Can somebody ID my rear differential on my 72 GMC 1500? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Kel's 72 GMC · 04-20-2010, 01:28 PM

im guessing the model numer is GM 58

i think the other number reads 3896838 or 3896839

Kel's 72 GMC · 04-20-2010, 01:29 PM

more pics

JimmyFloyd · 04-20-2010, 01:42 PM

Looks like a GM 12 bolt rear

To figure out if it is Posi, jack it up so both rear wheels are up and spin the tire. if the other side moves the same direction, posi, if opposite, then it's open.

To get the gear ratio, put a mark on the tire and one on the drive shaft. Spin the tire and count how many times the drive shaft turns during 2 full turns of the tire. You can use those numbers to figure out the ratio.

Kel's 72 GMC · 04-20-2010, 05:03 PM

its not a posi. Do you know what the numbers mean? So its just a GM 12 bolt not a DANA or any other brand

JimmyFloyd · 04-20-2010, 05:07 PM

Correct. GM 12 bolt.

http://coloradok5.com/axleguide.shtml

Kel's 72 GMC · 04-20-2010, 07:34 PM

is there a good site that has any info on the casting numbers for a GM 12 bolt for trucks. All ive found where for Camaros and Chevelles. Also any idea on the ratio?

*Sublvr72* · 04-20-2010, 08:58 PM

appears to be a standard GM 12 bolt as others have said. You probably won't know the ratio unless the service tag is still on the diff, which it appears that it's long gone. You can take the cover off and count the teeth on the pinion and gear to figure it out if you really want to know.

gkesseru · 04-20-2010, 11:33 PM

I find it easier and accurate to make a mark (via masking tape and magic marker) on rear yoke and diff housing. I put a piece of masking tape around the entire tire and make a start mark using a chair or something to point to the start mark on the tire. Turn the wheel until the drive shaft makes exactly one turn. Make a second mark on the tire masking tape for the distance the tire traveled. Cut off the tire masking tape exactly at the start mark. Lay the tape straight and measure the tire circumference (whole tape) and divide by distance tire moved (second mark). Multiply by two. That's the ratio. Don't multiply if you have posi.

neba · 04-21-2010, 08:32 AM

open the cover. the gear lube should be changed. then you can see what you have.

04-20-2010, 01:28 PM	#1
Kel's 72 GMC GM made with FORD paint Join Date: Oct 2007 Location: DeBary, Florida Posts: 127	Can somebody ID my rear differential on my 72 GMC 1500? im guessing the model numer is GM 58 i think the other number reads 3896838 or 3896839 Attached Images __________________ 1972 GMC Custom 1500, 2WD LWB, 1970 chevy impala SS 350 c.i., shaved door handles, Ford Paint (portofino blue), blue underglow, and much more. 1998 GMC Sierra 1500 ext. cab, 2WD SWB, 305 c.i. all original 165,000 miles and still strong (daily driver)

04-20-2010, 01:29 PM	#2
Kel's 72 GMC GM made with FORD paint Join Date: Oct 2007 Location: DeBary, Florida Posts: 127	Re: Can somebody ID my rear differential on my 72 GMC 1500? more pics Attached Images __________________ 1972 GMC Custom 1500, 2WD LWB, 1970 chevy impala SS 350 c.i., shaved door handles, Ford Paint (portofino blue), blue underglow, and much more. 1998 GMC Sierra 1500 ext. cab, 2WD SWB, 305 c.i. all original 165,000 miles and still strong (daily driver)

04-20-2010, 01:42 PM	#3
JimmyFloyd Registered User Join Date: Apr 2009 Location: Rochester, NY Posts: 1,458	Re: Can somebody ID my rear differential on my 72 GMC 1500? Looks like a GM 12 bolt rear To figure out if it is Posi, jack it up so both rear wheels are up and spin the tire. if the other side moves the same direction, posi, if opposite, then it's open. To get the gear ratio, put a mark on the tire and one on the drive shaft. Spin the tire and count how many times the drive shaft turns during 2 full turns of the tire. You can use those numbers to figure out the ratio. __________________ 1970 Blazer 350/SM465/205 SOLD! \| 1998 Z3

04-20-2010, 05:03 PM	#4
Kel's 72 GMC GM made with FORD paint Join Date: Oct 2007 Location: DeBary, Florida Posts: 127	Re: Can somebody ID my rear differential on my 72 GMC 1500? its not a posi. Do you know what the numbers mean? So its just a GM 12 bolt not a DANA or any other brand __________________ 1972 GMC Custom 1500, 2WD LWB, 1970 chevy impala SS 350 c.i., shaved door handles, Ford Paint (portofino blue), blue underglow, and much more. 1998 GMC Sierra 1500 ext. cab, 2WD SWB, 305 c.i. all original 165,000 miles and still strong (daily driver)

04-20-2010, 05:07 PM	#5
JimmyFloyd Registered User Join Date: Apr 2009 Location: Rochester, NY Posts: 1,458	Re: Can somebody ID my rear differential on my 72 GMC 1500? Correct. GM 12 bolt. http://coloradok5.com/axleguide.shtml __________________ 1970 Blazer 350/SM465/205 SOLD! \| 1998 Z3

04-20-2010, 07:34 PM	#6
Kel's 72 GMC GM made with FORD paint Join Date: Oct 2007 Location: DeBary, Florida Posts: 127	Re: Can somebody ID my rear differential on my 72 GMC 1500? is there a good site that has any info on the casting numbers for a GM 12 bolt for trucks. All ive found where for Camaros and Chevelles. Also any idea on the ratio? __________________ 1972 GMC Custom 1500, 2WD LWB, 1970 chevy impala SS 350 c.i., shaved door handles, Ford Paint (portofino blue), blue underglow, and much more. 1998 GMC Sierra 1500 ext. cab, 2WD SWB, 305 c.i. all original 165,000 miles and still strong (daily driver)

04-20-2010, 08:58 PM	#7
Sublvr72 Account Suspended Join Date: May 2004 Location: Simi Valley, CA Posts: 1,387	Re: Can somebody ID my rear differential on my 72 GMC 1500? appears to be a standard GM 12 bolt as others have said. You probably won't know the ratio unless the service tag is still on the diff, which it appears that it's long gone. You can take the cover off and count the teeth on the pinion and gear to figure it out if you really want to know.

04-20-2010, 11:33 PM	#8
gkesseru Registered User Join Date: Dec 2009 Location: California, Maryland Posts: 544	Re: Can somebody ID my rear differential on my 72 GMC 1500? I find it easier and accurate to make a mark (via masking tape and magic marker) on rear yoke and diff housing. I put a piece of masking tape around the entire tire and make a start mark using a chair or something to point to the start mark on the tire. Turn the wheel until the drive shaft makes exactly one turn. Make a second mark on the tire masking tape for the distance the tire traveled. Cut off the tire masking tape exactly at the start mark. Lay the tape straight and measure the tire circumference (whole tape) and divide by distance tire moved (second mark). Multiply by two. That's the ratio. Don't multiply if you have posi. __________________ 72 Fleet (died) 78 Step (sold) 55 Step named Fred (sold) 00 F150 68 Step (named El Scorcho by PO) Build thread for the new El Scorcho http://67-72chevytrucks.com/vboard/s...d.php?t=392932

04-21-2010, 08:32 AM	#9
neba Registered User Join Date: Feb 2010 Location: duluth mn Posts: 108	Re: Can somebody ID my rear differential on my 72 GMC 1500? open the cover. the gear lube should be changed. then you can see what you have.



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