How much is too much?

72Z15SS

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I've seen so many videos on this subject and just wanted opinions on this...
So How much diff fluid does everyone here put in their diffs when rebuilding or doing maintenance?
I fill mine just below the top of the case with the planetary gear sitting on top of the spider gears. Some people only fill them to the cross shafts... So what's the best solution for the Arrma Vehicles???
There's an old saying that "Opinions are like a**holes and every one stinks"! So lets hear yours...
 

72Z15SS

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That's about where I like it myself. Just below the water line! LOL
But seriously, some vids show the person letting the "air bubbles" float to the top but then they don't top it off, so what's the point>
 

jonUF02

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I just filled mine, filled to leave just enough room in the cup for the top gear displacement, then tightened the screws most of the way, held upright, remove bleed screw, tighten the main screws down and a tiny bit of fluid came out through the bleed hole. I also moved the gears back and forth a few times with the bleed screw out. I believe this got them completely filled with no air left.
 

72Z15SS

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I just filled mine, filled to leave just enough room in the cup for the top gear displacement, then tightened the screws most of the way, held upright, remove bleed screw, tighten the main screws down and a tiny bit of fluid came out through the bleed hole. I also moved the gears back and forth a few times with the bleed screw out. I believe this got them completely filled with no air left.
I've been filling them to the top of the of the spider gears then lay the planetary gear in place, rock them back and forth and swipe across the top of the case.
 

dpricenator

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BIG QUESTION.

What would be the effect of over filling it? There is a bleed off screw in the side of the diff. I assume that is so you can completely fill the diff carrier. But the guys over at team assc. told me not to over fill the diff on my RC18b and 18mt. They went as far as to say underfill it compared to what everyone says. I never ended up doing that work, so they are still stock. But I did fill the center diff, on my Kraton, completely. Then I had center diff issues. NOT that the two are related, as I also did a ton of other stuff in that area, so it could be a number of other causes to my current issue. I was just wondering why you wouldn't fill it all the way up.
 

Kauaiguy

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BIG QUESTION.

What would be the effect of over filling it? There is a bleed off screw in the side of the diff. I assume that is so you can completely fill the diff carrier. But the guys over at team assc. told me not to over fill the diff on my RC18b and 18mt. They went as far as to say underfill it compared to what everyone says. I never ended up doing that work, so they are still stock. But I did fill the center diff, on my Kraton, completely. Then I had center diff issues. NOT that the two are related, as I also did a ton of other stuff in that area, so it could be a number of other causes to my current issue. I was just wondering why you wouldn't fill it all the way up.
Thermal expansion
 

dpricenator

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Which explains the diff fluid on the chassis. thank you, I still haven't gotten to dig into her yet.
 

Andyshine77

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Thermal expansion is a very small part of the issue, over pressurization is the main concern, this applies to any gear box or transfer case. I don't see bubbles or foaming beaning a problem, this isn't a hydraulic system where air being compressed is a problem. I will say half full is likely more than enough, maybe 3/4 you have to leave plenty of air space.
 

slpcrf450x

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Which explains the diff fluid on the chassis. thank you, I still haven't gotten to dig into her yet.
I recently re-did my center diff. I had it almost completely full and it flung a little fluid out onto the chassis. I thought there was going to be something wrong with it but everything inside was fine and I filled it back up with a little less fluid and it has been fine.
 

crankestein

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I follow the same guidelines as Adam Drake, up to the cross hairs, where both pins meet up, enough to slightly cover them. Always done it this way and has never failed. I also add a drop of Boss 821 lube. Its super sticky, every time I service the diffs its glued to the gears and oil rings. Gives whatever oil I apply, 20k, 60k, a very sticky texture.
 
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