What differential rear end do I have??? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

hotwls13 · 05-10-2018, 06:46 PM

I was gonna replace the differential fluid in my new to me 1970 GMC LWB 1/2t. Don't see a drain plug or a fill plug (are they on the front of the differential?

Can we tell by outside pics what kind of gears etc I have?

My SPID is long gone (previous owner replaced the glovebox with chrome one). Original seat was replaced also so no paper/sheet in the back of the seat.

Mike C · 05-10-2018, 07:09 PM

12 bolt truck. No drain plug. Fill plug is on passengers side of case in front of axle tube.

**Steeveedee** · 05-10-2018, 07:12 PM

You have to take the cover off to drain the diff. You can inspect the gears while you are in there.

hotwls13 · 05-10-2018, 07:34 PM

Thanks guys. That wa s the only pic I had of the truck. After I posted it I figured fill must be on front. So if I have to take the cover off I’m also gonna need a new seal right? Any recommendations as to brand etc? Or just whatever my autozone has??

**Ironangel** · 05-10-2018, 07:50 PM

Fel-Pro RDS 13391...Any GL-5 MP Gear Lube in the 80w/90 will work...

hotwls13 · 05-10-2018, 08:47 PM

Quote:

Originally Posted by Ironangel

Fel-Pro RDS 13391...Any GL-5 MP Gear Lube in the 80w/90 will work...

Awesome thanks for the details!!

**Boog** · 05-10-2018, 10:15 PM

turn

Mike C · 05-11-2018, 08:30 AM

Once you get cover off you can get gear ratio from numbers stamped on OD of ring gear. You will also know if it has a limited slip and if so you will need to buy the friction modifier additive.

**69 MOD** · 05-11-2018, 12:09 PM

I'm not advocating not pulling the cover but if you jack the rear end up, put it in neutral and make a mark on your driveshaft you can make a very close estimate on gear ratio.

In one full rotation of your rear tire the driveshaft mark will go past 0 point X amount of times (short of 3 turns 2:54, just over 3 turns would be 3:04, 3-3/4 would be 3:73 etc.)

Backyard trick I picked up when I was a kid...
MOD

hotwls13 · 05-11-2018, 12:33 PM

Thanks again everyone for the tips. I usually drive new/newer vehicles, and usually have the maintenance done by a pro. So intentionally getting into stuff like this is not typical for me. haha

But, I want to know that the running gear is OK before I start driving it around and especially if I put my family in it. A good friend of mine who has been working on vehicles for over 50 years is my mentor and though he is kinda tired of working on them, he gives very good and important advice. He said "check and change the rear end oil" so that is getting done. Figure while I'm in there it's important to find out what I have.

Might be kind of a Duh question but, I'm guessing it's better to change/fill fluid while it's on level ground? Correct? But to see the side of the gear I may need to jack up the rear wheels to spin the gear??

I usually lift the rear with jack under the pumpkin. Should I put the rear on low jacks, pop cover off, drain, ID the gears, put cover back on, then drop wheels on the ground then add fluid?

Also, I've read where some people use a little RTV with the fel-pro gaskets?? Do or don't do that? And if I do, how long should I give the RTV to dry/set before filling with oil?

Lastly, what are the torque specs for the rear end bolts?

**Stocker** · 05-11-2018, 12:29 PM

If you decide to use a gasket sealer (I always do, gives me peace of mind), get Permatex 81182 -- it's made specifically for use with gear oil.

hotwls13 · 05-14-2018, 12:19 PM

Some external pics, hoping to get into the differential tonight.

custom10nut · 05-14-2018, 12:43 PM

Chiming in. I did this with mine last year, because the gasket failed. A light coat of the permatex will prevent any weeping.
On a side note:
Does anyone know where to get an "S-clip" these days?
We used to get them when I was a kid (like 35 years ago)
The clip will "tighten up" a one wheeler to make a poor man's posi

BCOWANWHEELS · 05-14-2018, 02:43 PM

forget gasket......... use permatex ultra black rtv. works a lot better as for diff lube use synthetic , you,ll never have to change it again ever. wash rear out with spray starting fluid.

**leddzepp** · 05-14-2018, 03:17 PM

Light coat of permatex on both sides of gasket. Let it tack over a bit, then install. Bolts do not need much torque at all, whatever service manual says. I’ve never had a leaking diff cover in my life doing it this way.

hotwls13 · 05-14-2018, 03:47 PM

Quote:

Originally Posted by leddzepp

Light coat of permatex on both sides of gasket. Let it tack over a bit, then install. Bolts do not need much torque at all, whatever service manual says. I’ve never had a leaking diff cover in my life doing it this way.

Ok thanks. Don't have a service manual. Did look in the pdf version, couldnt find torque specs for rear end. Friend said not tight, one handed on ratchet close to head of ratchet (if that makes sense).

**Steeveedee** · 05-14-2018, 05:37 PM

Bolt torque for the diff cover is specified on page 464. Dana 60 gets 35 Ft-Lbs.

hotwls13 · 05-14-2018, 06:41 PM

Quote:

Originally Posted by Steeveedee

Bolt torque for the diff cover is specified on page 464. Dana 60 gets 35 Ft-Lbs.

found it thanks. Page 646.

So I have a Dana 60??

**Ironangel** · 05-14-2018, 11:42 PM

Quote:

Originally Posted by hotwls13

found it thanks. Page 646.

So I have a Dana 60??

No, you have a GM-12T (12-Bolt Truck) What manual are you guys getting page numbers from? My 72 Service Manual dont have page numbers...? 3/8-16 cover bolts get 35 ft lbs, 5/16-18 get 23 ft lbs, and the 1/4-20 cover bolts get 18 ft lbs. Your GM 12T cover bolts get 23 ft lbs. Make sure the cover mating surface is clean and flat, same for the carrier housing. Use a can or two of Brake Clean to spray the inside of the housing clean, dont forget the drip pan. Check your vent while your under there, a clogged vent will lead to seal leaks. Vents located on the top right of the diff housing. Make sure you place jack stands under the axle before you go to work under there. LeddZepp gave a good method for sealing the cover gasket. The Fel-Pro gasket # for the 12T is RDS 13391. "There is no reason not to use a gasket and several reasons why you should"

**Steeveedee** · 05-16-2018, 06:04 PM

Quote:

Originally Posted by hotwls13

found it thanks. Page 646.

So I have a Dana 60??

Sorry! I wasn't even drinking! Glad you got it straightened out.

BCOWANWHEELS · 05-14-2018, 07:11 PM

12 bolt gm

hotwls13 · 05-15-2018, 12:55 AM

Here are the pics. I've got it all sealed back up, filling it back up tomorrow.

**leddzepp** · 05-15-2018, 12:59 AM

You have a 3.07 open diff. Non-posi.

43/14 = 3.07

I zoomed in on the numbers.

hotwls13 · 05-15-2018, 09:16 AM

Quote:

Originally Posted by leddzepp

You have a 3.07 open diff. Non-posi.

43/14 = 3.07

I zoomed in on the numbers.

Sweet thanks! I wasn't sure where to lookup the numbers. Any idea what the rest of it means?

So 3.07 is gonna be best possible mileage correct? I have a stock 350/350 with edelbrock intake/carb, headers, HD radiator.

My plans for this truck is get it driving/stopping 100%. It will be a weekend driver, trips to the dump, home depot etc. With the occasional (maybe 3-4 times a year) trailer pull of another 67-72 GM truck on uhaul trailer. I'm gonna add a curt class 4 hitch and a brake controller.

hotwls13 · 05-15-2018, 09:07 PM

Ugh!! Leaking!! I knew that diff cover was gonna be a problem.

Should I just order a new chrome one or try to fix this one?? Or load up on the RTV?? I know I'm gonna have to pop the cover off again, drain the oil (do I try to recapture it or buy 2 more quarts?), get a new gasket etc etc.

05-10-2018, 06:46 PM	#1
hotwls13 Registered User Join Date: Mar 2018 Location: Bakersfield, CA Posts: 308	What differential rear end do I have??? I was gonna replace the differential fluid in my new to me 1970 GMC LWB 1/2t. Don't see a drain plug or a fill plug (are they on the front of the differential? Can we tell by outside pics what kind of gears etc I have? My SPID is long gone (previous owner replaced the glovebox with chrome one). Original seat was replaced also so no paper/sheet in the back of the seat. Attached Images

05-10-2018, 07:09 PM	#2
Mike C Registered User Join Date: Dec 1999 Location: Austin, TX, USA Posts: 7,738	Re: What differential rear end do I have??? 12 bolt truck. No drain plug. Fill plug is on passengers side of case in front of axle tube. __________________ 44 Willys MB 52 M38A1 64 Corvette Coupe 68 Camaro 'vert LT1 & TH700 69 Z/28 355 12.6's @110 69 Chevy Short Step 4 1/2"/7" drop 72 Jimmy 4WD 4spd 4" & 35's 02 GMC 2500HD 4x4 Duramax

05-10-2018, 07:12 PM	#3
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,933	Re: What differential rear end do I have??? You have to take the cover off to drain the diff. You can inspect the gears while you are in there. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

05-10-2018, 07:34 PM	#4
hotwls13 Registered User Join Date: Mar 2018 Location: Bakersfield, CA Posts: 308	Re: What differential rear end do I have??? Thanks guys. That wa s the only pic I had of the truck. After I posted it I figured fill must be on front. So if I have to take the cover off I’m also gonna need a new seal right? Any recommendations as to brand etc? Or just whatever my autozone has??

05-10-2018, 07:50 PM	#5
Ironangel Senior Member Join Date: Feb 2010 Location: Falls City, Nebraska "100 Miles From Nowhere" Posts: 2,219	Re: What differential rear end do I have??? Fel-Pro RDS 13391...Any GL-5 MP Gear Lube in the 80w/90 will work... __________________ Michael of the clan Hill, "Two Seventy Two's" 71 1-ton Dually 350 4-Speed 71 C/50 Grain Truck, 350 Split-Axle 4-Speed 02 3/4 ton Express 14 Indian Chief Vintage 1952 Ford 8N, "Only Ford Allowed On The Property" "Be American, Buy American"

05-10-2018, 10:15 PM	#7
Boog laying low Join Date: Mar 2001 Location: Searcy, Ark. USA Posts: 13,659	Re: What differential rear end do I have??? turn Attached Images __________________ Boog 69 Chevy stepside, 358/T350, 4.11 posi, 4.5/4 drop, rallys, poboy driver primer is finer 91 Chevy sportside, Tahoe, Yukon & GMC Crewcab All GM..'nuff said. I stand for the flag and kneel at the cross

05-11-2018, 08:30 AM	#8
Mike C Registered User Join Date: Dec 1999 Location: Austin, TX, USA Posts: 7,738	Re: What differential rear end do I have??? Once you get cover off you can get gear ratio from numbers stamped on OD of ring gear. You will also know if it has a limited slip and if so you will need to buy the friction modifier additive. __________________ 44 Willys MB 52 M38A1 64 Corvette Coupe 68 Camaro 'vert LT1 & TH700 69 Z/28 355 12.6's @110 69 Chevy Short Step 4 1/2"/7" drop 72 Jimmy 4WD 4spd 4" & 35's 02 GMC 2500HD 4x4 Duramax

05-11-2018, 12:09 PM	#9
69 MOD Senior Member Join Date: May 2013 Location: Lino Lakes, MN Posts: 362	Re: What differential rear end do I have??? I'm not advocating not pulling the cover but if you jack the rear end up, put it in neutral and make a mark on your driveshaft you can make a very close estimate on gear ratio. In one full rotation of your rear tire the driveshaft mark will go past 0 point X amount of times (short of 3 turns 2:54, just over 3 turns would be 3:04, 3-3/4 would be 3:73 etc.) Backyard trick I picked up when I was a kid... MOD __________________ Chad '71 GMC 2500 Custom Ol Blue '72 Custom Deluxe Highlander Stuffed Olive '73 Trans Am '13 Silverado 2500 Duramax Storm Trooper

05-11-2018, 12:33 PM	#10
hotwls13 Registered User Join Date: Mar 2018 Location: Bakersfield, CA Posts: 308	Re: What differential rear end do I have??? Thanks again everyone for the tips. I usually drive new/newer vehicles, and usually have the maintenance done by a pro. So intentionally getting into stuff like this is not typical for me. haha But, I want to know that the running gear is OK before I start driving it around and especially if I put my family in it. A good friend of mine who has been working on vehicles for over 50 years is my mentor and though he is kinda tired of working on them, he gives very good and important advice. He said "check and change the rear end oil" so that is getting done. Figure while I'm in there it's important to find out what I have. Might be kind of a Duh question but, I'm guessing it's better to change/fill fluid while it's on level ground? Correct? But to see the side of the gear I may need to jack up the rear wheels to spin the gear?? I usually lift the rear with jack under the pumpkin. Should I put the rear on low jacks, pop cover off, drain, ID the gears, put cover back on, then drop wheels on the ground then add fluid? Also, I've read where some people use a little RTV with the fel-pro gaskets?? Do or don't do that? And if I do, how long should I give the RTV to dry/set before filling with oil? Lastly, what are the torque specs for the rear end bolts?

05-11-2018, 12:29 PM	#11
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,806	Re: What differential rear end do I have??? If you decide to use a gasket sealer (I always do, gives me peace of mind), get Permatex 81182 -- it's made specifically for use with gear oil. __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay

05-14-2018, 12:19 PM	#12
hotwls13 Registered User Join Date: Mar 2018 Location: Bakersfield, CA Posts: 308	Re: What differential rear end do I have??? Some external pics, hoping to get into the differential tonight. Attached Images

05-14-2018, 12:43 PM	#13
custom10nut Registered User Join Date: Apr 2015 Location: East Tn (In the heart of the Smoky Mtns) Posts: 1,903	Re: What differential rear end do I have??? Chiming in. I did this with mine last year, because the gasket failed. A light coat of the permatex will prevent any weeping. On a side note: Does anyone know where to get an "S-clip" these days? We used to get them when I was a kid (like 35 years ago) The clip will "tighten up" a one wheeler to make a poor man's posi

05-14-2018, 02:43 PM	#14
BCOWANWHEELS Registered User Join Date: Nov 2006 Location: KINGSPORT,TN. Posts: 3,035	Re: What differential rear end do I have??? forget gasket......... use permatex ultra black rtv. works a lot better as for diff lube use synthetic , you,ll never have to change it again ever. wash rear out with spray starting fluid. __________________ I BELIEVE IN JOHN 3:16

05-14-2018, 03:17 PM	#15
leddzepp Moderator Join Date: May 2007 Location: Southern Cal Posts: 20,115	Re: What differential rear end do I have??? Light coat of permatex on both sides of gasket. Let it tack over a bit, then install. Bolts do not need much torque at all, whatever service manual says. I’ve never had a leaking diff cover in my life doing it this way. __________________ 1972 C/10 Cheyenne Super SWB. Restored, loaded, slammed. 1968 C/10 50th Anniversary LWB. Unrestored, stock, daily driver/work truck. RIP ElJay RIP 67ChevyRedneck RIP Grumpy Old Man RIP FleetsidePaul

05-14-2018, 05:37 PM	#17
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,933	Re: What differential rear end do I have??? Bolt torque for the diff cover is specified on page 464. Dana 60 gets 35 Ft-Lbs. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA



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05-14-2018, 07:11 PM	#21
BCOWANWHEELS Registered User Join Date: Nov 2006 Location: KINGSPORT,TN. Posts: 3,035	Re: What differential rear end do I have??? 12 bolt gm __________________ I BELIEVE IN JOHN 3:16

05-15-2018, 12:55 AM	#22
hotwls13 Registered User Join Date: Mar 2018 Location: Bakersfield, CA Posts: 308	Re: What differential rear end do I have??? Here are the pics. I've got it all sealed back up, filling it back up tomorrow. Attached Images

05-15-2018, 12:59 AM	#23
leddzepp Moderator Join Date: May 2007 Location: Southern Cal Posts: 20,115	Re: What differential rear end do I have??? You have a 3.07 open diff. Non-posi. 43/14 = 3.07 I zoomed in on the numbers. __________________ 1972 C/10 Cheyenne Super SWB. Restored, loaded, slammed. 1968 C/10 50th Anniversary LWB. Unrestored, stock, daily driver/work truck. RIP ElJay RIP 67ChevyRedneck RIP Grumpy Old Man RIP FleetsidePaul

05-15-2018, 09:07 PM	#25
hotwls13 Registered User Join Date: Mar 2018 Location: Bakersfield, CA Posts: 308	Re: What differential rear end do I have??? Ugh!! Leaking!! I knew that diff cover was gonna be a problem. Should I just order a new chrome one or try to fix this one?? Or load up on the RTV?? I know I'm gonna have to pop the cover off again, drain the oil (do I try to recapture it or buy 2 more quarts?), get a new gasket etc etc. Attached Images

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