Out with Clutch Dogs: In with Ball Bearings in the OMC 6 HP

[fleetwin]

Frank
Totally agree. While running in forward, there is minimum pressure/friction against these two parts, the clutch dog pin hits its stop against the prop shaft which minimizes spring pressure pushing that prop shaft back toward the prop shaft bushing/seal carrier. Sailboat/tender use will definitely accelerate this condition.
And yes, it would be best if I/we can figure a way to "pin" this thrust bushing to the bushing/carrier, this will definitely be a key focus on the next one of these gearcases I repair. The bushing seems to want stay in place, which is good, I guess maybe there is less rotating friction between the gear and bushing then the bushing and housing. This unit, is from a ratty old 7.5hp I will keep for myself, so I will monitor it closely. I got this engine thanks to Dan Anderson who picked it up for $30 and sent it up to me with the help of Gary Mower. To look at it, one would not think much, pretty ratty. But, as I have learned, never judge a book by its cover. I started by seeing if I could even get the gearcase off, it pulled right down and the splines looked fine. I pulled the head and exhaust cover, everything looked fine in there also!
I will post some pictures tonight. It just baffles me how engineers thought this gearcase could survive without a thrust bushing/bearing between the reverse gear and prop shaft bushing.

[fisherman6]

Don,
You are correct in your thinking about the difference in friction coefficient between the gear and the thrust washer vs. the gearcase and the thrust washer. I’m assuming the thrust washer is phosphor bronze. I realize the gearcase is in an oil bath and the numbers I’m going to give are dry, but since the lubricant is so subjective, that seemsounds like the best for the sake of comparison. The friction coefficient between aluminum and steel is 0.61. The friction coefficient between phosphor bronze and steel is 0.35. I couldn’t readily find any references for phosphor bronze and aluminum, but being that the aluminum is a much softer and gummier material, it will certainly be higher. Theoretically the thrust washer would not rotate in this application, but… we all know theories don’t always work in the real world. If one could find a bronze thrust washer with a diamond knurled on one face and place the knurled face against the aluminum gearcase, it would certainly bite into the gearcase casting and prevent further rotation with a firm shift into reverse.

Bearing materials are a bit outside of my everyday engineering expertise. I am an industrial machine and automation design engineer so I do certainly have some familiarity with them and their applications.
___________

As far as how they could have thought this gearcase would survive. I also understand how things get designed to do a certain job in a certain way. Inevitably the end user will find a way to use the product in the real world in a way that it was not necessarily designed to be used. I could be off base in my thinking here, but it seems likely to me the this gearcase was designed for a fishing motor that spends nearly 100% of its time in forward gear. Reverse would be used only momentarily mostly when leaving or returning to the dock. For that purpose, it seems OK if not optimal. I’m guessing the heavy use of reverse in sailboat use and possibility of back trolling wasn’t given enough consideration in the design phase. I bet they realized like they had a problem in a lot less time than it took them to fix it. There is a lot of politics and accounting involved in basically scrapping a new design and replacing it with the previous one.

Whew! I may have gotten a little windy on that one myself. Everything below the line is just my theory based on what I know about engineering for industry.
-Ben

OldJohnnyRude on YouTube

[rudderless]

Also the reverse balls that are not engaged when going forward are thrown outward and interact with the inside of the reverse gear. Which is rotating the opposite way..so the balls rattle on the inside of the reverse gear when in forward. Thats why they use balls instead of pins. The balls can work in and out of the prop shaft without catching the inside of the reverse gear when in forward.

[fleetwin]

quote Fisherman6:

Don,
You are correct in your thinking about the difference in friction coefficient between the gear and the thrust washer vs. the gearcase and the thrust washer. I’m assuming the thrust washer is phosphor bronze. I realize the gearcase is in an oil bath and the numbers I’m going to give are dry, but since the lubricant is so subjective, that seemsounds like the best for the sake of comparison. The friction coefficient between aluminum and steel is 0.61. The friction coefficient between phosphor bronze and steel is 0.35. I couldn’t readily find any references for phosphor bronze and aluminum, but being that the aluminum is a much softer and gummier material, it will certainly be higher. Theoretically the thrust washer would not rotate in this application, but… we all know theories don’t always work in the real world. If one could find a bronze thrust washer with a diamond knurled on one face and place the knurled face against the aluminum gearcase, it would certainly bite into the gearcase casting and prevent further rotation with a firm shift into reverse.

Bearing materials are a bit outside of my everyday engineering expertise. I am an industrial machine and automation design engineer so I do certainly have some familiarity with them and their applications.
___________

As far as how they could have thought this gearcase would survive. I also understand how things get designed to do a certain job in a certain way. Inevitably the end user will find a way to use the product in the real world in a way that it was not necessarily designed to be used. I could be off base in my thinking here, but it seems likely to me the this gearcase was designed for a fishing motor that spends nearly 100% of its time in forward gear. Reverse would be used only momentarily mostly when leaving or returning to the dock. For that purpose, it seems OK if not optimal. I’m guessing the heavy use of reverse in sailboat use and possibility of back trolling wasn’t given enough consideration in the design phase. I bet they realized like they had a problem in a lot less time than it took them to fix it. There is a lot of politics and accounting involved in basically scrapping a new design and replacing it with the previous one.

Whew! I may have gotten a little windy on that one myself. Everything below the line is just my theory based on what I know about engineering for industry.
-Ben

Ben-
Yeah, OMC was just like any other company, cost was the only thing that trumped politics. I’m thinking you are correct, the engineers felt the design was "good enough" for most uses, but then they had no business marketing "sailmaster models" with high thrust props and extra exhaust reliefs. I could forgive this if after seeing higher than acceptable failure rates, they added some sort of bronze type thrust washer that could be retro-fitted to older models. In this case the politics probably kicked in thinking these small engines were not worth the engineering effort diverted from larger models along with the cost. This is a shame because these engines run very nicely and develop quite a bit more power than the conventional 6hp models they replaced, but I’m sure many customers went running to the competition after being socked with gearcase repair bills and driveshaft spline problems.

[fisherman6]

Yup Don. I believe we are on the very same page here. It is a shame for sure. I’d like to get one of those motors myself, but haven’t found a good deal on one. I would probably do the same thing you are doing with the thrust washer since I think that’s a great idea. I’m completely with you that OMC should have done something like this after realizing they had a real problem. Design changes are expensive, but sometimes not as expensive as the intangible losses incurred by a design issue.
-Ben

OldJohnnyRude on YouTube

[fleetwin]

OK, here are some pictures and explanations of this issue:

This is a picture of a typical reverse gear out of one of these units. Notice the shiny/galled looking area, this is the area that rubs on the aluminum prop shaft/seal bushing. Looks like the metal has gotten hot. Look carefully at the slight raised rib near the hub of the gear, the bronze thrust bushing I found must be carefully oriented over this rib or the prop shaft will bind up while the gear/shaft/bushing/bushing seal is installed into the gearcase.

This picture shows the reverse gear with the inexpensive bronze thrust bushing properly installed over the reverse gear. Notice the shiny inner aluminum rib on the prop shaft bushing/seal housing, this is the surface that the reverse gear normally "rubs" on. The friction and heat developed during normal operation slowly wears this aluminum surface down resulting in the burned gear lube often times seen during servicing along with the excessive prop shaft end play. The excessive prop shaft end play results in shifting/gear jumping problems, this includes the ball shift style gearcases. The excessive prop shaft end play is hard to detect because the clutch dog spring is preloading the prop shaft. The only way to really evaluate the prop shaft end play is to disassemble, remove the spring, then reassemble and evaluate prop shaft end play. Unfortunately, I can’t post the original dimension of this aluminum rib because I don’t have any of these prop shaft bushing/seal housings in new condition. This unit had almost 1/8" end play, the new bronze thrust bushing I found is about 1/8" thick as well. Once assembled with the thrust bushing, the prop shaft bound up, so the aluminum rib on the prop shaft bushing/seal hsg had to machined down about .020". Our chapter is so lucky to have DAVE BONO as a member, he popped the aluminum prop shaft bushing/seal hsg in his lathe and machined about .020" off in a matter of minutes. Now, with the new bronze thrust bushing installed, the prop shaft has about .010" end play. Please don’t be misled by this picture, it shows the bronze thrust bushing already in place on the reverse gear, I should have take a picture of these two pieces without the added thrust bushing. Remember, the bronze thrust bushing must be properly oriented over the raised rib on the reverse gear.

This picture shows the reverse gear with the bronze thrust bushing properly oriented the gear’s raised rib.

This picture shows the reverse gear, added bronze thrust bushing, and aluminum prop shaft bushing/seal housing properly assembled on the prop shaft.

This picture shows all the pieces IMPROPERLY assembled. The bronze thrust bushing is not properly oriented over the raised rib on the reverse gear.

Another picture of the pieces PROPERLY assembled.

This is a picture of the assembly reinstalled (bolts gently seated) without the clutch dog spring to evaluate prop shaft end play after the update/repair. This unit has about .010" end play now instead of almost 1/8". Again, it would be best to figure out some way of "pinning" the bronze thrust bushing to the aluminum prop shaft bushing/seal hsg so the bronze bushing does not rotate on the aluminum surface. But, there seems to be less friction between the gear and thrust bushing, so the thrust bushing seems to rotate on the gear face naturally.
I will post a link to the site where I found these bronze thrust bushings, they cost only $3 apiece. Much less expensive than replacing the aluminum prop shaft bushing/seal housings that are no longer available new. I’m hoping this repair will reduce the friction and minimize friction generated gear lube failure as well…
Don

[frankr]

I couldn’t help but notice the striking similarity between that gearcase design and the Chrysler Sea King I just posted. Nothing new under the sun.
viewtopic.php?f=2&t=11731

[b-dittmar]

quote FrankR:

I couldn’t help but notice the striking similarity between that gearcase design and the Chrysler Sea King I just posted. Nothing new under the sun.
viewtopic.php?f=2&t=11731

Yes but the Chrysler has a bearing for the gear.

[fleetwin]

Here is a link to the supplier for the bronze thrust washers:
http://www.bronzebushings.com/catalogse … =tt+1800-1

[fleetwin]

Another thing to consider when servicing this style gearcase…..
Sitting home with a "broken wing", can do very little except think about stuff.
I found the impeller had shreaded/melted/just the hub left when I pulled this gearcase off the engine. Needless to say, I went "on the hunt" for impeller pieces while pulling the powerhead apart. Nothing in the water tube, saw nothing in any of the water passages either….This debris had to go somewhere, doesn’t seem like it all could have been expelled from the cooling system….
This gearcase has a hidden "chamber" under the plastic cover. The water is drawn up from the screen in the exhaust snout, it goes through this "chamber", then through the plastic top, before ending up under the impeller plate.

I feared some of the debris could be down inside the screen, or in the passage, and did my best to try to blow it out. I even back flushed with parts washing fluid which expelled a few impeller pieces out of the top passage…I couldn’t help but fear that more debris could be hidden down there, but couldn’t see much with a light. I started to think about how the gearcase water passage was laid out under that pesky plastic cover. I really wanted to avoid pulling that plastic cover off, these plastic covers are a royal pain to clean, and tough to reseal onto the gearcase. Salt water really destroys these covers, so I am used to just replacing them. Availability and pricing has curbed that practice! Try replacing that plastic screen in the gearcase, ha! $100 worth of some universal screen is the replacement here.
I wrestled with this for too long before deciding I better pull the cover off, and look what I found:

A lesson learned for sure. These pieces could have plugged up the system or damaged the new water pump!

[Author]

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