Pressure vs Vacuum Test of Lower Unit

[DennisG01]

I don't have any issues - at least not as of the last time I changed the gear oil (last Fall). But, I thought it might be a good idea to do a pressure test once a season as some PM. Granted, something could fail mid-season, but it can't hurt, right?

Anyways, I have the service manual and it talks about pressure testing. But there is nothing in there about vacuum testing. Is there a reason for that? Does the lower unit not hold much vacuum (as compared to pressure)?

[boscoe99]

Usually there is a pressure build up within the lower unit. I don't recall there ever being a low pressure within the lower unit vis a vis ambient air pressure. Gear oil and the internals of the lower unit getting hot cause a pressure rise within the lower unit.

The propeller shaft and drive shaft seals are positioned so as to be more effective at keeping water out than keeping oil in. Therefore, if the seals will hold an internal pressure of 9.9 psi, using the F300XCA as an example, then it would seem reasonable to me to expect that the seals would hold an external pressure that is 9.9 psi, or more.

By doing the worst case condition test, if it passes then it is presumed that the least case condition will also pass.

Try it and see.

[DennisG01]

Thanks, will do.

Merc has stopped publishing anything in regards to pressure (and I'm not sure they ever published vacuum, either). Now, they basically just say "if there's a water/oil leak, fix it". Basically, the way things are now designed they will fail pressure tests, even if everything is A-OK. Or, at least, that is my understanding, now. I'm not sure I agree with that line of thinking, though! But then, I'm not a Merc engineer, either!

[rodbolt17]

unlike mercs and most others.
Yamaha puts the double seals in place with both lips(spring side) out.

[DennisG01]

Hmmm... Reading back through (and thinking more about it... whether that's good or bad is TBD) what the two of you said...

Maybe the theory of "if the seals will hold an internal pressure of 9.9 psi, using the F300XCA as an example, then it would seem reasonable to me to expect that the seals would hold an external pressure that is 9.9 psi, or more" doesn't hold true? Meaning, the seals are kinda like a one way valve (I'm thinking "duck bills" on a Vacu-Flush system). If too much pressure would builds up in the gearcase, oil could seep past the seals. But, the same "seep" may not be true for water trying to go the other way. In other words, fluid can somewhat easily flow in one direction, but it would be harder to flow in the other.

I don't know... maybe I'm not explaining myself clearly enough. But, if that's true, then "in theory" the gearcase should hold MORE vacuum than pressure. Or... maybe internal vacuum is not the same as external pressure?

[boscoe99]

My intuition is telling me that the seals should hold more pressure on side A than they will on side B. Reason being that when pressure is applied to side A the pressure is acting to force the lip of the seal to conform to the shaft. It makes a tighter seal. When pressure is applied to side B then the pressure is acting to force the lip of the seal away from the shaft.

If the seal will hold 9.9 psi when pressure is applied to oil side B then it seems that water side A could withstand an even greater pressure.

Yamaha apparently wants to keep water out moreso than keeping oil in. Lose a bit of oil, no harm no foul. Get a bit of water in, might not be good.

Am I missing anything?

Now all in all I would prefer the BRP system that uses a reservoir to ensure there is adequate gear oil to the lower unit, at all times and under positive pressure. Less chance of water getting in. Less chance of the lower being run on low oil.

[DennisG01]

My intuition is telling me that the seals should hold more pressure on side A than they will on side B. Reason being that when pressure is applied to side A the pressure is acting to force the lip of the seal to conform to the shaft. It makes a tighter seal. When pressure is applied to side B then the pressure is acting to force the lip of the seal away from the shaft.

If the seal will hold 9.9 psi when pressure is applied to oil side B then it seems that water side A could withstand an even greater pressure.

Yamaha apparently wants to keep water out moreso than keeping oil in. Lose a bit of oil, no harm no foul. Get a bit of water in, might not be good.

Am I missing anything?

Now all in all I would prefer the BRP system that uses a reservoir to ensure there is adequate gear oil to the lower unit, at all times and under positive pressure. Less chance of water getting in. Less chance of the lower being run on low oil.

I agree - that is what I was trying to say... although not as eloquently!