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

[Don]

Funny you should mention this topic, I am getting ready to post a thread concerning some of the engineering issues on both styles of these gearcases used in the 4-8hp engines with full gear shift….
In any event, this "ball bearing clutch system" was introduced in 1988 on the 4deluxe thru 8hp models, replacing the conventional clutch dog system used on earlier models. But, it is important to note that OMC went back to the conventional clutch dog system on these engines in the early 90s, so I’m thinking this system performed no better than the clutch dog system it replaced, maybe it was cheaper to produce. Those balls are surely inexpensive, but the gears lugs are subject to wear just like on the clutch dog system. I am trying to remember, but I think some of the ball failures included the balls actually being sheared in half and escaping into the gearcase. So, most failure/repairs will probably include at least one gear.
I will start my thread here with yours, and try not to go on and on like I am prone to do.
In any event, I remember doing routine servicing to these gearcases (both styles) many years ago, and always noticing that gear lube seemed black/burned. This was not necessarily a case of water intrusion, just burned up/black/thick lube. Back then, I never paid much attention, just changed the lube. It’s only in later years that I have pondered just what the cause(s) could be for lube failure in this little light duty gearcase. It is only in the past few years that I have discovered the problem.
Look at item #1, the prop shaft bushing seal carrier, then look at item #8, the reverse gear. The back face of that reverse gear rubs directly on the inner raised rib of the prop shaft bushing/carrier. There is no bronze thrust bushing/bearing used here. Now notice the spring that preloads the clutch dog/balls forward or the prop shaft aft, adding even more friction between these surfaces. I’m no metallurgist for sure, but the steel gear surface rubbing on that aluminum rib is going to make a lot of heat and friction between those two surfaces. Like I say, I am used to the reverse gear on most gearcases being cushioned by a bronze bushing or actual thrust bearing preventing this type of friction/wear. And, in this situation, this problem is amplified by the spring inside the prop shaft which adds even more pressure between these surfaces.
Now, how does this situation relate to shift system problems. Eventually, the inner aluminum rib is worn down by the hard gear surface resulting in excess prop shaft end play. This prop shaft end play results in inadequate dog/ball engagement in both gears. The problem is that the servicing techs (myself included) just replaces the dog/balls/gears thinking the damage is a result of improper shifting. The repair never lasts long before shifting problems reappear, or in extreme cases, the engine won’t even shift correctly right after the (incomplete) repair.
So, why is it that us technicians never notice all the end play in the prop shaft? Very simply, because that darn spring holds the prop shaft tight against the reverse gear so you can not easily notice evaluate the excessive prop shaft end play. In order to evaluate the prop shaft end play, the spring must be removed from the prop shaft. Not a real big deal, just removing that bushing seal carrier. Unfortunately, the balls fall out into the case on the units with the ball shift system. But, on the other hand, you wouldn’t be looking for excessive end play unless the unit was in for shifting problems, so it would have to be pulled down anyway.
The obvious fix is to replace the prop shaft bushing/seal carrier, this part is very expensive if you can find one, keeping in mind that the problem will reoccur, especially on engines used as sailboat auxiliaries or high hour tender use. I have been pondering how the heck to repair this problem properly, without having to replace the original prop shaft/seal bushing. I just starting googling outlets for bronze bushings and found one that fits perfectly between the reverse gear and the worn aluminum prop shaft/seal bushing. The gearcase I am currently working with had about 1/8" prop shaft end play. The bronze thrust bushing I found online is about 1/8" thick as well. The prop shaft bound up when the unit was gently assembled, so I had about .025-.030" machined off the raised aluminum rib of the prop shaft bushing/seal carrier. Now, there is about .010" prop shaft end play and there is a bronze thrust bushing between the reverse gear and prop shaft bushing/seal carrier.
I have taken some pictures and will post them later this week….Don