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.

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.

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.

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??

Re: What differential rear end do I have???
 

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

Re: What differential rear end do I have???
 

Awesome thanks for the details!!

Re: What differential rear end do I have???
 

turn

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.

Re: What differential rear end do I have???
 

Op, a factory Lsd will look like this.

Re: What differential rear end do I have???
 

Turn

Re: What differential rear end do I have???
 

Thanks guys. I'm a total newb at all of this. All of these gears look like drilling equipment to me. Haha.

So, I have a 12 bolt and when I get the cover off, where specifically are the numbers I'm looking for??

Just want to be sure I get all the info I need while in there.

Also, I picked up gear oil (80-90w) and the fel-pro gasket last night. What' proper procedure for putting the new gasket back on?? Do I need sealant as well? If so what type?

Just don't want to make things worse by getting in there and screwing something up.

I plan to take a bunch of pics but don't want to go back in if I don't have to. ;)

Re: What differential rear end do I have???
 

Numbers in the red block. I don't believe felpro gaskets require sealant.

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

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.

https://www.permatex.com/wp-content/...6/02/81182.jpg

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?

Re: What differential rear end do I have???
 

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

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

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.

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.

Re: What differential rear end do I have???
 

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).

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.

Re: What differential rear end do I have???
 

found it thanks. Page 646. ;)

So I have a Dana 60??

Re: What differential rear end do I have???
 

12 bolt gm

Re: What differential rear end do I have???
 

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" ;)

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.

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.

Re: What differential rear end do I have???
 

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.

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.

Re: What differential rear end do I have???
 

If you get a new chrome one, be sure to lightly scuff the mounting surface to give it some grip. The longer number preceding the other two numbers is the part number (begins with 3). A weekend driver with light hauling and trips to the dump is fine. I have 3.07 in my 68 and that is exactly what I use it for.

Re: What differential rear end do I have???
 

I have this coming tomorrow:

https://www.oreillyauto.com/detail/b...r/9894/4514825

Do I use the gasket that comes with it, get a fel-pro, or get green RTV?? With the metal brake line in the way, I could see how a line of RTV would be a pain trying to get the cover on under the brake line.

As far as scuffing the mounting surface, will a red scotchbrite pad work or should it be more like 80grit?

Torque is 20lbs, got that.

I'm gonna be an additional $49 for the cover kit, an additional $16 for 2 more quarts of gear oil and possibly another gasket and more permatex (I used ultra black I had lightly on the felpro).

I just don't want to do this again. :/

Re: What differential rear end do I have???
 

I would use the gasket that comes with it (get a Felpro if it gives you peace of mind), and coat all surfaces with the stuff I posted in post #14. It's the best I have found to seal against gear oil leakage.

Re: What differential rear end do I have???
 

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

Re: What differential rear end do I have???
 

pulled my blazer out ....gear oil all over the floor....looks like I think gasket blew or i hit rear end when car was lowered.

one time when it was at a shop getting air ride installed...67-72 member said rear end didn't look original, might have been swapped out at 1 time....how can I confirm what rear end I have? is there markings on the outside to tell?