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Old 12-06-2012, 11:19 PM   #1
DizzyD
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help please

I'm trying to figure out the gears on my 78 gmc c15 with 12 bolt. The casting number on the axle is 3896839N. I have tried searching this forum and the internet with no luck. Any help is greatly appreciated.
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Old 12-06-2012, 11:27 PM   #2
frog210
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Re: help please

the only way to really do it would be to take the top off and count the teeth
have you check you spid sheet?
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Old 12-06-2012, 11:50 PM   #3
geezer#99
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Re: help please

Jack one wheel off the ground. Put a chalk mark on it at 6 oclock. Right at the bottom. Spin the tire around one revolution. Count the number of times the yoke on the driveshaft spins around. Multiply that number by 2 which will equal your gear ratio.
Easy Peazy!!
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Old 12-07-2012, 05:56 PM   #4
DizzyD
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Re: help please

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Old 12-07-2012, 05:58 PM   #5
DizzyD
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Re: help please

Not sure what the spid sheet is but I will just pop the top off and do it that way. Thanks for the help gents.
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Old 12-07-2012, 07:26 PM   #6
88Jimmy4x4
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Re: help please

before you remove cover, loosen the fill plug on top to make sure it isnt frozen so you can refill with lube after your done
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Old 12-07-2012, 11:48 PM   #7
motornut
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Re: help please

my 79 has a 1G80 12 bolt locker
it's got a 3.4 gear
where was your number? front of the green circle?
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Old 12-08-2012, 01:42 PM   #8
DizzyD
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Re: help please

No the numbers I found were on the front facing passenger side of the axle. But then again I did have to do some serious scraping with a spatula and scrubbing with a stiff brush to get all that gunk off the axle. It didn't help i was also doing it at night so visibility was not the best. Will take another look at it today.
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