CONTINUOUSWAVE

When you change seals in a gear case you then test the seals with air pressure and vacuum. Should you put in oil and drain it- or test the case dry?

I've done this on a sterndrive gearcase. It was dry but the pressure was low and soapy water was used on the outside much as we used to use to find a leak in a tire tube. I don't recall the pressure used but I turned my regulator to a very low setting. The manual should give you a recommended pressure.

I did the test dry to avoid a mess if there was a leak, but on pressure vessels we always did a hydrostatic test. We were using much higher pressures and I believe the hydrostatic test was to prevent any damage.

The sterndrive was a Mercruiser Alpha drive.

Outboard lower units are tested dry. The use of soapy water helps tremendously in finding any leaks.

Check the service manual from the manufacturer to get the specified positive and negative pressures to be used in testing the gear case seals. As I recall, the pressure is typically low, just a few PSI. The pressure is to be held for ten minutes or so, and checked for leak-down. The gear case has to be empty during the test. If there were fluid in the gear case, when applying a negative pressure the fluid would tend to be sucked into the pressure tester.

OK- dry it is. Thanks. I was unsure because It took a lot of pressure to find the leak. I guess the residual oil helped the seal resist leaking. When I drained the oil last Fall I got butterscotch looking gunk. Letting it set in a jar allowed the water to settle out and I knew I had a leak. At first the recommended 15psi pressure did not show anything. I then submerged the entire lower unit in a slop sink and pumped it up to 30 psi. Still no leak even while turning the drive and prop shafts. It was only when I moved the shift shaft that the air bubbles showed. So I replaced the shift shaft seals and I am about to test but I could not remember if it should be dry or wet. Thanks again.