New diff gears, why the "break in"?

Posted on Friday, October 13th, 2017 at 12:22 am

To me, something’s not making sense here. Been Jeeping for a few decades, owned 9 different Jeeps and, have changed gears in diffs plenty of times. But, when I changed gears in our ’04 Rubi, we never did any form of a break-in. Took it out, right from the git-go and used the Jeep. Drove it on freeways, off roaded it, took it to Moab and more, all within the first 500 – 1000 miles. Never had any trouble with those.

Now, one thing more. There are HOW MANY vehicles manufactured every single day in this country alone, quite a few thousand, and, each and every one of them has a BRAND NEW SET OF GEARS, IN A BRAND NEW DIFFERENTIAL. And yet, there’s ZERO requirements or manufacturer’s instructions to break those in from the git-go.

So, what’s different with the gears and bearings that’s installed in a gear change, to the gears and bearings of a brand new car/truck/JEEP/motorhome/anything?

I’m getting brand new 4:56s installed today (changed from 3:73s) at our local dealer and, I’ll find out tomorrow when I pick the Jeep up if they tell me to “break them in”. Now, if any of you have done a gear change recently, many of you have maybe been instructed to:

1. For 20 minutes, drive normally around town.
2. Park it for a complete cool down of the gears
3. Drive for another 20 minutes and cool down again.
4. Drive at freeway speeds for around 20 minutes and, again, cool down completely.
5. Do it once more at least, then a cool down.
6. And finally, at 500 miles, change the gear oil.

There is no such instructions for a brand new car/truck/Jeep etc. To me, gears are gears. What comes in a brand new diff, on a brand new car/truck/jeep etc, is the same thing that you use when you change to a new set of diff gears for your reasons. Am I missing something here?

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