I have a 12 bolt rear - The 1947 - Present Chevrolet & GMC Truck Message Board Network

gintaras · 05-02-2009, 08:40 PM

Today I counted the bolts on my rear diff.. it's a 12 bolt. Is that something special? I KNOW it is not posi.. can I convert it to posi, or would it be best to buy a positraction rear end and swap it?

The Big Green · 05-02-2009, 08:47 PM

Yea, you can just buy a posi unit for it.

gintaras · 05-02-2009, 10:47 PM

thanks for the reply.

I don't know what ratio I have, so I jacked up the rear and turned the driveshaft by hand, but I only get about 1 and 1/2 turns from the driveshaft for one complete tire rotation.. I get the feeling that I need to multiply that by 2, so I assume I have a 3.08:1?

5150.4.67 · 05-03-2009, 12:59 AM

Pull the cover off and have a look. The numbers you are looking for are stamped on the ring itself. For example 38:10. This would be a 3.80 ratio. Now that you're this far, you might as well pull the axles and put in that posi!

**JJanzen** · 05-03-2009, 09:13 AM

If you will look on the forward side of the passenger side of your rear axle, you will find a (3) digit code. It will be (3) capital letters. Once you have that code, go to this link and then "Axel Codes". It will tell your what your differential is. A lot easier than pulling the cover and counting teeth.

http://www.americanclassic.com/paint_test.asp

LONGHAIR · 05-03-2009, 09:29 AM

Quote:

Originally Posted by JJanzen

If you will look on the forward side of the passenger side of your rear axle, you will find a (3) digit code. It will be (3) capital letters. Once you have that code, go to this link and then "Axel Codes". It will tell your what your differential is. A lot easier than pulling the cover and counting teeth.

http://www.americanclassic.com/paint_test.asp

That will tell you what was originally in there, but this is a 40 year old truck, it could be anything. Counting or looking for the numbers that are stamped into the ring gear itself are the only ways to know for sure.....it could use some new lube anyway, that is one of the most neglected things on any truck.

Quote:

Originally Posted by 5150.4.67

Pull the cover off and have a look. The numbers you are looking for are stamped on the ring itself. For example 38:10. This would be a 3.80 ratio. Now that you're this far, you might as well pull the axles and put in that posi!

It's just not as easy as that. You really should have a Pro install something like that.

**WorkinLonghorn** · 05-03-2009, 10:23 AM

Yeah,you just multiply by two so if it is REAL close to 1 1/2 revs,then it's a 3.07-1 ratio (my book from 1969 doesn't show a "3.08" for these trucks).
To be sure,you can count the revs necessary for 10 revs of the wheel and divide by 5. This of course is with one wheel on the ground with an open (non-posi) rear.

cdowns · 05-03-2009, 10:43 AM

converting with new parts posi carrier gearset bearings seals shims etc plus insallation labor runs $1200-$1500 if you can reuse the original axels

5150.4.67 · 05-06-2009, 12:49 AM

Correct, have it done by a pro. If you want it to last, that is.

05-02-2009, 08:40 PM	#1
gintaras Registered User Join Date: Aug 2008 Location: Schererville, IN Posts: 110	I have a 12 bolt rear Today I counted the bolts on my rear diff.. it's a 12 bolt. Is that something special? I KNOW it is not posi.. can I convert it to posi, or would it be best to buy a positraction rear end and swap it? __________________ 2004 VW R32 450 HP money pit (faster than a Viper in the 1320 ) 2001.5 VW Passat beater 1972 Custom Deluxe Highlander 69k miles and counting, all original, even the paint

05-02-2009, 08:47 PM	#2
The Big Green Registered User Join Date: Oct 2007 Location: Fort Smith,AR Posts: 1,305	Re: I have a 12 bolt rear Yea, you can just buy a posi unit for it.

05-02-2009, 10:47 PM	#3
gintaras Registered User Join Date: Aug 2008 Location: Schererville, IN Posts: 110	Re: I have a 12 bolt rear thanks for the reply. I don't know what ratio I have, so I jacked up the rear and turned the driveshaft by hand, but I only get about 1 and 1/2 turns from the driveshaft for one complete tire rotation.. I get the feeling that I need to multiply that by 2, so I assume I have a 3.08:1? __________________ 2004 VW R32 450 HP money pit (faster than a Viper in the 1320 ) 2001.5 VW Passat beater 1972 Custom Deluxe Highlander 69k miles and counting, all original, even the paint

05-03-2009, 12:59 AM	#4
5150.4.67 Registered User Join Date: Dec 2007 Location: Modesto, Ca Posts: 411	Re: I have a 12 bolt rear Pull the cover off and have a look. The numbers you are looking for are stamped on the ring itself. For example 38:10. This would be a 3.80 ratio. Now that you're this far, you might as well pull the axles and put in that posi!

05-03-2009, 09:13 AM	#5
JJanzen Senior Member Join Date: Feb 2008 Location: Denver, CO Posts: 252	Re: I have a 12 bolt rear If you will look on the forward side of the passenger side of your rear axle, you will find a (3) digit code. It will be (3) capital letters. Once you have that code, go to this link and then "Axel Codes". It will tell your what your differential is. A lot easier than pulling the cover and counting teeth. http://www.americanclassic.com/paint_test.asp __________________ Blue72 Red72

05-03-2009, 10:23 AM	#7
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,861	Re: I have a 12 bolt rear Yeah,you just multiply by two so if it is REAL close to 1 1/2 revs,then it's a 3.07-1 ratio (my book from 1969 doesn't show a "3.08" for these trucks). To be sure,you can count the revs necessary for 10 revs of the wheel and divide by 5. This of course is with one wheel on the ground with an open (non-posi) rear. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, 2nd owner, Work-Truck supreme. Best $300 I ever spent.

05-03-2009, 10:43 AM	#8
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: I have a 12 bolt rear converting with new parts posi carrier gearset bearings seals shims etc plus insallation labor runs $1200-$1500 if you can reuse the original axels __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

05-06-2009, 12:49 AM	#9
5150.4.67 Registered User Join Date: Dec 2007 Location: Modesto, Ca Posts: 411	Re: I have a 12 bolt rear Correct, have it done by a pro. If you want it to last, that is. Last edited by 5150.4.67; 05-06-2009 at 12:49 AM.



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.