Mechanical 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

Split Stabilizers
Flywheel Issues
Failed Driveshaft
Corroded Exhaust Housing
Broken Cranks

Split Stabilizers

Most smaller OMC motors utilize a system of 6-7 stabilizers to attach the powerhead and exhaust housing the transom bracket/clamps.  There are 3 just underneath the motor pan (lower cowel); one aft, one starboard, one port.  At the bottom of the exhaust housing/leg, there are usually two screws that run laterally and two screws running bow to stern on the lower bracket which hold 3-4 other rubber mounts.  These lower screws are usually the ones that become stuck because they are always submerged in water, particularly in salt applications.  There are usually 4 more rubber mounts below the lower cowel/motor pan to further reduce vibrations.  The fail over time as well.  Basically anything that is rubber, will fail over time.  Not much you can do about this.

Now when these things fail it leads to safety issues as remember that these points are what keep the motor attached to your transom!  I see folks running motors all the time with broken motor mounts, and usually they are the same people who are being a little too cheap in terms of maintenence.  This is absolutely one area that you DO NOT want to be pushing your luck on.  Granted, the chances of the motor breaking loose are low (unless in various obvious, serious situations), but why chance the safety of you or your passengers over a few bucks?

Here are some pics of broken motor mounts.  You may not be able to see them, but if you have the motor mounted and can physically move the powerhead/upper pan forward or backward more than a 1/2 inch, and possibly see/feel a 'thud' each time, well, you likely have some broken motor mounts.  Picture #1 shows an aft stabilizer that has broken; there should be a screw stud on both ends of it.  This is what keeps the top of the motor from shaking forward and backward.  If you drop a motor on the skeg, these often split in half.  So say you accidentally put too much weight in the back of your boat and you go to unhook the trailer from your vehicle, and the tongue swings up in the air.  What hits the ground first?  The skeg of the motor, as the whole system pivots on the trailer wheels like a teeter-totter (1st class lever).  Now you have to replace this motor mount (yes, I have accidentally done this before...easy to do with an aluminum boat).

Picture #2 shows a lateral mount that has split.  You can see the rubber has vulcanized and has cracks, but where the transom bracket attaches via the stud, the rubber has separated from the mount and no longer offers a solid connection.  These are common situations.  The powerhead/lower cowel should have some play involved to help damper normal vibrations of the motor under normal circumstances.

The three horizontal pics are a front, rear, and side picture of a lower motor mount on a 9.9/15hp motor.  It was totally shredded.  The one on the left is the deteriorated one, on the right is a normal one.  The last photo is a forward lower mount that has separated from the lower unit/exhaust housing on a 30hp motor.  Commonly, this happens when trailered improperly.

stabilizer1  stabilizer2


Flywheel Issues

The flywheel serves a few very important functions.  First, it usually has a magnet installed in it which is used as part of the ignition system to generate spark and combustion.  Second, it helps balance the forces produced by the powerhead and prevent premature gearcase wear.  Even more importantly, flywheels help maintain the inertia created by the spinning crank for overall engine economy.  So when you are a high throttle/RPMs, when you throttle back, if you did not have a flywheel spinning with inertia, the motor would slow down rapidly. 

This means that everytime you needed to accelerate or decelerate, the motor would have to work that much more rather than using the inertia of the flywheel to gradually slow down/speed up.  This could possibly lead to faster mechanical failure of the internal components of the motor as well. 

Flywheels sit on top of the crankshaft and utilize a taper to hold it in place.  They need to be installed with specific torque values so they can function as intended.  DIYers generally don't know what these are, or simply do not have the tools to remove/install correctly.  Again - don't get into something that is over your head.  There really isn't a margin of error here, it has to be correct 100% of the time.  A flywheel (woodruff) key is installed in the crank taper to insure the flywheel sits in the correct location so that the timing of the ignition system is right.

Here is a picture of the keyway of a flywheel that has been wallowed out.  This should be perfectly square under normal circumstances.  A number of things can cause this, but generally this type of wear is due to sudden shockwaves being transmitted throughout the motor which can stem from any number of sources.  If a flywheel is torqued down the wrong amount, this could possibly lead to a worn flywheel.  This cannot be used any longer as if the flywheel changes it's position even by a 1/2 mm, then the timing of the motor will be severely affected, which only applifies the whole issue in very short order.  To the scrap yard this went!

wallowed keyway

Below are two pictures of a flywheel where the woodruff key has sheared off and melted into the flywheel keyway.  Again, incorrect torquing or an imbalanced motor can lead to this.  The taper on the crank and flywheel orifice should be perfectly clean as well.  Often times DIYers will put grease or pentrating oil on the tapers thinking this will make it easier to remove/install the flywheel at future junctures.  This is totally wrong. 

The flywheel is supposed to stick to the taper, and usually comes off with a loud 'pop.'  I've removed flywheels before that were so loose you could almost remove them by hand (which is absolutely wrong).    In these two cases, if the broken key can be chipped out without damaging the keyway, the flywheel could be used again.  Removing the broken pieces from the crank orifice is a different situation.

key1  key 2

A more obvious issue is if the flywheel is damaged.  Below is a flywheel with a few teeth missing.  Depending on the motor type you have, this may not mean anything if the manual starter is employed and utilizes bosses on top of the flywheel, independant of these teeth.  But if you have an automotive type manual starter with a pinion gear, or an electric start model, now you have a problem, as the missing teeth will not allow either to work properly.


Failed Driveshafts

This is a rare situation, in fact so rare, that some other long-time (in many cases, retired) mechanics have conferred that you might only see this once or twice in a lifetime under normal circumstances.  Driveshafts are made of stainless steel.  So literally the metal has failed, but that is extremely uncommon.  Driveshafts can and do break when the motor encounters a collision, and this is something I have seen many times.  There is usually evidence in other parts of the motor of damage as well such as the gearcase or exhaust housing, or the mounting hardware.  But to have a driveshaft fail on it's own under normal usage is generally going to be a manufacturing defect. 

Generally with a collision the driveshaft will simply snap in half.  Remember the rotary forces created by the crankshaft are being transmitted down the driveshaft (so you are applying a twisting force), into a pinion gear, and then to the prop shaft and various gearcase components.   When the prop is suddenly stopped, the prop will spin in the prop hub and/or break the shear pin.  These two provisions generally prevent damage to the motor, but not necessarily give it 100% protection; it's the best you can do.  If the collisions is of great enough magnitude the driveshaft is going to take on this sudden stop.  There are a lot of forces being generated here!

Here is a picture of a driveshaft that broke under normal usage (no collision) at the impeller key.  The customer was boating along when suddenly he lost all power and the motor started winding out at high RPM.  Well the reason for the high RPM is because the powerhead had nothing to work against with the broken driveshaft.  Upon receipt, I would shift the motor and here the gearcase click into gear, yet I could turn the flywheel without seeing the prop turn in any direction.  Further diagnosis became suspect when I removed the gearcase off the motor to see the driveshaft still mounted in it's proper position.  Off came the impeller housing and the problem was very obvious.  Of all the places a driveshaft could fail, I suppose this would be the best spot due to the smaller amount of materal left to absorb the rotary forces, due to the impeller key orifice (the little metal peg, which holds the impeller/water pump in place).


Corroded Exhaust Housing

The exhaust housing is often referred to as the lower 'leg' of the motor. In fact what it is responsible for is protecting the driveshaft in an enclosure, the shift shaft, the water tube which delivers cooling water up to the powerhead, and finally directly exhaust down from the powerhead to an under water orifice.  Perhaps the most obvious function is noise suppression and keeping exhaust emissions away from an operator.  If you've ever owned an outboard where the shift shaft linkage is accessed via a little 'door' on the exhaust housing, and this cover has been lost or removed while the motor is running, then you quickly learn just how important the housing is.

Without a properly enclosed housing, every internal combustion explosion sends out a deafining pop.  It literally sounds like an automatic shotgun going off and is an absolutely miserable thing to be around.  If you aren't underway, you quickly find yourself inhaling exhaust fumes too.  Even modern day motors with next to no emmisions still give off an odor, and your clothing and hair will get saturated with it.

Here is an upper exhaust housing for a 15hp motor that has two holes in it.  After carrying out all the repairs necessary I was running the motor on it's initial test trial.  It was vary obvious there was something wrong, as exhaust fumes started going everywhere in a very unusual way.  After some investigation it was clear what the source of the problem was.  Well this also meant I had to do a full disassembly of the motor to replace the exhaust housing.  Who knows how these holes got here.  And how did the previous operator keep using the motor thinking this was normal?

You can see the upper driveshaft after I had removed the powerhead, the gearcase was still mounted on the exhaust housing.  It ended up being removed and having the entire exhaust housing assembly replaced.

exhaust housing holes

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