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Discussion Starter · #1 ·
Hello;

I need to replace front and rear differential oil on 78 RC . I got this truck not long ago and dont know much about if it stock or not. So the question is what type of oil do i get for the front and rear diff. Could I use syntetic motor oil. And also the transfer case - what type of oil I need for there ??? this is 4 manual

thanks :)
 

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gear oil, not motor oil

t-case could be oil or ATF depending on what case you have

and spill about half a quart of oil on your shirt. That distinctive gear oil smell will be there forever.

and get a Chiltons or Haynes for your truck, they're not expensive and will tell you which it should need
 

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Motor oil is too thin for the axles, but there is synthetic gear oil. The synthetic gear oil is good stuff but is more expensive.

One thing I'm gonna mention here, you might want to check inside the axles to see if it might have a limited slip differential. If either of your axles does have a LS it's important to know what kind it is. Some limited slips require a friction modifier to be added in the gear oil to operate correctly, and some do not work well with synthetic oils.

If you have a full time 4wd or you have a NP203 transfer case. The correct oil to use is 10W30 or 10W40 motor oil. (Go with the lighter 10W30 oil for winter)

Ed
 

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get 90 wieght gear oil for both front and rear diffs. as far as transfer case, if you have approx. a 73-79 with the 203, those take straight 10w-30 MOTOR OIL not gear oil. as far as procedure they are the same for the diffs. take all bolts off of diff, and pry covers off. re-seal the rear diff. with gasket sealer and tourque to specs (sometimes it wont seal so you'll have to take the whole thing off and start again. a good trick is after you have the cover with sealant in place, finger tighten all bolts, and let it sit for ten minutes, wrench down a little, wait ten minutes, then torque them all the way). you can buy a gasket for a dana 44 front, put sealent on it, install and torque to specs. the transfer case is trickey, because the drain plug is also a bolt that holds on the rear cover for the font driveshaft. it is located on the back of the t-case on the right hand side and it looks like a circle. of the two lowest bolts that hold it on, the one on the left hand side is the drain plug. unbolt that one and let it drain. then bolt it back up, undo the square headed nut above and to the left of it, and thats the filler hole. i trust you know what to do with that. drive it around and pray for no leaks, cause thats the most frustrating thing after all that work.
 

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Discussion Starter · #5 ·
This is all very helpfull, but how do figure if they have LS? And if they are it require different gear oil, correct??
 

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yreah u needa gear oil modifier if it has the LS to check u have to pull the covers off and have a look . there is another way to do it with the wheels off the ground .
 

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Floyd said:
This is all very helpfull, but how do figure if they have LS? And if they are it require different gear oil, correct??
Sorry but I can't help you much here. You can start by looking at the build sheet or sticker. If you have a Sure Grip rear axle, the sticker should indicate it.

An aftermarket LS will be much harder to determine because there are alot of different kinds available.

You can take a pic and post your diff here, or look up some sites and try and compare, or have it brought to a 4 wheel drive shop with experianced 4wd mechanics.

Ed
 

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Safely get both tires on the axle off the ground (both front or both back)... turn one tire by hand and watch the other tire....... if the other tire turns in the SAME direction, you have a limited slip diff. If the tires turn in OPPOSITE directions, then it's not, it's "open" or "standard".
Mike
 

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If you take the cover off and look inside the gear carrier, an open diff will only have spider gears inside (you will see one gear on the top, bottom, left, and right) A LS will have spring packs in one form or another. In other words, if you only see 4 gears and the crosspin in the middle, it's open (regular 85W90 gear oil). If you see anything else in the middle, it's a LS (Specially marked "Limited Slip" 85W90 gear oil)
 

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Discussion Starter · #10 ·
thanks everyone,
I think I'll open it and look inside, I'm thinking of doing the front one first( since it has small leak)
hey Tazmaniak, is that valid for both axels?
 
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