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Drive Shaft Seal

"If you have oil in the bellows, it comes from the drive shaft seal. That's where it comes from."
Replacing the Drive Shaft Seal is Easy. Setting the Bearing Pre-load is pretty easy.

On these two units, one drive shaft had some pitting on the sealing surface. The seal is held still in the carrier and the shaft spins. Fortunately, the new seal did not leak. Neither did the old seal for that matter. The unit that leaked oil into the bellows had a seal that moved during pressure test.
Water in the bellows can promote rust on the yoke where the drive shaft seal rides. The yoke can be replaced or it can be turned down (machined on a lathe) and a sleeve installed to renew the surface. Another option is to turn and grind the yoke and specify and install a seal having a smaller inside diameter. I would just replace the yoke.
A word about pressure testing.
It seems that pressure testing is pretty standard practice. Or, at least that's what you might read on the internet. However, the service manual does not call for pressure testing or vacuum testing the outdrive. Pressure testing does not guarantee a leak free unit. And as you can see in this video, there is at least one proven risk of failure. Pressure test or not? You decide for yourself. As for me, if the thing leaks oil, I'll find the leak and fix it. If there is water in the oil, I'll find the leak and fix it. I might use a pressure test to do that. If I have the thing apart, I'm replacing all the seals.
Something to consider when Setting Bearing Pre-Load
The bearings are pressed onto the Pinion Gear. When pressing on or pressing off the bearings, the bearings do not slide smoothly down the shaft of the Pinion Gear. Sometimes they pop and jump down in descrete steps. Tightening the Pinion Nut to set the Pre-load attemps to move one of the bearings on the Pinion Gear shaft. The bearing close to the gear should already be bottomed out. If you crank down on the Pinion Nut and the bearing never moves and then install the unit, the bearing could pop down tight the first time the engine is cranked. The bearing would then be too tight. Some folks will use a bearing separator to move that bearing a little bit before beginning the drive shaft bearing preload procedure. Watch the Drive shaft Bearings video. I measure the pre-load before I take the pinion nut off and write that value down. I don't try to exceed that value when setting the pre-load on the used bearings. I find I can feel when the nut is bottoming out to its original torque.
I use an inch pound torque wrench that has a scale of 0 to 30 inch pounds. The one called for in the service manual is 0 to 150 inch pounds. It is difficult enough to tell the difference between 7 inch pounds and 9 inch pounds when the scale is 1 to 30. It is impossible to tell the difference on a scale of 1 to 150. I bought my used 0 to 30 inch pound 1/4 inch drive torque wrench on eBay for around $100.

Drive Shaft Seal

Sunset on Oneida Lake
Sylvan Beach, New York