Vacuum vs Pressure? Alpha One G1

[Fiat4Fun]

Howdy,
Working on my 1986 Mercrusier Alpha One , Gen 1.
Had my lower end replaced under warranty, and my mechanic checked both the upper and lower. When we checked the lower, we were able to get 15lbs of pressure, and 15" vacuum. But the upper got the 15lbs of pressure, but would only go to 9" of vacuum, and then drop to 7". Tried to track down the specs in my mercruiser manual, and found nothing about vacuum. The local mercruiser shop said it was fine, but my mechanic was concerned about it, since he is a OMC kinda guy, and thought that the alpha one should be the same.

Any thoughts or opinions on this?
Thanks in advanced.
Bob

 

[NHGuy]

A drive is not really a pressure vessel. You just want to keep the oil in and the water out.
7# vacuum is a little low, but I'd take it. Since it holds good pressure it won't be able to suck in water as it cools after use, there will be very slight pressure on the inside til it gets down to air temperature at which point there will be zero pressure.

 

[ktbarrentine]

Not really designed to hold vacuum (or to be tested by a vacuum test; ergo that's why you find little about it), but the grey-beards I've talked to say 7" hg is about the limit you want to take it to for a vacuum test. Anything more and you risk actually doing more harm than good because the seal (particularly the yoke shaft seal) is only designed to work "right" one way. My alpha-one hold 7" hg just fine (each half tested individually, then again after assembly.

 

[Fiat4Fun]

Thanks for the information. Is there anything in the factory manual about this? I could not find anything. Any other input is greatly appreciated.
Thanks

 

[dubs283]

the factory manual states to only pressure test, a vacuum test will fail due to the design of the yoke seal, that is how the vacuum is released when the drive cools down

pressure test to 15 psi for 10+ min

 

[Grub54891]

I've tested many that only hold 5 lbs vacuum,15 lbs pressure. No issues.

 

[ktbarrentine]

5 lbs vac? or 5 inch hg vac? its a subtle difference. (vac gauges I use read out in inches of mercury, hg) (1 inch hg vac = .491 lbs/sq inch neg pressure)