1970 MC-1 gear oil blockage.

[Fasterskis]

Restored classic that I have had for a year. Runs good but a little crunchy engaging in gear. Tried to do my first gear oil change. I’ve done it in an outboard but never a I/O. Removed top breather plug, removed bottom drain plug. Surprised not much fluid came out. Maybe .75 qt. And it came out slow. Let it drain for 30 minutes and pumped in warmed up new oil. Got to the .75 qt level and it became very hard to pump. All comments say be patient keep pumping. Finally it blows the hose connection off the pump. Got a small oil shower and lubed all my tools! Put hose clamps on both ends of my pump hose and tried it again. Still very high pressure but after 4 pumps and a little wait, it “seemed” to be slowly be going in. After 5 -6 cycles of this, I realize the fluid level in the bottle isn’t going down. More research. Tried opening the drain and pumping in from the breather hole in hopes of clearing blockage. Tried turning prop in and out of gear. It easily took another .25 qt and again became very hard to pump. Slowly kept trying to pump it in and this time the metal hose connector that screws into the breather hole comes apart and this time I get a full plastic from the hose and projectile vomit from the breather hole. Hat, hair, glasses, shirt and pants all covered. Now I’m not happy and I try the last tip I read about putting an air hose on it. Turned compressor down to about 10psi and held the rubber tip nozzle against the upper breather hole. Didn’t sound like any air was going in so I pulled it away to check my pressure. Bad idea. Got another high pressure spray of oil in the shirt. After yet another clean up I tried this a couple more times because it seemed like it should work. Defeated I have come here asking for assistance.

 

[Bt Doctur]

depending on its age you may have 2 independent oil cavities to drain and fill

 

[kenny nunez]

I am also in agreement with the Bt doc. What may have happened in the past the lower was replaced with a later style that had the transfer port.
I have had to drill and tap a vent port in the side of the later models which is easy to do.

 

[Fasterskis]

Thank you guys! Now that I have it out of the barn I can see that this would be the case. But I’m still not sure of which is the drain vs the vent for the lower unit. Neither of which are labeled like the “fill” and “vent” for the top unit.

There are two likely suspects here. (See photo)

 

[kenny nunez]

The vent is the one just above the cavitation plate. The larger one above it was for a flushout. It would connect with a threaded hole in the water pump base plate.
So just remove the vent plug and pump the oil in.
There should be 2 plugs on the upper, pump oil in one side until it comes ou the other side.

 

[Fasterskis]

The vent is the one just above the cavitation plate. The larger one above it was for a flushout. It would connect with a threaded hole in the water pump base plate.
So just remove the vent plug and pump the oil in.
There should be 2 plugs on the upper, pump oil in one side until it comes ou the other side.

Thank you so much. I never considered that it might be two separate oil cavities. I’m all set now.