Lower unit pressure/vacuum Test

[raglover]

What are the suggested values on pressure/vacuum tests for once big twin lower units?

I have always used 7 psi on the pressure, but read that I should then step up to 16 psi

Vacuum? Inches of mercury or kpa?

I usually put the lower unit in water for the pressure just to get a visual if bubbles appear but I am unclear on what to do to find vacuum leak

Finally is there a preferred time to hold seal?

Thanks

Allen

[RICHARD A. WHITE]

I recall this coming up on the old boards…. I check 12 -15 psi both ways… I use a bicycle pump with a home made contraption to pressure check, and a brake line bleeder and one of those yellow lower unit fill connectors to pull a vacuum.

Frank I recall had the numbers, pretty sure he will correct my error, because I am not 100% positive. I usually do like 10 minutes for each as well….

http://www.richardsoutboardtools.com
classicomctools@gmail.com

[fleetwin]

Well, recommendations from OMC have changed over the years. The thing to remember is to use low pressure/vacuum such as 3-6PSI to test seals. Using the higher pressure/vacuum (16-18PSI) can help marginal seals seal up better. It is best to submerge the gearcase in a bucket of water when pressure testing, be sure to turn shafts and move shift rods while in the water to help isolate leakage areas. The higher pressure/vacuum settings are used to test gaskets/orings. I hesitate to recommend doing the vacuum tests on gearcases with single lip seals, the vacuum might overcome the seal lip, pulling air past it, showing a "leak". In any event, don’t submerge the gearcase in water for vacuum tests, you won’t be able to see leaks and water will be pulled into the gearcase. I would say that 10-15 minutes is adequate for holding a particular pressure/vacuum reading. Keep in mind that faulty equipment/leaky hoses are often to blame for leakage problems, don’t drive yourself crazy (like I have done) trying to fix a gearcase leak when it is the tester that is leaking!

[beerman57]

For finding pressure test leaks I use a spray bottle with dish soap and water.

[chris-p]

Yeah, I don’t bother with vacuum tests either, and as Don stated, don’t use too much pressure, or it will not be accurate. 3 to 6 psi is adequate.

[raglover]

Thank you. I have a lower on a big twin I revealed and it holds pressure for hours at 7 psi. Turning shafts and shifting in the bucket but winds up with water I the oil after running.

I will try lower pressure first. Then perhaps a low pressure vacuum next. If that fails high pressure?

Shafts are good and not chewed up. Also no abnormal playin the shafts…..a little stumped.

[dave-bernard]

to much pressure can seal the seals tighter. that is why you do vacuum also.

[fleetwin]

Did you turn the shafts and move the shift rod while pressure testing? Submerge the gearcase in a bucket of water while testing. 3-6PSI will check the seals/shift oring, 16-18PSI will check spaghetti seal or oring on seal housing.

[raglover]

Thanks, yes I have been turning shafts and shifting under pressure in the bucket, but it has to be there, the leak that is

Fleetwin, when you ,emotion going to 16 psi for the spaghetti and o ring I assume the seals should hold that too? They will just seal tighter?

I have seen those fail at the 7 psi point too but have never gone higher. At this point I guess
Since I know there is a leak I need to test until failure……systematically of course

[fleetwin]

Yes, the added pressure actually forces the seal lips tighter against the shaft.

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