Converting OMC 5.5/7.5hp gearcase to ’58 10hp Gearcase

[chris-p]

I am converting a 5.5/7.5hp OMC gearcase to be used with a 1958 10hp Johnson (never did have a gearcase for this motor). I realize the shift rod will need to be changed, but what about the drive shaft? I know they have different part numbers, but not sure if that is simply because the 5.5/7.5hp shaft has the hole drilled in it for the carbon seal assembly. Im wondering, if I pull the pin and use it, do the crank splines differ? Cannot think off hand, and Im sure someone on here has done this.

[beerman57]

I had/have an extra QD19 driveshaft, but I just looked at it and I have it marked as a ’62’ ten hp. What I’m thinking is that when I resealed the ’62’ lower unit, I compared driveshafts and they were identical, but the QD shaft was in a little better shape. Anyway, it only has the 4 big splines at the top, if that helps at all.

[chris-p]

If you have it out and accessible, can you measure it end to end for me? Just curious if they are and exact match, minus the hole for the carbon seal.

Thanks!

[dave-bernard]

if a 18hp will fit I have a nice one.

[chris-p]

No the 18hp gearcase is much larger, and a 5 bolt unit.

The 10hp OMC used a 5 bolt gearcase up to the end of 1957. Then in 1958, they started using the smaller 4 bolt gearcase from the 5.5 and 7.5hp models. It made it lighter, and a little quicker, but more prone to damage.

You need a few more parts than I had first thought would be needed to convert a 5.5/7.5 gearcase to make it work on the 4 bolt 10hp motors.

I was originally hoping the drive shaft from the 5.5/7.5 would work, and I would leave the shock absorber in. Im now starting to realize I need to swap out the drive shaft completely, and bearing plate under the water pump.

Might have to start digging through parts boxes! I

[retiredoz]

Starting to sound like finding a 10-hp lower unit would be easier.

[chris-p]

Yeah for anyone doing this, just find a good original gearcase! I like to do things the hard way. Plus I usually have the parts to make it work.

So for those searching this in the future what I believe to be true is:

If you have a gearcase from a AD10M, or a CD-13A through CD-22M, you can make it work on the QD-19 through QD-24M by:

1. Changing out the shift rod to a 305319 rod
2. Changing out the drive shaft to a 305368 shaft
3. Changing out water pump housing (pn 310000 or 305302) complete with spacer and o rings on top (305552 for spacer)
4. If you have a QD-19 through QD-21, you can use the gear sets from the CD-12 through CD-22 (pinion gear changed on the CD- 22), or the AD-10 through AD-12
5. The SS impeller wear plate can stay, but the drive shaft housing and seal assembly will need to be swapped to the 377186 assembly with drive shaft seal 303345. The gasket for the seal assemblies is the same.
6. Changing the prop to a 377635 which is the original 8 1/4 x 8 1/2 3 blade. Drive pin obviously the same as same prop shaft.

So moral of the story, just buy a complete case.

Stay tuned for tomorrow overly difficult restoration 😉

[beerman57]

25 & 9/16

[retiredoz]

Don’t you need a shear pin now, as opposed to a drive pin ? Without the shock absorber there’s no ‘give’..

[chris-p]

The 10hp prop has a rubber hub to take up any impact and protect the powerhead.

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