Why does the oil need changing after gears?

[SS376]

Just had 3.91's installed at dealer. Taking it easy, heat cycling, etc, and planning on having the diff oil changed at 500 or so miles (even though dealer claims not needed).

The question is, why am I doing this?

Does anyone here, actually KNOW why a new gear install needs to be broken in and have the oil changed at 500 miles? Nobody says to do this when the car is brand new, so what's the difference?

I know everybody has an opinion, but 9 times out of 10 it's just because they heard it from someone else. Anyone actually know why getting new gears is any different from when the car itself is new (and the gears are obviously new as well)???

[EM1/SS]

I changed the oil on my original gears at 1500 miles. I plan on doing the same for 3.91's. By waiting for them to be broken in, you get any metal particles out that are floating around.

[Todd in Vancouver]

This is what I understand and please feel free to correct it if you know better.

When the pinion and ring gear are installed you set a pattern on those gears. While you are breaking them in the gear pattern wears in and gives you that nice shining pattern that you see on the gears. That metal that comes off while the pattern is wearing in suspends in the oil and after X miles and the pattern is set you need to get that metal suspended in the oil out of there.

[SS376]

I understand these replies but this is not what I'm asking.

Why is it any different when you buy a car new vs when you change the gears?

Anybody know why installing new GM gears for some reason requires more care than the original GM gears the car came with? There would presumably be metal filings there too.

[Todd in Vancouver]

Originally Posted by SS376

I understand these replies but this is not what I'm asking.

Why is it any different when you buy a car new vs when you change the gears?

Anybody know why installing new GM gears for some reason requires more care than the original GM gears the car came with? There would presumably be metal filings there too.

I changed the diff oil on my car after a few thousand miles from buying it new. While this isn't excessive amount of metallic filings it sure doesn't hurt getting it out of my diff.

[MagnumForceGB]

Some people say it impacts the noise gears make. If the gears are installed correctly it doesn't matter how they are broken in. When we did a 3.73 swap we were easy the first 3 drives, then we got on it. 5000 miles later, no noise and still on the oil we had put in during the install.

[JCunningham]

they dont tell you to do it when its new because they dont want to tell you you have to spend money on it 500 miles after you buy it. + there price is robbery. cost $12 and takes 5 minutes and they charge $150. When you change it you will see why it should be done by 500 miles. It will be metalic black. The gears have to wear in to each other.

[camguynj]

Originally Posted by SS376

I understand these replies but this is not what I'm asking.

Why is it any different when you buy a car new vs when you change the gears?

Anybody know why installing new GM gears for some reason requires more care than the original GM gears the car came with? There would presumably be metal filings there too.

See this re: EOM vs after market gear lapping & phosphrous coating

http://www.jeepforum.com/forum/f9/ne...cedure-360238/

[SS376]

Originally Posted by camguynj

See this re: EOM vs after market gear lapping & phosphrous coating

http://www.jeepforum.com/forum/f9/ne...cedure-360238/

Very interesting. Probably, since I got GM factory gears (presuming they are the same 3.91's that go right into the 1LI), I don't really need to go through the 500 mile oil change, etc. But I'll probably do it anyway.

[Mr_Draco]

Originally Posted by SS376

I understand these replies but this is not what I'm asking.

Why is it any different when you buy a car new vs when you change the gears?

Anybody know why installing new GM gears for some reason requires more care than the original GM gears the car came with? There would presumably be metal filings there too.

Because most people don't actually read the manual. The manual tells you to change the rear diff at 1,500 miles. Most people don't because they don't know that you are suppose to. Now the 500 miles vs 1,500 miles is more of a matter of opinion and taste but you should definitely so it by 1,500.

[SS376]

Originally Posted by Mr_Draco

Because most people don't actually read the manual. The manual tells you to change the rear diff at 1,500 miles. Most people don't because they don't know that you are suppose to. Now the 500 miles vs 1,500 miles is more of a matter of opinion and taste but you should definitely so it by 1,500.

It says 15,000 miles, not 1,500.

[Mr_Draco]

Originally Posted by SS376

It says 15,000 miles, not 1,500.

Nope. Read the Break In chapter. It says after the 1,500 mile break in procedure is complete it is advised to change the rear diff fluid.

[Highland Ranger]

Page 11-7, severe schedule of the 2014 manual 3rd printing says 15,000 miles for rear axle fluid change. It does not delineate between zl1 and other models.

The break in section does not mention the diff ( page 9-19) other than to say fluid should be changed before racing.

Can you give me a page number and version of manual you are looking at?

[SS376]

Originally Posted by Mr_Draco

Nope. Read the Break In chapter. It says after the 1,500 mile break in procedure is complete it is advised to change the rear diff fluid.

It doesn't say that. It references changing the diff oil if the car is used for racing or competition purposes. First diff oil change is recommended at 15,000 miles.