turn your propeller either direction to change the orientation of the clutch dog and avoid the lock up. 

Before doing any of this, remove the plug leads off your motor, or better yet, remove the spark plugs completely.  There is such a thing called 'runaway motor phenomenon.'  It is rare, but your motor can in fact run without the plug leads attached to the spark plug wires if enough carbon/fuel build up is gunked up in the combustion chamber, and static electricity/charge decides to detonate that gunk.  This can and will happen with no warning, rhyme, or reason.  And most people like their fingers/hands just right where they are.

Now if you are doing either of these things (turn the flywheel or the propeller) and you still can't shift, then you may have a real problem.  I have run into a few situations where the pinion gear, which sits at the bottom of the driveshaft and articulates with both the forward and reverse gears constantly, has broken a tooth (or two).  This could happen for any number of reasons, but if it does and that tooth happens to find it's way between other gear teeth, then your gearcase can quickly become blown up internally.  If somehow you luck out and it finds it's way to the bottom of the gearcase where the magnetic pickup is (most gearcases have a magnet which picks up any metal shavings that inevitably accumulate from gear wear), you may not  notice any problems, but you will eventually at some point.

The other scenario is that tooth could become lodged in such a way that it prevents the shift cradle from actuating properly, and stop you from being able to shift forward, or reverse, or at all.  Here is a picture of a pinion gear with a broken tooth.  The gearcase would not shift into forward, and after opening it up and replacing the pinion gear, it worked properly.


Before you get worked up, make sure you don't have your throttle all the way up to the point where the low idle lockout isn't engaged.   Many motors post 1960 have a cam/mechanism which prevents a manual starter from working when you are throttled up high.  The reason for this is if you accidentally turn off the motor and are in forward gear, then try to start the engine up, you could easily fall out of the boat (or a passenger), then the motor makes a hard port side turn, circles around, and you get run over by the propeller and die.  This happens every year in every state due to broken remote steering cables (make sure you check them regularly). 

These basic safety features also prevent you from shifting out of forward gear into neutral or reverse when at wide open throttle.  What happens when you slam your brakes on in your car without wearing a safety belt?  You go out the windshield.  Well in a boat, you fly out of the boat (or a passenger), then get run over by your own boat, and die.  Again, boating isn't for everyone.  You have to have at least >some< brain activity to be a good skipper.

Why O-Rings Are Important

Another customer brought in a motor (which he subsequently left behind) where he stated he had put the motor away and fogged it, but after letting it sit for a few months, had become stuck.  I was able to move the flywheel with great effort and a long socket wrench (no cheater bar).  The cylinders appeared OK with visual inspection, but I suspected he had somehow overheated it and caused a bearing to get stuck or fail, or perhaps the gearcase had not been changed out and water infiltration had somehow made the driveshaft and internals get stuck.  The other possibility is that the impeller was very stuck or broken down, and jamming up the internals of the impeller housing.

On trying to remove the gearcase to eliminate that as a source, the driveshaft had become seized up in the crank (or at least we both though at the time).  To carry out more repairs would have cost more than what the motor was worth, so I reluctantly accepted it as a parts donor.  This motor had been used over in Kittery, ME, with frequency, and this particular customer had stated he could not get it to run consistently well and had fooled with the oil mixture.  That obviously was a very large red flag for me, and indicated that he plain had no idea what he was doing, because the oil mixture (or potential lack thereof) would not be a source for a motor running poorly, unless he was putting way too much oil into the fuel.

So off came the powerhead.  5 broken gearcase screws, 1 broken powerhead mounting screw, 2 broken inner exhaust tube screws snapped.  Last was the lower powerhead/upper driveshaft water seal bearing.  Surprisingly, all 3 screws came out easily, and so did the driveshaft...still seized up in this bearing housing!  Voila, the powerhead turned over as normal, and the source of the issue was clearly this seized housing.

1 2 3

(Left to right) You can see the upper driveshaft seal bearing seized onto the driveshaft.  The driveshaft itself came out the crank easily as normal.  The lower aspect of this bearing had become so infiltrated with salt and standing water, that as this crystalized it slowly squeezed down on the driveshaft when the motor was left to sit.  But this is unusual.  After holding the entire bearing and driveshaft head under a 600° flame for a minute or two, the aluminum expanded enough where I was able to use a vice as a narrow passage to ram the bearing off the driveshaft.  This took about 50-70 hard blows, so the bearing carrier obviously needed replacing once finally removed.

The last picture shows just how bad the salt intrusion had gotten, but the 'smoking gun' was the complete lack of an o-ring at the neck of the driveshaft.  This is the source of the problem; either the customer or a lazy marine tech had serviced the impeller and forgotten to replace the driveshaft o-ring and/or grease the splines.  The o-ring is responsible for preventing water from working it's way up to the splines and making everything get stuck, the grease further discourages this.  It is also an indication that the customer probably didn't carry out normal service maintenence.  Again - another reason to make sure these jobs are done by a good mechanic. 

I ended up scraping out the exhaust housing in lieu of a replacement (too many salt issues), but the powerhead itself was turned over correctly and showed excellent compresion, along with several other components.  So this motor ended up being a transplant to a happy ending to this particular story, and is still trolling the waterways today with a new operator.

Odd Driveshaft Situations

Driveshafts are responsible for transmitting the forces created in the powerhead to the gearcase, creating propulsion.  Logic would point to reason that they are one of the more susceptible areas of the motor to high applications of torque (or rotary forces, twisting motions).  Here is a driveshaft from a gearcase that had been in a collision.  The gearcase itself had a partially broken ventilation plate, and required full inspection (consequently, it could not be returned to service). 

You can see in the picture that the splines are actually twisted; the pinion gear (which goes on the bottom of the driveshaft and engages your forward and reverse gears) was actually still OK.  This driveshaft could no longer be used because clearly the metal had actually twisted.  You can see where the splines of the pinion gear actually 'chewed' up the splines on the lower driveshaft during the jolt of force.  It's sort of like bending a piece of wire; if you keep bending it back and forth, eventually it will break.  You don't want a weak driveshaft.

Basically the motor was in forward gear winding out, and then the prop suddenly stopped due to the collision.  Then started spinning again.  Now props are designed to slip around their hubs in these situations, but remember your boat is flying forward.  All that force has to go somewhere!  In this example, a good portion of that force went into the driveshaft splines.  There's no way to predict what is going to happen, it just does.

twisted driveshaft

Driveshaft Bearing Failure

Larger gearcases need beefier components to handle the torque and stress placed on their components.  One way to handle the extra HP put out by the powerhead is the implementation of bearings at the driveshaft.  Just below the impeller (water pump) housing is a SS plate, and below that plate is a bearing housing.  In most motors, tiny shims are used to line up the pinion gear with the forward and reverse gears in a very precise manner.  The roller bearing, which is pretty much the same type of thing you find in your trailer tire hubs, helps stabilize the driveshaft and along with the bearing housing and seals, gives the driveshaft some extra support (say, about 1/3rd the length up from the lower splines of the shaft). 

I had a customer bring in a 40hp motor where he said the motor would not stay running once he shifted it into gear.  He had owned the motor for a long period of time, and been doing a fair job of keeping up with normal maintenence.  Out using it one day, it started to make a whining noise, lost power, and stalled.  He was able to restart the motor, but it would stall as soon as it went into gear.  So running fine in neutral, but quitting in gear?  It is easy to surmise that neutral = NO LOAD.  In gear = LOAD, so evidence points in the direction of a gearcase issue.

Pulling the gearcase down off the motor, the next step was to pull the gearbox apart and see if it was a matter of stripped out gears, a failed pinion, who knows what?  After 'gutting' the gearcase, I found the gears to be in good shape, and was left saying, what is the issue here?  These larger gearcases have a pinion gear with a nut holding it to the driveshaft, rather than splines like their smaller counterparts.  Pulling the driveshaft bearing housing, it became very obvious what the problem was - the bearing had disintegrated. 

These bits and pieces you see in the gearcase driveshaft orifice are metal shards, and you can see the roller bearing cage had failed, the needle bearings came loose, and were instantaneously shredded.  Keeping in mind the forces applied in a split second; the driveshaft is spinning at 3, 4, 5,000 RPM, and now it can start wobbling out of sync, and the needle bearings are tumbling around.  The flat discs are the shims, which are set at a precise relationship using a special calibration tool.

The picture you see with the sparkles are metal shards, and then you can see what the individual pieces (or remnants) looked like when spread onto the work bench.  The bearing retaining ring was still mostly there, albeit scored and scratched from metal shards being abbrasive. 

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Anti-Ventilation Plates (sometimes call cavitation)

This is the large flat plate located directed above the prop.  These are most important from 1/2 to full throttle range.  Their main purpose is to prevent air from entering into the spinning prop, which can cause slippage, but also ventilation.  Cavitation is a different situation, which is influenced by many different factors.  A more in depth explanation can be found all over the internet, here is one example.  Keep in mind that correct installation of the motor plays a major role in the effectiveness of this plate, along with the size of your vessel, load, and prop size.  Generally the plate should be the same height as the bottom of your boat, but this doesn't guarantee effectiveness.  Talk to an experienced mechanic to make sure your motor is set up the right way.

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