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04-20-2010, 01:28 PM | #1 |
GM made with FORD paint
Join Date: Oct 2007
Location: DeBary, Florida
Posts: 127
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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
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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 |
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
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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 |
Registered User
Join Date: Apr 2009
Location: Rochester, NY
Posts: 1,458
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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.
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1970 Blazer 350/SM465/205 SOLD! | 1998 Z3 |
04-20-2010, 05:03 PM | #4 |
GM made with FORD paint
Join Date: Oct 2007
Location: DeBary, Florida
Posts: 127
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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
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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 |
Registered User
Join Date: Apr 2009
Location: Rochester, NY
Posts: 1,458
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Re: Can somebody ID my rear differential on my 72 GMC 1500?
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1970 Blazer 350/SM465/205 SOLD! | 1998 Z3 |
04-20-2010, 07:34 PM | #6 |
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?
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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 |
Account Suspended
Join Date: May 2004
Location: Simi Valley, CA
Posts: 1,387
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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.
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04-20-2010, 11:33 PM | #8 |
Registered User
Join Date: Dec 2009
Location: California, Maryland
Posts: 544
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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.
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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 |
Registered User
Join Date: Feb 2010
Location: duluth mn
Posts: 108
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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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