this is the other drive the one that had no problems.Starboard, I just pulled the top cap to swap them and now i have 2 drive issues.
At this point since i won't be going boating now i just may pull the fork on the noisy drive

If it had an air leak then wouldn't water enter, wouldn't you see a stream of oil sitting in the water engine off, and wouldn't the level of the bottle drop. What pumps the oil up to the top bearing in the cap? That air gap between the vent hole and the top of the drive, that air is supposed to work its way out through the bottle.
According to a merc mechanic you fill the drive to the vent hole then you make sure the level in the bottle is correct. After a few runs the level in the bottle will drop because there is air inside the top of drive and eventually that will come out.So you will have to add oil. I used to do it that way but now I drain the oil from the drive and i remove the cap on the oil bottle. The bottle should drain also if it is working correctly. Then i fill from the bottom of the drive and after it comes out the bleed hole i cap that and i keep pumping until i see 1/3 of the bottle filled. This way it gives room for expansion.You may have to add a bit a few times. I have friends that don't even take out the bleed port. They just fill it to the bottle mark and that's it. I think that oil level in my drive saved the drive from blowing up.

I have always just filled them from the bottom until the reservoir was half full. Never saw the need to remove the top plug.

Today I took the noisy drive apart. I removed the shift fork for a test on the rattling noise. When i pulled the top cap off I noticed the race on the cap was different than the other drive. The race was wider and thinner and when it was on the cap it was lower than the casting. I was told that when you install these caps the race presses against another type of washer with needle bearings and it pushes down on the gear assembly setting also lash. So how could the race that was on the cap sitting below the casting push down on anything. I looked up these race's on Mercruiser parts and they make 3 different sizes. starting with .091, .094, .097 thicknesses. I got a new cap for the other drive that had the cracked tower race and the guy whom I bought it from sent me a bunch of extra stuff. He sent me the needle bearing washers and all these races brand new from Mercruiser No charge. So i started with the .091 race and placed it on the cap. Now it is slightly raised above the casting. Tomorrow will be the real test on startup. see pics + video





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took it out today...was easy but putting it back maybe an issue...please read my last post interesting what i found
thanks

I thought you had a drive mechanic build these drives?

Is this a 1990's year model,,thin floor housing that has newer thick floor gears installed.