12 bolt rear end: posi? - Chevelle Tech
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post #1 of 18 (permalink) Old Apr 16th, 12, 4:43 PM Thread Starter
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Dan
 
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12 bolt rear end: posi?

Hi guys,

I have a rear end which i believe has 2.56 gears. i am curious how to tell if its posi. if i turn either wheel the other should also turn? or if i turn the drive line it, both should turn?

I am tryin to run a 12 bolt posi with 3.73 gears. so if this is a posi, is it possible to swap the gears myself or is there necessary machine work? whats the easiest cheapest way to get a 3.73 posi?

THANKS!
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post #2 of 18 (permalink) Old Apr 16th, 12, 4:52 PM
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Re: 12 bolt rear end: posi?

With the rear tires off the ground, if you spin one tire the other should spin the same direction(unless it's worn out). Also, if you have a buddy that can hold the other tire, spin it and see if there is resistance. You can make a mark on the driveshaft and on the tire and find the approx gear ratio also. Rotate the tire one revolution, and watch the driveshaft to see how many time it spins. 2.5 times would give you 2.56, etc. Hope this helps.

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Big Shot Plate
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Best#'s- 1.35-5.95-120-9.59-141
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post #3 of 18 (permalink) Old Apr 16th, 12, 4:57 PM Thread Starter
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Re: 12 bolt rear end: posi?

thanks for your response, do you know much about swapping gears? what machine work is necessary or how difficult?
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post #4 of 18 (permalink) Old Apr 16th, 12, 5:05 PM
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Re: 12 bolt rear end: posi?

There's no machine work involved.
If you do have 2.56's then you have a 2 series carrier and you need a 3 series carrier to put 3.73's in.
Here's the carrier breaks:
12 bolt ED32088 2 2.73:1 & down
12 bolt 30140PM1 3 3.08:1 to 3.73:1
12 bolt EDB30174 4 3.90:1 & up

uıɐbɐ ʎɐqǝ ɯoɹɟ pɹɐoqʎǝʞ ɐ buıʎnq ɹǝʌǝu ɯ,ı
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post #5 of 18 (permalink) Old Apr 16th, 12, 5:15 PM
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Josh
 
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Re: 12 bolt rear end: posi?

edit: Mike has "treed" me on a few things^^^
First you need to find out for sure what ratio you have. There are different "series" carriers for ratios in the 2's, 3's, and 4's. If you want to go to 3.73 gears, you really need a 3-series posi carrier. If you happened to have a 3.08 or 3.36 posi unit, you would be in good shape and could get it rebuilt instead of a new unit. Taking the cover off would tell the story also. Setting up a ring&pinion requires a few specialty tools, like a press, dial indicator, pinion depth tool, and inch-pound torque wrench. Not for a novice. Hope this helps.

1970 ElCamino
440ci "Baby" BBC
Big Shot Plate
TH400 w/Brake
12 bolt
Best#'s- 1.35-5.95-120-9.59-141
Getting it done with 10"
2015 Pinks All Out 8-9sec Winner
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post #6 of 18 (permalink) Old Apr 16th, 12, 5:27 PM
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Brad
 
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Re: 12 bolt rear end: posi?

Like stated, jack up rear and spin a wheel. Posi both will spin same way. Open one will go one way and the other will go opposite direction. Changing gears is one of the things best left to an experienced person. Really I know good mechanics that bow out on this. To get ratio take off cover and count teeth on ring and pinion. Divide ring # by pinion# will give ratio. With the price of fuel may want to stick to the 2.56 I run 3.73 with 1-1 no overdrive trannny and sucks the fuel. Enjoy
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post #7 of 18 (permalink) Old Apr 16th, 12, 5:30 PM Thread Starter
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Dan
 
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Re: 12 bolt rear end: posi?

what does this mean, if i need a 3 series carrier, can i buy the gears and carrier at once and install it all? or does this mean my rear end wont work?
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post #8 of 18 (permalink) Old Apr 16th, 12, 5:32 PM Thread Starter
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Re: 12 bolt rear end: posi?

Quote:
Originally Posted by Qwiktripz1320 View Post
edit: Mike has "treed" me on a few things^^^
First you need to find out for sure what ratio you have. There are different "series" carriers for ratios in the 2's, 3's, and 4's. If you want to go to 3.73 gears, you really need a 3-series posi carrier. If you happened to have a 3.08 or 3.36 posi unit, you would be in good shape and could get it rebuilt instead of a new unit. Taking the cover off would tell the story also. Setting up a ring&pinion requires a few specialty tools, like a press, dial indicator, pinion depth tool, and inch-pound torque wrench. Not for a novice. Hope this helps.

great response, thanks....so any clue on what it costs to have one rebuilt? i can buy a rear end for 500 that has 3.36's i believe. would i be better off (save more time and money) buying this and having it rebuilt or having the current rear end built which has the 2.56's?
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post #9 of 18 (permalink) Old Apr 16th, 12, 5:35 PM
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Re: 12 bolt rear end: posi?

Jacking up the car and spinning the tires will not tell you what you have. I have seen many posi rear ends that would spin the tires in opposite directions and I've seen standard rear ends that would spin both tires in the same direction because the differential was damaged or the axle tubes were bent. The ONLY way to be sure of what you have is to remove the cover and look inside. This is also the only sure way to know what gear ratio you have.

To set up the gears you must know how to set the pinion depth, bearing preload and backlash to get the correct contact pattern. You also must know how to read the contact pattern so that you will know what adjustments need to be made to the pinion depth and backlash. These adjustments are made with shims and must be within +/- .001 inch. There are a few tools that you need to do this job.
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post #10 of 18 (permalink) Old Apr 16th, 12, 5:35 PM Thread Starter
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Re: 12 bolt rear end: posi?

Quote:
Originally Posted by Brad Richardson View Post
Like stated, jack up rear and spin a wheel. Posi both will spin same way. Open one will go one way and the other will go opposite direction. Changing gears is one of the things best left to an experienced person. Really I know good mechanics that bow out on this. To get ratio take off cover and count teeth on ring and pinion. Divide ring # by pinion# will give ratio. With the price of fuel may want to stick to the 2.56 I run 3.73 with 1-1 no overdrive trannny and sucks the fuel. Enjoy
i was told that with my 396, a 700r4 trans and 3.73 i would get pretty good gas mileage like above 15mpg. not sure if i can run this set up with the 2.56's or if it would give better or worse mpg...do u know?
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post #11 of 18 (permalink) Old Apr 16th, 12, 5:37 PM Thread Starter
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Re: 12 bolt rear end: posi?

Quote:
Originally Posted by big gear head View Post
Jacking up the car and spinning the tires will not tell you what you have. I have seen many posi rear ends that would spin the tires in opposite directions and I've seen standard rear ends that would spin both tires in the same direction because the differential was damaged or the axle tubes were bent. The ONLY way to be sure of what you have is to remove the cover and look inside. This is also the only sure way to know what gear ratio you have.

To set up the gears you must know how to set the pinion depth, bearing preload and backlash to get the correct contact pattern. You also must know how to read the contact pattern so that you will know what adjustments need to be made to the pinion depth and backlash. These adjustments are made with shims and must be within +/- .001 inch. There are a few tools that you need to do this job.
so as they have stated above, i cant count teeth to find the gear ratio by opening it...what do i look at on the inside to know if its posi or not? once i have it open, thanks
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post #12 of 18 (permalink) Old Apr 16th, 12, 5:45 PM
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Re: 12 bolt rear end: posi?

Tire hieght will be a factor as well....Say you can get a 12 bolt 3.36?...I have one of those and it came from a 68 El Cam SS. Don't see many IMO would be a great gear for your setup. Would be a great improvement over the 2.56. But I understand the need for speed....
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post #13 of 18 (permalink) Old Apr 16th, 12, 7:02 PM
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Re: 12 bolt rear end: posi?


This is what the Eaton Posi looks like. If you see the springs in the center then it's probably an Eaton. If there are no springs then it's probably a standard differential. I have seen people remove the springs from the Eaton, so take a picture and post it if you are not sure.

The 3.36 is a 10 bolt gear. The 12 bolt is a 3.31.
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post #14 of 18 (permalink) Old Apr 16th, 12, 8:45 PM
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Re: 12 bolt rear end: posi?

A couple things:
3.73's would be great with a 700r4. If you have 2.56 gears AND an overdrive, it will possibly bog the engine at normal highway speeds, giving you worse fuel mileage. The 0.7:1 overdrive will make cruising a 3.73 almost exactly like a regular 3 speed transmission cruising with a 2.56. (3.73 x .7 = 2.61 )

Also, if you have (or acquire) a posi unit that is a 4 series carrier, most ring and pinion manufacturers make a 3.73 gear for 4 series carriers.

_____________________________________
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post #15 of 18 (permalink) Old Apr 16th, 12, 9:00 PM
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Re: 12 bolt rear end: posi?

Big Gear Head. Have done research and you are correct in saying that the 10 bolts had 3:36 and the 12 bolts had 3:31.from factory... The one I have is in fact a 3:36 in a 12 bolt and must be an after mkt is only explanation. Come up dead end.. with mostly Vettes running them.
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