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Welcome to the Team your Highness (I am only a lowly British baron by birth, or I will be if my dad dies before me).

Application codes are stamped on the axle tube at time of assembly in one of three plants. The gear set installed at the factory is stamped into the tube (in code) and another letter may or may not be stamped indicating the inclusion of a positraction if you had one from the factory. Also included in the stamped marks is the date of assembly and which plant makes it.

Look for the stamped letters and numbers on the passenger side on the front of the tube about the middle. You may need to wire brush off road grime and rust to read the information stamped there. If you can take a clear in focus photo (usually requires a digital camera instead of a cell phone or a tablet) that would be the best; other wise just post what you find and we can interpret the numbers and letters for you.

Big Dave (Baron of Dawyck in Peebles Scotland)
 

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There are couple of different codes to determine the rear the codes here are the codes for a 10 bolt 8.2" EDF 21ST WHICH IS GEARED 2.73:1 & DOWN
EDB30116 WHICH IS 3.08:1 &UP
then codes for a 10 bolt 8.5" 410409n 2.56:1 &down
410409n 2.73:1&up
Then the 12th bolt codes are ed32088 2.73:1& down
30140pm1 3.08:1 to3.73:1
Ed30174 3.90:1& up
 

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There are couple of different codes to determine the rear the codes here are the codes for a 10 bolt 8.2" EDF 21ST WHICH IS GEARED 2.73:1 & DOWN
EDB30116 WHICH IS 3.08:1 &UP
then codes for a 10 bolt 8.5" 410409n 2.56:1 &down
410409n 2.73:1&up
Then the 12th bolt codes are ed32088 2.73:1& down
30140pm1 3.08:1 to3.73:1
Ed30174 3.90:1& up

Those casting numbers are located on the carrier which he will have to open the inspection cover to read. If the cover is off he could count the teeth and divide the pinion gear into the ring gear to obtain the ratio.

Big Dave
 

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The casting number on the front is gbg1402, what does this mean, couldn't get a clear answer on internet.
A bit late to the party, as King hasn't been around for about 5 years, but I noticed everyone seemed to interpret is question as wanting to decipher gear ratio of the differential, but I didn't read it that way.
Taking what he wrote, literally, my answer would be that it is exactly what you said it was King.
A casting number.
It would correspond with a certain mold used in the casting of that particular differential housing.
It has not real rhyme or reason, other than saying that housing was cast using that mold.
It could be used to name the place and approx. date of the casting, as molds usually don't last to long, from the way I understand things, and different foundry's will catalogue uniquely, so the number would be particular to the specific foundry.

….however, if King WERE asking about how to decipher gear ratio, I would suggest getting some oil and pulling the diff cover.
Then slowly rotate the ring gear. Eventually you'll see some writing on it.
Part of that will be two numbers separated by a colon ( : ).
Those numbers are the number of teeth on the ring gear and the pinion gear.
Divide the smaller number by the bigger number and that will give you your gear ratio.
For instance, if you see the numbers, "37:11", you'll find you have a set of 3.36 gears in that rear end, because 37/11 is 3.363636363636364.


Elvis
 
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