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Old 04-19-2008, 01:55 PM   #1
fredf1980
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Question Identify a rear end

I have an axle/differential marked GM79 on one axle and the numbers 3896839 on the other axle. It appears to be possitrac with a ratio of about 3 and a quarter( a little less).The wheel pattern is 6 lug. I'm trying to figure out what this may fit.Year? Truck? Whatever I can. Anyone have any idea where to look (or know themselves)?

Last edited by fredf1980; 04-19-2008 at 05:36 PM. Reason: forgot something
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Old 04-22-2008, 10:11 AM   #2
Pontiac1976
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Do you know if it's a 12 bolt or 10 cover. It bing a 6 bolt most likey out of a 3/4 4x4 has to see if I can find some more info.
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77GMC 1/2 ton Pu 454 has been my daily driver since 98-04, other than oil changes tune up. I put new fuel pump timing on the motor. UP date front clip to 84 clip an 89van F bumper was put on. I took out the 454 to try to save on fuel. I put 85 305 SS motor, still using the 77 3core rad. I had to do work to the right brace for the bumper and drill new holes, on the bumper. I had to make new top braces to hold the top of the rad. It's a flatbed homemade steel bed all this. Make a great work bench
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Old 04-22-2008, 12:29 PM   #3
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12 bolt cover. Any ideas with that?
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Old 04-23-2008, 06:13 PM   #4
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pretty sure it 1979 three should be a code on the diff that will tell you the diff size or get the # off the in side gears. pretty sure the 3896839 code is the GM diff housing cassting #. I have a few but can't come up with that one.


It's a little hard to see the #'s on the ring gear but you can make them out this is a open 3.42 diff . There should be a 3 code can be two letter one number or one letter two numbers. if you can see them still.

this will give you a idea where and what what #'s
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77GMC 1/2 ton Pu 454 has been my daily driver since 98-04, other than oil changes tune up. I put new fuel pump timing on the motor. UP date front clip to 84 clip an 89van F bumper was put on. I took out the 454 to try to save on fuel. I put 85 305 SS motor, still using the 77 3core rad. I had to do work to the right brace for the bumper and drill new holes, on the bumper. I had to make new top braces to hold the top of the rad. It's a flatbed homemade steel bed all this. Make a great work bench
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Old 04-23-2008, 06:29 PM   #5
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Thank You very much Pontiac1976. I will let you know if I find the numbers. This is a huge help. Your picture is exactly what is in my shed (looks) . Fred
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