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Snapped Main vertical shaft 15hp 2-stroke short shaft

2

2001rotax800

Member
So I am half way through my hunting season and have some serious motor issues. The other day while traveling wide open the motor hit hard and stopped, almost like the top end seized, but it fired right up again. I have been noticing the engine not reving up as high and having a "whine" to it, well the problem showed itself yesterday. The main shaft snapped just above the spline gear in the lower end.

I have dismantled the lower unit, surprisingly all gears and bearings look great with the exception of the thrust washer that sits above the pinion gear, and the shaft guide bushing. It appears that the play that developed allowed the main vertical shaft to wear groove and eventually allow the pinion to bind up on the gears snapping the main shaft.

2 Questions:

1. Anyone ever see this before?

2. I was able to press the lower shaft guide out, but I cannot seem to figure out a way to remove and replace the upper.

Thanks
 
So after further research I have discovered that there is a "Design 1", and a "Design 2". Design 2 has a needle bearing in place of the bushing on the lower most part. I guess I answered my own question "If anyone has seen has seen this before?" Apparently Mercury has, which is why "Design 2" now includes a needle bearing.
 
That is a driveshaft design flaw not a lower unit flaw. That lower unit is the same one that fits the 6/8/9.9 and 15 hp motors it is not a great design. The design 1 and 2 lowers are both good just make sure you never have water in them and change the lower unit fluid once or twice a year.
 
How is it a design shaft flaw?

The new version uses a needle bearing instead of a bushing on the lower guide portion. My bushing became so worn, it wore it allowed the driveshaft and spur gear to shift ever so slightly and create massive heat(Stainless drive shaft is blued locally) and eventually became so misaligned that it bound up, and with the inertia of the fly wheel snapped the shaft a it weakest point..
I am working on retrofitting the lower failed bushing with the new version needle bearing.
 
I have seen both designs snap the driveshaft like that. The drive shaft tapers down to much where it goes into the pinion gear it's a weak spot on a less than robust lower unit. I have seen motors that hit rocks snap that off. The bushing became worn because either the gear lube was not serviced regularly or water was in it. I have seen well cared for design 1 lowers last the life of the motor and still going. Design 1 and 2 use the same housing, the bearings will fit right in there. I have seen 70 hp motors throw the upper bearing and lock everything up and not snap the driveshaft.
 
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Well thanks for the feed back. I am finally getting around to fixing this (since hunting season is coming up). How should I approach this? I have all the parts and bushings out of the lower end already. Thanks for your advice.
 
Blueghost said:
Do you have any pics of where the shaft broke? Did you hit anything before it snaped?
Click to expand...
Post #1 tells you where it broke it's where they all break.
As far as putting it together again I would look at the parts break down carefully and see if I could rebuild it as a design 2 lower. I am pretty sure it can be done but I have never done it.
 
For the cost of these parts I would buy a used lower unit or pick up a whole parts engine. I have two of these 9.9's any way to tell Design I from II from outside? Parts diagram also show variance in water pumps...
 
Blueghost said:
For the cost of these parts I would buy a used lower unit or pick up a whole parts engine. I have two of these 9.9's any way to tell Design I from II from outside? Parts diagram also show variance in water pumps...
Click to expand...
Where are you going to find a good used one? lot of powerheads for those not lower units. It is not a great lower unit
 
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