The Stovebolt Site Forums The Shop Area General Truck Talk Newbie questions - rear axle and transmission fluids

My son's 51 3100 has a 72 Suburban rear end. Does anyone know how to clean and replace whatever fluid is in there. I think it's straight grease, but not sure. There's what appears to be a big grease nipple at the top - is this a vent?
Also, the transmission is original - what kind lubricant is in there and how do I replace it.
Thanks very much and sorry for the stupid questions, Rob

Well 1411506 (in the future, we will all be called by numbers instead of names, ol' 1411506 is just ahead of the game), do you mean fluid for the pumpkin? If so, I'm thinkin' a 90 weight gear oil would be good. Change it like you're changing oil on your engine. You could clean the little bits of crud in there while you've got the cover off.
And is this a manual tranny you're talkin' about? If so, pretty much the same applies. Although there are differing opinions on what weight to use and hopefully those folks will chime in. I used 90 in my 4 speed 'til I yanked it and replaced with T5 which uses ATF.
-101869 (my name in the future)

Hi Mugsy - thanks for the advice. The rear axle is where I still have a question - I can see that I have to take the cover off, as there is no drain plug, and while I'm in there, clean everything out. However, there's no fill plug - just what looks to be a larger than average grease nipple on the top - how do I fill it with gear oil without a fill plug? And what is that big nipply thing on top? a vent of some kind?
Thanks, Rob (1411506)

If the rearend is the Eaton H072 (think salad bowl, or big Ford 9 inch), you can just take the lowermost diff cover bolt out, as the hole goes all the way inside the diff, and let 'er drain that way.

Of course, with not much more work you can do what mugsy said, to check the gears & oil out.

As for the SM420, there should be 2 plugs...a fill (upper) and a drain (lower). Pretty self explanatory

Hmmm...I didn't see your last comment until I had already replied. That sounds like a 12 or 14 bolt...they have fill plugs on the side of the housing, not the diff cover.

I agree, the nipple-looking thing is most likely your breather.

Good luck!

I just went out and checked - 12 bolts - no filler holes anywhere - I'm stumped.....

Look up front in the cast area around the pinion. There is usualy a plug right in the passenger side of the housing about level with the u-joint. Most like this are set up to fit the square end of a 3/8" ratchet into. I usualy use a short extension to get the ratchet head away from the housing and make it easier to spin.

Yeah, I don't think a nipple on top is what you're lookin for. If this was where you fill from, your whole pumpkin would be full of fluid. Fill plugs are usually around the halfway point between top and bottom and you fill until fluid is level with bottom of hole.
If it's not on the cover, it probably is on the forward side as rcurrier says and probably does have a square hole for 3/8" socket extension. My Pontiac is set up this way. I had to dig and uncover some dirt to find it.
-David

obvoiusly this is something that you have never done before. No problem, here's the basics. Once you've got the drain pan under the diff. housing and you're ready to get gear oil drippin' off your elbows, start by removing the bottom most diff. cover bolt. This way you know if one of the bolts goes all the way into the housing, thus making it a cover bolt/drain plug (chances are, NOT). If nothing comes out, remove all but the top few cover bolts, start prying the cover off at the bottom, a little at a time so you don't have oil all gushing out at once. When the bulk of the oil is out, remove the remaining cover bolts. When the cover is off, you may be surprised at just how clean it is in there. If you're not an experienced rear-end guy, (automobile rear-end, not the other) you probably don't want to be dinkin' around in there anyway so don't be takin' anything apart to clean it. As far as the cover goes, clean the inside of it, get rid of old gasket (on cover and housing) and look for any metal filings. If you see some, you need to ask someone else because I'm NOT a rear-end guy (automobile OR other) a new gasket should be available at your local parts store, if not, get some gasket material and make your own. Some guys just put a bead of silicone gasket maker on and call it good. Your fill plug should be on the side of the diff. housing, in front of the axle. It is a small plug about 1/2in. in diameter. It doesn't have a head on it so if it's covered in dirt you probably won't be able to see it. It should have a square hole in it that your 3/8 drive ratchet will fit into just like a socket. (man my fingers are gettin' sore!) The plug should be about the same height as the drive shaft where it meets the diff. This is called the pinion. I just use 80W90 run of the mill gear oil. Fill the housing through this hole, (it takes patience) fill it until you can't fill any more. (up to the bottom of the fill hole) As far as your transmission, is it man. or auto.?