When to change factory diff oil?
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There are really no set break-in intervals for differentials. Amsoil published a study stating that it is best to change it before 5,000 miles. Since you're going to use Redline, here is one of their FAQ's:
Me personally, I would drain it between 1,000-2,000 miles.
Me personally, I would drain it between 1,000-2,000 miles.
Most of the "run in" will have occured the first 1000 miles. That's when I change mine. I think it's a waste to use synthetics in the initial fill, and, I see no harm in saving this fluid and running it through a very tight filter, then putting it back in. The fluid isn't "used up", it's just that there is some extra particles in it...due to no filter mechanism. I always stick a very strong magnet in the drain plug socket hole...I'm sometimes amazed at the fuzz I find on the plug at the next fluid change.
Kestas
Staff member
I changed the diff fluid on my neighbor's 02 Silverado with 40K. There was a magnet inside the cover, but it was glued on. I couldn't get the cover clean without removing the magnet. I figured the magnet did it's job to scavenge the break-in debris and left it off.
Synthetic for factory fill diff fluid is used for any of two reasons:
- Most owners rarely think about changing the diff fluid for the life of the car, so a long-lasting fluid is filled at the factory.
- Some manufacturers are trying to bump up the CAFE ratings of their models. Synthetic helps a little, with not a lot of extra cost for the 1-2 quarts. Plus, the manufacturers have to use factory fill in the cars presented to the EPA for qualifying.
In a perfect world, the manufacturers would use a cheap grade of diff fluid for break-in, then require it to be changed out with a high-quality synthetic after break-in.
Synthetic for factory fill diff fluid is used for any of two reasons:
- Most owners rarely think about changing the diff fluid for the life of the car, so a long-lasting fluid is filled at the factory.
- Some manufacturers are trying to bump up the CAFE ratings of their models. Synthetic helps a little, with not a lot of extra cost for the 1-2 quarts. Plus, the manufacturers have to use factory fill in the cars presented to the EPA for qualifying.
In a perfect world, the manufacturers would use a cheap grade of diff fluid for break-in, then require it to be changed out with a high-quality synthetic after break-in.
"In a perfect world, the manufacturers would use a cheap grade of diff fluid for break-in, then require it to be changed out with a high-quality synthetic after break-in."
Amen.
Amen.
"In a perfect world, the manufacturers would use a cheap grade of diff fluid for break-in, then require it to be changed out with a high-quality synthetic after break-in."
Exactly! And, they would also provide a more exact window for determining when most of the break-in was complete.
Exactly! And, they would also provide a more exact window for determining when most of the break-in was complete.
If it were my BMW I would first review what the owners manual recomends. BMW uses selected additives in the initial diff fill and it is a mistake to dump it early. If the manual has no recomendation then I would change at ~ 25,000 miles with OEM or OEM approved fluid. Mighty fine vehicle you have and I'd take NO chances out guessing the factory on oil & fluids. Ed
Minimal axle break-in takes between 500-1000 miles on a lightly used car, so I wouldn't change before then. Full break-in could take up to 5K miles if the car is used lightly. Break-in is mostly work hardening the gear teeth. Excessive heat tends to anneal (soften) the gears, so you want to avoid that. New gears tend to generate lots of friction initially, so there is some danger of getting them too hot. The syn oils tend to reduce heat by reducing friction, so they are good for break-in. For that first 1000 miles, avoid towing, heavy loads and long runs at high speed (especially in hot climates).
It's often best to break in a gear set in stages. That first 1000 mikes is important, but with a light load, only a small part of the gear faces are being used. With increased loads (from weight, towing or acceleration), some deformation occurs, so those tooth contact areas are increased and those new contact areas need some break in too. So, you start off easy, then you can gradually increase the severity of use, taking care not to let the axle get overly hot.
After break-in, most cars can easily go 30-50K on diff oil, or more, even if it's ordinary dino oil. Ordinarly driving isn't much of a load and doesn't generate much heat. If its a top quality oil, you can go lot farther safely. You're just wasting oil without any real benefit if you change it more often than necessary.
The more hypoid offset the axle has, the hotter it will run naturally. That's an element to keep in mind if you are trying to be cost effective in gear oil choices. An old Ford 9-inch, with a 2 3/8 inch offset, will run some 80-100 degrees hottter than a Dana axle with only a 1 1/8 inch hypoid offset. In normal use the Dana axle will be much easier on the oil, so you can use a "lesser" oil.
Continuous high speeds, mountain driving, loads (carried and towed) all tend to increase axle oil temp. After putting an oil temp gauge on my pickup and monitoring it for the past two years, I will never own another truck without installing one. There were times I could have made some nice "hypoid fries!"
It's often best to break in a gear set in stages. That first 1000 mikes is important, but with a light load, only a small part of the gear faces are being used. With increased loads (from weight, towing or acceleration), some deformation occurs, so those tooth contact areas are increased and those new contact areas need some break in too. So, you start off easy, then you can gradually increase the severity of use, taking care not to let the axle get overly hot.
After break-in, most cars can easily go 30-50K on diff oil, or more, even if it's ordinary dino oil. Ordinarly driving isn't much of a load and doesn't generate much heat. If its a top quality oil, you can go lot farther safely. You're just wasting oil without any real benefit if you change it more often than necessary.
The more hypoid offset the axle has, the hotter it will run naturally. That's an element to keep in mind if you are trying to be cost effective in gear oil choices. An old Ford 9-inch, with a 2 3/8 inch offset, will run some 80-100 degrees hottter than a Dana axle with only a 1 1/8 inch hypoid offset. In normal use the Dana axle will be much easier on the oil, so you can use a "lesser" oil.
Continuous high speeds, mountain driving, loads (carried and towed) all tend to increase axle oil temp. After putting an oil temp gauge on my pickup and monitoring it for the past two years, I will never own another truck without installing one. There were times I could have made some nice "hypoid fries!"
I change the differential fluid (front & rear) every 30,000 miles.
Kestas
Staff member
Break-in is actually a burnishing of the gear teeth. It wears down the high spots and smoothens the surface to reduce microflaws that can initiate spalling, and "matches" the mating gear teeth so the two gears become a matched set. This allows higher load capacity and/or longer service life of the gear set. It is not so much a work-hardening phenomeneon.
If enough heat was produced during service to anneal, or temper the steel, then you have big problems.
Otherwise, your post was fine.
re: Breakin
Here is info from a GM service bulletin
Here is info from a GM service bulletin
You always give transmissions,differential and transfer case's 10,000 miles to break in completely. They do not finish these items anywhere near as finely as engines!
Kestas: Your "burnishing" is nothing more than cold forging the surfaces of the teeth. It goes hand in hand with the heating and cooling cycles needed to complete the process of work hardening (or strain hardening) the 8620 steel used in R&Ps. Also, the surface temps at the contact points of hypoid gears can easily reach an annealing temperature. That's where a good friction-reducing lubricant comes in, the flow of which also cools the metal. I have at least two photos of gear sets that were destroyed by improper break-in (e.g. working the axle too hard too soon). Heat is what killed them. First the temperature reduces the lubricant qualities of the oil, the temps rise even more and that aforementioned surface temp gets out of hand. One of the failure modes is when the surfaces of the gear teeth reach an annealing temp, soften and begin to wear rapidly. Here's a particularly concise explannation from a gear manufacturer, Yukon Gear:
"In order to make them run cooler and quieter, new gears are lapped at the factory. However, they are not lapped under the same pressures that driving creates. The loads generated while driving force any microscopic high spots on the gear teeth back into the surface of the metal. This is called "work hardening". Work hardening is similar to forging in the way that it compresses the metal molecules into a very compact and hard formation. This can only be accomplished if the metal surfaces are lubricated and the gear temperature stays cool enough that the molecular structure does not change. If the temperature of the metal gets hot enough to change the molecular structure, it will soften the surface instead of hardening it. This may seen like a balancing act, but it all happens easily and passively as long as the oil keeps the gear cool while it is breaking in."
Anyway, this is one of those instances where we both get to be right
"In order to make them run cooler and quieter, new gears are lapped at the factory. However, they are not lapped under the same pressures that driving creates. The loads generated while driving force any microscopic high spots on the gear teeth back into the surface of the metal. This is called "work hardening". Work hardening is similar to forging in the way that it compresses the metal molecules into a very compact and hard formation. This can only be accomplished if the metal surfaces are lubricated and the gear temperature stays cool enough that the molecular structure does not change. If the temperature of the metal gets hot enough to change the molecular structure, it will soften the surface instead of hardening it. This may seen like a balancing act, but it all happens easily and passively as long as the oil keeps the gear cool while it is breaking in."
Anyway, this is one of those instances where we both get to be right
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