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Old 08-19-2011, 04:30 AM
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Diff fluids...

Long story short... I've got a leaking rear diff and I'm a little short on research time.

For the rear I've read that I need 7 pts of 75W-140 and I can use black permatex to seal the cover.

For the front I'm seeing 90W hyphoid (sp?) gear oil... What is it? Why is it different? Can I use the same oil as the rear and if not why not?

I know the answers are out there I just haven't had the time to find them.

Thanks for the anticipated help!
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Old 08-19-2011, 06:30 AM
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Yes you can you black RTV silicon sealant. Front and rear diff's take different fluids. If you are running a limited slip rear don't forget to have some of the additive on hand. Some synthetics have it already but sometimes it takes a little more.

Last edited by PGreenSVT; 08-19-2011 at 08:10 AM. Reason: spelling
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Old 08-19-2011, 07:20 AM
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I run the same in both diffs with no problems (75w140 and limited slip lube in the rear). I also use the black RTV (it came in a tube like a caulking tube).
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Old 08-19-2011, 08:28 AM
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I just swapped both of mine to Amsoil not too long ago. Im a dealer PM me if you are interested in using it.
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Old 08-19-2011, 10:22 AM
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Can u use the red rtv high temp stuff to seal it?
75w140 rear plus limited slip- how much do I need
What do I use for the front and how much do I need
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Old 08-19-2011, 01:15 PM
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Rear Diff fluid: 75w140 synthetic gear oil
Front Diff fluid: 80W90 (if you can find syn close to this, use it)

The "book" calls for 8 ounces of friction modifier added to the rear differential. Check whatever syn oil you buy for the rear before buying the modifier. Many of them come with the friction modifier as part of the formulation, and you would not add any to it. If the particular oil you buy does not have it in the oil already, don't just buy 8 ounces and pour it in there and call it good if you want your limited slip diff to work The best way to do it is to add about 1/2 of that to the rear diff and drive it for a couple hundred miles. One of the times you are out and have been driving it down the highway for awhile, get off and look for a parking lot. Come to a full stop, turn the wheel hard in one direction and slowly roll into the throttle. If you can feel the truck actually vibrating or feel a chatter, add an ounce, drive for a couple hundred miles and try again. (I add to the diff by removing the rear ABS/VSS sensor on top of the diff) You really want to find the balance between too much friction modifier and not enough. The goal is to get just enough modifier in the diff to stop the chatter when the oil is hot, and no more. Too much and the diff could act like an open diff, not enough and you can feel the chatter.
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Old 08-19-2011, 04:37 PM
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Thanks guys!

I got in a twenty minute read on gear oils and hypoid gears at work today and with the replies here I'm a little more enlightened in the diff department. I didn't have a clue but now I do... sounds like everything about this truck, no clue until I need to know something!

So the GL-5 rating seems to be the standard for gears hypoid (which I've learned is the name for the type of gear and how the teeth mesh) or not. It also seems the pressure the oil is under between the gear teeth is the big difference in gear oils compared to a motor oil or ATF. The friction modifiers (from my limited 20 minute understanding) are what help to increase an oils performance under extreme pressures which reduces the chatter or vibration. I'm guessing this is felt when the limited slip is "slipping".

Now that I've been thinking about it and learning a little I would think that the reason for the difference in weights for the front versus rear diff fluids is that the front spends most of it's time unloaded. When you're in 4wd it sees a load but not as much as the rear gears do and would see lower temps as well. Am I way out in left field or have I learned something again? I'd like to hear some more comments on this stuff, I've got more to learn.

Now that I've done some reading it seems there are trucks (maybe not the same truck though... Dodge...) running 75W140, 75W110 or 75W90 both front and rear. Any thought on this?
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Old 08-20-2011, 06:20 AM
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Quote:
Originally Posted by Fishermedic View Post
The friction modifiers (from my limited 20 minute understanding) are what help to increase an oils performance under extreme pressures which reduces the chatter or vibration. I'm guessing this is felt when the limited slip is "slipping".
Actually, it is the opposite. The chatter is the clutches engaging when they shouldn't. The modifier is to help the clutches "slip" when the limited slip should not engage. Which is the reason you want to find that balance. Too much and the clutches will slip when you do not want them to, and not enough and you get the chatter.
Quote:
Originally Posted by Fishermedic View Post
Now that I've been thinking about it and learning a little I would think that the reason for the difference in weights for the front versus rear diff fluids is that the front spends most of it's time unloaded. When you're in 4wd it sees a load but not as much as the rear gears do and would see lower temps as well. Am I way out in left field or have I learned something again? I'd like to hear some more comments on this stuff, I've got more to learn.
The front diff, when engaged, sees half the load placed on the drivetrain. If it didn't it would bind without the presence of a "viscous coupling" which is designed to slip a little and is used in transfercases for all-wheel drive SUV's and cars. The VC acts sort of like a differential within the transfercase so that all 4 wheels can be connected, yet allow all of them to spin at different speeds (such as in a corner). The absence of the VC is the reason why you do not want to run around town, on dry pavement, with the 4WD engaged. Doing that would cause premature wear and place alot of stress on the drivetrain when cornering (which can result in expensive repairs).
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Old 08-21-2011, 07:24 AM
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Quote:
Originally Posted by Fishermedic View Post
Now that I've done some reading it seems there are trucks (maybe not the same truck though... Dodge...) running 75W140, 75W110 or 75W90 both front and rear. Any thought on this?
I run 75w140 synthetic in both front and rear diffs with 0 problems. It has the same viscous rating when cold as the standard 75w90, but I would prefer the protection of the higher rating, particularly since it gets quite warm here in the desert of AZ. It won't hurt anything to run the 75w140 in both diffs, but the reverse isn't true. 75w140 is the minimum you can run in the Sterling 10.25 rear end, so you can't run the 75w90, 75w110 or any lesser gear oil.
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Old 08-21-2011, 08:49 AM
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Make sure you take the time to clean both the diff cover surface and the axle prior to putting them back together. It is also a good time to paint the diff cover if you so plan to. This link is more of a how to clean link. If you need amounts or types of fluids to use I prefer amsoils page. It is also a good time to change the transfer case oil. 1 bolt to drain and 1 to refill, no extra RTV and only need more oil and it is very easy to do especially with winter coming.
How to, Welcome to guzzle's Rear Differential Maintenance Web Page
Amsoil, AMSOIL Online Product Application Guide TRUCKS+EXCURSION+P
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