Advice on filling gear boxes...slight overfill?

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Chris B.

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Doing maintenance on my 1997 GMC Sierra and have a few questions. My pinion seal started leaking and I replaced it. I replaced it 7 years ago but only 10,000 miles were driven since then. I have always filled my front and rear differentials and transfer case the same way. I fill them with a hand pump till the fluid comes up to the bottom of the fill hole and then pump a few more times till it is starting to flow out the fill hole. Then I plug it up. I do this to make checking easy, all I have to do open the fill hole a crack and if oil starts to seep out or drip, I know it is still 100% full.
I'm wondering if this slight over filling is causing my seal to leak and fail. I don't know if it failed because of overfilling or from age and sitting a lot?
Is this practice safe or am I causing any issues?

Many thanks!
 
Good question, I do roughly the same. Fill until a slow tricks starts, let it drain out, then pump abut more in, and as it is flowing, cap it.

I figure it gets me in the lower half of the fill plug opening, and ensures any junk has flowed out.

I cant imagine its that bad, and have never observed any signs of overfill, like aeration.

But im interested in what others might think.
 
Well, if while the case is overfull and the fluid is expending enough, I guess you could cause the seal to fail but, it's hard to say really!
 
It's not a big deal. Just let the excess drain out. You could be below the plug some as long as you can feel it with your finger. It will splash around and everything gets lubricated. I don't see the point in purposely overfilling it.
 
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I fill until it trickles out and wait before replacing the cap.

Obviously, level ground is a must too.
 
Depends on the gearbox.

Conventional Chevrolet? Sure, no problem.

But, on the angle gear (front differential) on my Volvo XC, the proper fill of 700cc is about 1/4" below the fill plug. Filling it to the fill plug causes aeration and foaming that leads to it leaking and oil coming out the breather. A common Volvo forum discussion topic...
 
Overfilling can sometimes have bad consequences. I have to "overfill" the rear diff on my Cherokee because it is shimmed up to account for the lift. If I fill-to-spill, it's going to only have about 1/2 of the fluid it should have. I have to park it facing down hill and fill it that way.

My dad's truck has the ZF5 transmission in it. A lot of people overfill that transmission because they are super noisey. A few people have ruined their clutch by putting too much fluid in it to the point that it comes out the input shaft - blows the seal right out.

I don't believe your slight overfill is causing any issues with your pinion seal. IME, overfills on differentials will usually start coming out of the axle seals first. I think you are having issues because the truck is not used frequently. Or maybe there is some play in the pinion bearing that is causing the seal to leak.
 
For reference, Mercedes modified their oil fill specification from 7.5 to 7 quarts for one engine, citing the original fill was too hard on a seal.
 
Originally Posted By: Chris B.
Doing maintenance on my 1997 GMC Sierra and have a few questions. My pinion seal started leaking and I replaced it. I replaced it 7 years ago but only 10,000 miles were driven since then. I have always filled my front and rear differentials and transfer case the same way. I fill them with a hand pump till the fluid comes up to the bottom of the fill hole and then pump a few more times till it is starting to flow out the fill hole. Then I plug it up. I do this to make checking easy, all I have to do open the fill hole a crack and if oil starts to seep out or drip, I know it is still 100% full.
I'm wondering if this slight over filling is causing my seal to leak and fail. I don't know if it failed because of overfilling or from age and sitting a lot?
Is this practice safe or am I causing any issues?

Many thanks!


There are some instructions where they want it left a small distance below the fill hole. The Amsoil website should specify it. I think this was for my 2001 Jeep GC.
 
I have noticed that as long as the lube is within 1/2 -3/4 of the fill hole then level is OK , More tends to make the pinion seal drip. This allows for expansion when warm
 
I'd be concerned about thermal expansion and then the higher internal pressures it could develop during really hot days or long drives.
 
Funny you say that. I slightly overfilled my 05 Equinox and it actually weakened the seal and has caused some leaking. Granted this is an older vehicle so the gaskets are inherently weaker but it wasn't until I drained and filled that this occurred. Many other circumstances could have caused this but I did overfill slightly too.

Long story short is a little isn't an issue as long as the gaskets are in good shape. preferably I would keep the vehicle as level as you can when filling
 
Originally Posted By: Chris B.
Doing maintenance on my 1997 GMC Sierra and have a few questions. My pinion seal started leaking and I replaced it. I replaced it 7 years ago but only 10,000 miles were driven since then. I have always filled my front and rear differentials and transfer case the same way. I fill them with a hand pump till the fluid comes up to the bottom of the fill hole and then pump a few more times till it is starting to flow out the fill hole. Then I plug it up. I do this to make checking easy, all I have to do open the fill hole a crack and if oil starts to seep out or drip, I know it is still 100% full.
I'm wondering if this slight over filling is causing my seal to leak and fail. I don't know if it failed because of overfilling or from age and sitting a lot?
Is this practice safe or am I causing any issues?

Many thanks!

No, your practice/methodology is excellent, and is not causing the issues.
Prolonged sitting after putting (the oil seal) into use is probably the cause in your case.
In machines where input pinion seal habitually leaks ...... supposedly due to 'overfill' , would leak too at 'proper' fill level or underfill levels.
These leaks are not caused by thermal expansion of heated gear oils , nor is it caused by air foaming or poor quality oil seal materials/design. It is generally caused by poor pinion shaft layout with highly 'angled' pinion shaft center-line inclination, say > 15 degrees from horizontal lines.
JMHO.
blush.gif
 
My Escalade front and rear diffs have 2 different "full" procedures. The front (and TC ) is what you describe. The rear fill plug is on top so you can't use the overflow method. Regardless you have a breather, so there will not be any pressure issues. Frothing or aeration possibly..
 
Originally Posted By: zeng
.. These leaks are not caused by thermal expansion of heated gear oils , nor is it caused by air foaming or poor quality oil seal materials/design. It is generally caused by poor pinion shaft layout with highly 'angled' pinion shaft center-line inclination, say > 15 degrees from horizontal lines.
JMHO..


Thank you
 
Originally Posted By: Astro14
Depends on the gearbox.

Conventional Chevrolet? Sure, no problem.

But, on the angle gear (front differential) on my Volvo XC, the proper fill of 700cc is about 1/4" below the fill plug. Filling it to the fill plug causes aeration and foaming that leads to it leaking and oil coming out the breather. A common Volvo forum discussion topic...


Ive heard of such setups before... just seems idiotic. Put the fill hole 1/4" lower, or go to the next size larger plug and let nature make setting the level easy...
 
Originally Posted By: Y_K
Originally Posted By: zeng
.. These leaks are not caused by thermal expansion of heated gear oils , nor is it caused by air foaming or poor quality oil seal materials/design. It is generally caused by poor pinion shaft layout with highly 'angled' pinion shaft center-line inclination, say > 15 degrees from horizontal lines.
JMHO..


Thank you

My pleasure, mate.
Apology for 'judgemental' comments....... from time to time.
27.gif
 
Originally Posted By: Chris B.
Does the front diff or transfer case have a breather?
front diffs do. My solution to the Rats leaky front diff is to top it off before planned use.
grin2.gif
 
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