1968 Mercury 3.9HP Vintage Outboard Lower Unit Gearcase Seal Leak Test

In this video, we perform a simple leak test procedure on a vintage Mercury outboard lower unit gearcase. The equipment used for this test is a hand-pump leak tester similar to those used for bleeding brake lines with a gearcase drain plug fitting. The maximum pressure to apply to a leak test is 8 psi. The gearcase seals are not designed to hold more than this.
Especially on older outboards like this one, the seals break down and eventually fail as the rubber dries out and loses resiliency. This test procedure conclusively determined that the shift rod seal is bad on this antique outboard.
The same process can be used on other makes, models, and model years to determine if gearcase seals are holding or not.

[@dobrodaddy-o]

hi where did you get the fitting that screws into the lower unit? thanks

[@danmcginn772]

Hi James,could you tell me which direction i turn the retaining nut off that holds the oil bearing carrier in(prop shaft) on this motor,i have a 4 hp which is the same.Thanks again

[@brentolson8531]

Mine was full of water 👍🏻