850xs gear case into 1500xs gear case project

[milkdud]

Ok I have a bare XS gear housing that I wanted to put my good 1.78 or 2.0 gears from my std 1500 into. I am finding out that I may have a 850xs gear case instead of a 1500xs gear case.

None of my driveshaft/lower water pump bases work in this case. The 850's were set up with helical gears and are slightly different parts in there.

Has anyone tried to change the 850xs gear ratio from 2.3 to 2.0? or even 1.78?

I never thought that there would be such a difference but there seems to be.

I have 2.0 and 1.78 gears to use and two different types of 15 inch drive shafts. (roller and taper).

Any ideas ?

Thanks

[89HST]

I have 1.78's in mine now.

[milkdud]

I have 1.78's in mine now.

Was it a direct swap? Did you just put 1.78 gears in there instead of the 2.3's? Any other things needed?

Its looking like I need to find an old 850 lower unit to cannibalize parts from. My 1500 parts are not much use at this point.

Thanks
Conrad

[milkdud]

Well I thought Id update what I could find out.

This case turned out to be a 850xs and the short of it is there is not any easy way to put 1500xs internals inside of it.

The 850 units had spiral cut 2.3 gears, that means the drive shaft bearings were set up to pull down towards the prop shaft and thus had different bearing and different driveshaft.

The 1500's had straight cut 2.0 or 1.78 gears and they push up away from the prop shaft naturally.

I think Mercury forecast someone trying to do what I am trying to do and they made it very hard to swap. All the bearings have slightly different OD's and ID's then the others. Even the dive shafts are slightly different. The 850 bearing actually slides on from the pinion side with the driveshaft lip above it. The 1500 slides on from the crank side with the driveshaft lip below it. Because they are controlling opposite direction of forces.

Externally they look the same and above the lower water pump base they are the same.

The only way I can make this case work is if I have a machine shop bore out the case under the water pump .35 inch to accept the larger sized 1500 tapered bearing. I would also have to address additional shim spacers to get the bearing up higher. As the 850 bearing naturally sits lower than the 1500.

Also when raising the bearing up in the 850 case you run into blocking the oil passage to the upper bearing. It also would have to be modified to get past the new location.

Some may ask why not just use the 850 drive shaft components and swap out the 2.3 spiral gears for 2.0 or 1.78 straights. All you need to do is flip the tapered roller bearing over to control the force in the opposite direction. This is somewhat true but if yo did that particular bearing is not designed to work upside down. Nor is the drive shaft. On the 1500 tapered roller bearing there is a special extra collar on the bottom of the bearing cage that supports the driveshaft from falling down when not in use. Also the 850 driveshaft would let the bearing work its way off if the forces were in the opposite direction.

So I am left with a choice. I could bore out the case to accept 1500xs components. Or just letting it sit and someday make it into a 2.3 850 xs case when I find a donor 850 lower unit.

89HST mentioned he used XR6 gears, which I assume are spiral cut. I don't have XR6 gear nor do I have any 850 parts :( 89HST will have to elaborate when he can, describing how he did it. Not sure exactly his procedure.

If someone finds anything I said incorrect please advise me. If there was a ball bearing with a .985id and 2.25od on the market I may be able to use it. But so far I cannot find one.

**NOTE: The bearing I refer to is the one right under the water pump that sets the pinion height and supports the drive shaft. Can be a roller bearing or a ball bearing depending on what year produced**