[stinky jinx]

I have a 12 bolt rear end (posi) that I just changed the gear oil in. The gear oil is coming out of the upper breather valve.
Here is what I did. After removing the old oil, I put the cover back on with a new gasket. I put the posi additive in first (Trans-X Posi Additive) then I put the gear oil in (Stay-Lube 80W90... API GL-5 Plus) It took about a gallon and a quart. I brought the oil up to a 1/2 inch below the fill plug.
At this point I took it out and drove it around. I took a look underneath for any leaks around the gasket. That is when I noticed it coming out around the upper valve.
It seems to blow out when I am driving, then once I am stopped, the oil that came out, drips off the axle onto the ground. About a tablespoon each time out.
It does not continue to leak after I park. I rechecked the oil level. It is still 1/2 inch below the fill plug.
Any idea why this would happen? Is there a incompatibility with these oils?

[Crosley]

Originally Posted by stinky jinx

I have a 12 bolt rear end (posi) that I just changed the gear oil in. The gear oil is coming out of the upper breather valve.
Here is what I did. After removing the old oil, I put the cover back on with a new gasket. I put the posi additive in first (Trans-X Posi Additive) then I put the gear oil in (Stay-Lube 80W90... API GL-5 Plus) It took about a gallon and a quart. I brought the oil up to a 1/2 inch below the fill plug.
At this point I took it out and drove it around. I took a look underneath for any leaks around the gasket. That is when I noticed it coming out around the upper valve.
It seems to blow out when I am driving, then once I am stopped, the oil that came out, drips off the axle onto the ground. About a tablespoon each time out.
It does not continue to leak after I park. I rechecked the oil level. It is still 1/2 inch below the fill plug.
Any idea why this would happen? Is there a incompatibility with these oils?

is the fill plug in the cover? Is it possible the cover upside down with fill plug at a higher point?

5 quarts( gallon plus one quart) is too much oil for the 12 bolt diff

[Chazmac]

Crosley is right about the oil quantity.
Was this filled with the axle level? Did you fill it with the rear up and nose down?? The diff assembly must be filled with the car level to the ground. On end or one side higher than the other is a no-no

[stinky jinx]

The cover is in the correct position. It is a Moser rear cover and it has "USA" at the top and "Moser" in the middle. See picture below.....
When I rechecked the level, the car was on the ground...no jacks...and the car is level, front to back.
I agree that a gallon and a quart seemed to much to me, but I figured it needed to be filled to the level of the fill plug.

[Henry Highrise]

Can that name plate be removed and turned around? If so it looks like the cover is on upside down. I have never seen a filler plug up that high. Too much gear oil in your rear end is the problem...its building up pressure and blowing it out the vent.

[BstMech]

Is there a fill hole in the front side of the diff housing?

[302 Z28]

I agree, looks like the cover is on upside down. But I wonder if the stud girdles will still match up if you change it.

Vince

[Chazmac]

The cover is on corrrectly. The problem is the fill hole in the cover is too high for the diff. Use the FACTORY fill hole to check the proper level. It is located in the differential casting, along the pinion centerline, behind the pinion yoke.
The proper oil fill level for ANY diff is the centerline of the axles. The fill hole in the cover you have is significantly higher than that.

[stinky jinx]

Originally Posted by 302/Z28

I agree, looks like the cover is on upside down. But I wonder if the stud girdles will still match up if you change it.

Vince

If you notice, the typical Chevy curve is just above the filler plug, so the cover is right side up. And I believe it is shaped like that to allow enough space for the gear cluster. And as far as the name plate being removable......No.

But now that I take a closer look at the position of the filler plug compared to the original one, it "is" higher on the cover and I will bet they moved it up to allow for the "Moser" to be where it is on the cover. I looked again at the instructions, and it only talks about cover bolt, and stud girdle torque numbers. It says nothing about how much, in volume, gear oil to put in, or even makes one aware of the reposition of the plug.....not good.
Let me ask this question then, how much ( in quarts ??) "should" be in a 12 bolt Chevy rear end? I have always just filled it to the plug line.

[stinky jinx]

Originally Posted by Chazmac

The cover is on correctly. The problem is the fill hole in the cover is too high for the diff. Use the FACTORY fill hole to check the proper level. It is located in the differential casting, along the pinion centerline, behind the pinion yoke.
The proper oil fill level for ANY diff is the centerline of the axles. The fill hole in the cover you have is significantly higher than that.

I will have to agree with you Chazmac. I mistakenly made the assumption that the hole was in the correct position. Now that I look at what you pointed out about the height of the fill plug, it is obvious it is much higher, a point I wish Moser would have brought to my attention. I am going to use the fill hole at the pinion yoke. I will just open it up and let it drain until it is level with the bottom of the hole.
Thanks for the info

[Crosley]

good to read problem solved.

you would think Moser would provide some information on the fill hole

[stinky jinx]

Originally Posted by Crosley

good to read problem solved.

you would think Moser would provide some information on the fill hole

Ditto...
thanks to all

[stinky jinx]

Another thought...
now that it has become obvious that I inadvertently put to much gear oil ( maybe 3 pints to much) in, and since I put a Posi additive in. Is the mix of additive and gear oil off by to much? Or can I just drain down the mix that is in there to the level it should be, and not worry about the mix?
Or do I need to start from scratch

[66GMC]

Typical capacity for most passenger / light truck is approximately 3 Litres (or US quarts).

If you have added the additive to 5 Qts, then you have effectively diluted the mix by what ... 40%?

ASSUMING that the oil that you have added is plain-vanilla 80W90 Hypoid oil (I went looking for specs on "Stay Lube" and came up empty) that does not ALREADY contain the additive (Many do ... including Pennzoil Gear Plus)...
Pennzoil Spec Sheet I would suggest erring on the side of caution, and add another 1/2 bottle of friction modifier.

From what I understand, too much additive won't hurt anything ... but not enough will cause clutch chatter. I work as a Parts Manager at a Ford dealership. Ford recommends pre-soaking new diff clutches in 100% friction modifier, and then adding another bottle with the fluid. Following this logic is what I base my "too much is better than too little" opinion on.

It might not hurt to consult with an expert (eg specialized Driveline shop) to ask thier opinion as well.

HTH,
Don

[stinky jinx]

Originally Posted by 66GMC

Typical capacity for most passenger / light truck is approximately 3 Litres (or US quarts).

If you have added the additive to 5 Qts, then you have effectively diluted the mix by what ... 40%?

ASSUMING that the oil that you have added is plain-vanilla 80W90 Hypoid oil (I went looking for specs on "Stay Lube" and came up empty) that does not ALREADY contain the additive (Many do ... including Pennzoil Gear Plus)...
Pennzoil Spec Sheet I would suggest erring on the side of caution, and add another 1/2 bottle of friction modifier.

From what I understand, too much additive won't hurt anything ... but not enough will cause clutch chatter. I work as a Parts Manager at a Ford dealership. Ford recommends pre-soaking new diff clutches in 100% friction modifier, and then adding another bottle with the fluid. Following this logic is what I base my "too much is better than too little" opinion on.

It might not hurt to consult with an expert (eg specialized Driveline shop) to ask thier opinion as well.

HTH,
Don

Don
like the Pennzoil "Gear Plus 80w90 GL-5....The Sta-Lube 80w90 GL-5 I used, says it is a limited-slip gear oil.
But they do go on to say "some manufacturers require a additional additive for their diffs"
What do you think ? Do I even need to add more additive?