Differential gear conundrum

My78truck

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I want to change the fluid in my rear diff (even a girl can do this!!! ;D) as I cannot remember if, or when, it has ever been changed (I am sure it has over 43 years at some point). I always was told it had limited slip, how do I figure out if it does or doesn't because I know that if it is limited slip, I need to use an additive.

So here is the conundrum......it was ordered with a 4.11 ratio, the RPO build list has it at a 3.40 ratio and on the diff there is a GM8 which when I look it up says that is for a 2.56 ratio. So, how do I know WHICH I have? There are a series of numbers like a serial number on the diff. There is no tag attached to any bolt.
HELP!!!!!
 

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Yur pullin the cover off... when its off just roll it around and look in the carrier? :shrug: Also the GM8 i dont think means anything, my 78' 12 bolt had GM8 and it was a 3:07 gear.
 

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Personally I'd just jack it up and take off the cover. Overall ratio will be stamped on the ring gear and you can turn the wheels while it's in the air to know if it's open or LSD
 

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I want to change the fluid in my rear diff (even a girl can do this!!! ;D) as I cannot remember if, or when, it has ever been changed (I am sure it has over 43 years at some point). I always was told it had limited slip, how do I figure out if it does or doesn't because I know that if it is limited slip, I need to use an additive.

So here is the conundrum......it was ordered with a 4.11 ratio, the RPO build list has it at a 3.40 ratio and on the diff there is a GM8 which when I look it up says that is for a 2.56 ratio. So, how do I know WHICH I have? There are a series of numbers like a serial number on the diff. There is no tag attached to any bolt.
HELP!!!!!

I'm confused too... If it was ordered with a 4.11, why would the RPO be for a 3.40? Where do you see GM8 at? Is it the code stamped into the passenger side axle tube?
 

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Like others have said, If you have the cover off you'll see exactly what the ratio is and whether it is locking or not. If you have a G80, it doesn't take an additive anyways.
 

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I'm confused too... If it was ordered with a 4.11, why would the RPO be for a 3.40? Where do you see GM8 at? Is it the code stamped into the passenger side axle tube?
Its a big casting on the center near the tube
 

My78truck

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I'm confused too... If it was ordered with a 4.11, why would the RPO be for a 3.40? Where do you see GM8 at? Is it the code stamped into the passenger side axle tube?

Actually it is raised lettering "GM8" on the drivers side axle tube.
 

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Actually it is raised lettering "GM8" on the drivers side axle tube.
OHHHHH the casting number... got it.

You should have a STAMPED number on the top of the passenger side tube that will actually tell you EXACTLY what it came with originally. Of course, since it is 40+ years old, you might have something completely different in there. I couldn't see the stamping on my 1980 14BFF until it was removed from the truck and had been scraped, degreased, scraped again, and then a final pressure washing. It turned out to be the original "RHJ" code which was for 1980 K20/K30 4.10 Non-locking. You can find the code list here: https://www.gmsquarebody.com/threads/squarebody-rear-axle-codes.4185/#post-109731
 

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And this is what the STAMPED number looks like on the passenger side tube. The first three letters are what you should be interested in.
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My78truck

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Thank you, thank you, thank you. I saw some stamped numbers on that tube but didn't have a light and I need to clean it up a bit to get a good read. I will let you know how it goes.

One more question, do you suggest I get a real gasket or use the RTF gasket maker when I reassemble.
 

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Thank you, thank you, thank you. I saw some stamped numbers on that tube but didn't have a light and I need to clean it up a bit to get a good read. I will let you know how it goes.

One more question, do you suggest I get a real gasket or use the RTF gasket maker when I reassemble.

I've used both with good success. You'll want a good amount of brake cleaner also to help clear out all the junk that is likely to be built up in there. I'd also recommend pouring a little bit of marvel mystery oil or seafoam in and then go on an extended drive to help clean out old oil deposits. I use good old supertech 80-90w from walmart in my square as it is GL5 rated. I use more expensive valvoline 75w-90 in my wifes' daily that we use for towing.
 

My78truck

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I've used both with good success. You'll want a good amount of brake cleaner also to help clear out all the junk that is likely to be built up in there. I'd also recommend pouring a little bit of marvel mystery oil or seafoam in and then go on an extended drive to help clean out old oil deposits. I use good old supertech 80-90w from walmart in my square as it is GL5 rated. I use more expensive valvoline 75w-90 in my wifes' daily that we use for towing.

Thanks again. Tomorrow starts the adventure.
 

My78truck

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And this is what the STAMPED number looks like on the passenger side tube. The first three letters are what you should be interested in.
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Here are mine. FINALLY!!!!! found them. Was worried that my truck was the lucky one without numbers

You must be registered for see images attach
 

Bextreme04

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Here are mine. FINALLY!!!!! found them. Was worried that my truck was the lucky one without numbers

You must be registered for see images attach

well, there you go... so likely the original axle at least. Which makes it a 3.40 non-locking in a 74-79 C10
 

My78truck

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well, there you go... so likely the original axle at least. Which makes it a 3.40 non-locking in a 74-79 C10

Thanks so much for clarifying and your help. I was looking at another list trying to decipher and it had my axle as a K=GM of Canada D=Cadillac A= January. Very confusing.

Now, once I pop this pumpkin, what is the best way to clean the internal gears in place. Or should I?
 

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