Just one more thing. I mentioned a vacuum test earlier. Here?s why (and why I have a ?vac? side on my test gauge).
Standard lip seals generally only seal fluid in one direction, the ?lip-side?. As the pressure increases (to a reasonable level) the lip will be pressed down on the shaft and forms a tight seal. And this brings up another issue. A seal may be good at higher pressure, but leak at very low pressure. To that end, I recommend pressure testing at 3 or 4 psi as well as ?full? pressure (between 10 and 15psi). And I like to spray a little 'soapy water' around the seal areas while they are under pressure (both low and higher pressure). A very slight leak may not show quickly on the gauge, but a big bunch of soapy bubbles is a dead give-away.
If fluid is pushed from the ?non-lip? side, the seal will allow the fluid to pass at a much lower pressure. This is why there are ?back-to-back? seals on the lower drive shaft (water pump base) and the propeller shaft. The inner seals? lip faces the oil and stops oil leaking out of the drive. The outer seals? lip faces out and stops water passing into the drive. A pressure test only tests the drives? ability to stop oil getting out. The vacuum test tests the drives? ability to stop water getting in. I usually pull about 9-10 inches of vacuum. On older drives you may get a vacuum leak at about 6 inches from the lower shift shaft seal. At 6 inches, I accept that as ok, any less gets it replaced. I expect a new one to hold around 10 inches easily. You will also note that there is only one seal at the front of the drive-shaft housing, facing in (to the oil). That?s because the only thing on the other side of the seal should be air (in the bellows), hence there is no need of a second seal.
Chris.....
BTW. 'Back-to-back' seals must have grease pushed into the cavity between them, or the outer one will burn up. Also, while pressure and vac testing, rotate the shafts (Prop, drive and shift shafts).
