I hope this is in the right forum. If not, I trust the moderator will move it to a more appropriate one.
I having trouble comprehending the necessity of filling the lower unit with gear oil by the procedure that seems to be universally accepted, that is filling from the lower hole until it runs out the top, then trying to start this tiny plug, which is at an odd angle, and where you can't really see, before any, or much oil runs out.
I've heard all kinds of reasons, air bubbles, the necessity to flush out contaminants, etc., but can't really see the validity of the arguments.
If you drain from the lower hole, and flush with fresh oil after draining, I don't see how flushing from the bottom up would be any better. In fact, the most likely contaminant would be water, which is heavier than oil and would be at the bottom of the gear case. You would be pushing it back up through the gears. Any metal contaminants would be drained or flushed out the bottom. Filling from the bottom would push these back up in the gears.
As for air bubbles, 80-90 weight oil is not all that thick. Put some in a clear container and blow in it with a straw. The bubbles come to the top, right? We've filled automotive gear cases for years from the top, sometime with much heavier oil, and it's not a problem with them. The only difference is that they are vented, or at least some are. I'm not certain if all auto differentials and transmissions are vented, anyway.
I just spent over an hour Sunday changing the oil in my outboard, what should have been a 20 minute job, so I had plenty of time to ponder this. Admittedly, this was my first time doing this, and with practice, I might get better at it.
If there's a question in this, I guess it would be why can't I go back now that the oil has had time to settle out any bubbles and top it off, if it needs it? With oil running out the top and bottom (I put the top plug in before disconnecting the pump), I'm not certain it's full.
Thanks, John
I having trouble comprehending the necessity of filling the lower unit with gear oil by the procedure that seems to be universally accepted, that is filling from the lower hole until it runs out the top, then trying to start this tiny plug, which is at an odd angle, and where you can't really see, before any, or much oil runs out.
I've heard all kinds of reasons, air bubbles, the necessity to flush out contaminants, etc., but can't really see the validity of the arguments.
If you drain from the lower hole, and flush with fresh oil after draining, I don't see how flushing from the bottom up would be any better. In fact, the most likely contaminant would be water, which is heavier than oil and would be at the bottom of the gear case. You would be pushing it back up through the gears. Any metal contaminants would be drained or flushed out the bottom. Filling from the bottom would push these back up in the gears.
As for air bubbles, 80-90 weight oil is not all that thick. Put some in a clear container and blow in it with a straw. The bubbles come to the top, right? We've filled automotive gear cases for years from the top, sometime with much heavier oil, and it's not a problem with them. The only difference is that they are vented, or at least some are. I'm not certain if all auto differentials and transmissions are vented, anyway.
I just spent over an hour Sunday changing the oil in my outboard, what should have been a 20 minute job, so I had plenty of time to ponder this. Admittedly, this was my first time doing this, and with practice, I might get better at it.
If there's a question in this, I guess it would be why can't I go back now that the oil has had time to settle out any bubbles and top it off, if it needs it? With oil running out the top and bottom (I put the top plug in before disconnecting the pump), I'm not certain it's full.
Thanks, John
