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1968 Johnson 3hp JHL23-M Gearcase Test Pressure

S

Swarlos

Regular Contributor
I'm suspicious that the gearcase on my 1968 Johnson 3hp, JHL23M has a leak. I got a little pump with a gauge on it to test if it holds pressure, but I don't know how much pressure to apply. Any thoughts would be helpful.
 
So that's all, eh? I test 10 to 15. Maybe I'm replacing seals that are still gonna last awhile. However, I have noticed if they don't leak at 3 or even 5 pounds, they still don't leak at 10 to 15. The shift shaft seal/o-ring, (not applicable here), has to be exercised and then you will see the pressure drop. Be sure to rotate and wiggle shafts while pressure or vacuum is applied. Have lower unit drained too.
 
Agreed, Pappy.....only sure fire way to ensure success. How much vacuum do you recommend?
For years my reasoning was this: How can water get inside gearcase if the lube doesn't first escape? Can you answer that for me and our readers? Hint.....has to do with improper filling?
 
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Well for one thing, they are not totally full of oil if filled properly to the vent. Some air space is left for expansion/contraction.
Pressure tends to push the seal lips tighter against the shaft, possibly hiding a leak.
Vacuum has the opposite effect. Most older motors are poorly sealed against vacuum anyway. That's why they are using back-to-back seals now.
 
The reason you vac test is...............gearcase and oil gets hot from exhaust and load shear forces. when you stop, the gear case cools down and creates a vac in gearcase. If a seal is worn/faulty it will suck water in. Same principle as canning food....
 
Pressure test was good to 10 psi for 20 minutes while spinning the flywheel/prop. What level of inhg should I use for vacuum test?
 
So 3 to 5 for pre test, then 15 for final. Get about 20 on my 350 Chevy Quadrajet at idle. A little trick. Ever adjust timing on an automobile engine after modifications like camshaft and compression ratio changed? Certainly can't go by the book. You use a vacuum gauge and advance timing while idling until you reach max vacuum.....then retard timing to loose1 inch of vacuum. Old school stuff. Worthless information for 99.9 percent of vehicles on the road today. Works for the stuff I drive, however.
 
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