1973 Rear Differential oil

[BigBird28]

So my rear diff is leaking. I believe it's the classic stock Ford 9inch but am not certain. I bought the pinion and main assembly gaskets to replace but until I get around to that I want to keep the oil topped off as best I can. What rear differential gear oil do you guys use? I'm looking at 80w90 MasterPro GL5 from Oreilley's but any input would be appreciated.

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[Toothaker]

You can use any non-synthetic 80w90 lube in your differential. I happen to use Lucas 80w90, but especially since yours is leaking I doubt the brand makes much difference.

Yes, the 1973 Tbird has the 9" differential.

 

[BigBird28]

You can use any non-synthetic 80w90 lube in your differential. I happen to use Lucas 80w90, but especially since yours is leaking I doubt the brand makes much difference.

Yes, the 1973 Tbird has the 9" differential.

Beautiful, I'll pick some up then to keep it topped off. I've heard it can be a real hassle to remove the rear diff (involving pulling the axles) so my plan is just to jack it up and replace the seals on jack stands. Do you know how to best check the diff level and how much oil the 9 inch holds? I know some diffs you can see the fluid level via the fill hole but any input would be appreciated.

 

[Toothaker]

You fill the oil up to the fill port. It's about 65% to 70% up the side of the housing, so you just put oil in until no more will go in.

I don't recommend changing the front seal while the center section is still in the housing. All the instructions for setting the preload assume you are working on the pumpkin/chunk/center section while it is out of the housing. Yes, pulling the axles are a pain, but it's just a matter of pulling the wheels off and then removing four nuts on the retaining plate and yanking out the axles. For what it's worth, I just changed the front seals, along with the axle seals and the wheel bearings on my 1970. I'm not an experienced or professional mechanic, just a weekend warrior, and it's not as big a hassle as it seems.

You will need a bar torque wrench that measures inch pounds. A click torque wrench won't work. I bought one, and it is a 1/4" drive. But the socket was an 1 1/8", which I have in 1/2" drive. So I had to buy a 1/4" to 1/2" drive adapter to connect the tiny torque wrench to the huge socket. You tighten, then check the preload, then tighten some more until the preload is right. All this turns the gears, and having the axles in place will throw everything off and make it impossible to get right. That was the only specialty tools I needed, since I didn't replace bearings.

 

[BigBird28]

Front

You fill the oil up to the fill port. It's about 65% to 70% up the side of the housing, so you just put oil in until no more will go in.

I don't recommend changing the front seal while the center section is still in the housing. All the instructions for setting the preload assume you are working on the pumpkin/chunk/center section while it is out of the housing. Yes, pulling the axles are a pain, but it's just a matter of pulling the wheels off and then removing four nuts on the retaining plate and yanking out the axles. For what it's worth, I just changed the front seals, along with the axle seals and the wheel bearings on my 1970. I'm not an experienced or professional mechanic, just a weekend warrior, and it's not as big a hassle as it seems.

You will need a bar torque wrench that measures inch pounds. A click torque wrench won't work. I bought one, and it is a 1/4" drive. But the socket was an 1 1/8", which I have in 1/2" drive. So I had to buy a 1/4" to 1/2" drive adapter to connect the tiny torque wrench to the huge socket. You tighten, then check the preload, then tighten some more until the preload is right. All this turns the gears, and having the axles in place will throw everything off and make it impossible to get right. That was the only specialty tools I needed, since I didn't replace bearings.

Thanks for that bit on the fill port. I have a hand pump arriving today so I'll give that a go later this week. I too am a "weekend" (and sometimes lunch break) warrior mechanic.

Front seal being the pinion where the driveshaft connects with the differential? What about the larger rear seaI of the differential? Could that be replaced and fluid replaced without taking the diff out?

Also what's your preferred jack point for the rear end? Only got this guy last May and have been primarily focused on electrical the past few months. Now it's at a point where things seem good enough so I'm starting to address the leaks. Rear diff first, then planning to swap the various pulleys, belts, and pumps up front. Eventually a full front end rebuild is in order, dual exhaust, proper tuning and mods to get this thing over 300HP.... big plans but starting small for now.

I need to save up before I shell out for a another specialty tool. And preferably have a garage I can work in rather than the street in front of my house haha.

 

[Toothaker]

Yes, I was talking about the front seal next to the pinion. If you have a leak at the differential gasket, that won't require any specialized tools. It will require the removal of the axles. The axles lock the center section in place - or at least hold it closely enough that you won't be able to clean and replace the housing seal. People that know more about this than me say to not use a replacement gasket. Use Permatex Ultra Black RTV instead.

When I lift the rear end, I lift from the middle of the differential, and then I place jack stands on each side.

You could replace "1973" with "1970" and be telling my story. I'm fortunate to have a garage, though.

 

[OneAmongOthers]

Don't use that RVT slop. Get the proper gasket and a little RVT is fine for holding on the gasket while you close it up. You will also need new copper crush washers too. Pulling the axle is a piece of cake. Easiest one I have ever taken out. Just jack up the car to take the weight off the axle and pull out the rear springs. Does the 73 Tbird have coils or leaf? Get a second jack to support the axle. Pull the wheels and there is just four bolts that hold on the axle shaft brackets. After that remove the rubber brake hose and breather tube from where its connected on the frame. Get a new brake hose while you are at it. I would recommend against replacing the pinion seal unless its leaking. Also check the outer bearings. On mine the axle shaft bearings where in front of the axle tube seals. Bearings where sealed. I have them inspected and as long as the bearings don't have any play I wouldn't touch them as they are a pain in the ass to remove.

9" rears are one of the strongest rears around. Extremely reliable. Once you get the "chunk" off the axle housing just check the ring/pinion gear play. As long as it's in spec I wouldn't touch it. I believe these axles are limited slip. Count the teeth on your splines Some came 29 teeth and some 32. Also Ford did a weird thing with the Tbird and had a 9 1/2" style ring gear on them instead of the 9" one. A 9" ring gear will fit into a 9 1/2" axle housing, but a 9 1/2" ring gear will not fit into a 9" housing. Also look at the numbers on the case and look at the ribs. Some cases where nodular iron and some where not. Hopefully you axle tag is still on it. Lastly when you are disconnecting your drive shaft check the U-joints. Not sure if its the same on a 73, but on a 69 the driveshaft was a double cardian joint.

 

[BigBird28]

Ah thank you guys for the incredible insight and pictures! Right now I'm considering just a full rebuild of the rear end but I'll save that for another thread. While we are on the topic of the rear diff, I'm with you guys now. When I get to this, 100% I'll pull the axles. When I have the diff opened up, what should I look for in terms of wear? I'd prefer not to rebuild it but if it must be done, then it shall be done.

 

[OneAmongOthers]

Ah thank you guys for the incredible insight and pictures! Right now I'm considering just a full rebuild of the rear end but I'll save that for another thread. While we are on the topic of the rear diff, I'm with you guys now. When I get to this, 100% I'll pull the axles. When I have the diff opened up, what should I look for in terms of wear? I'd prefer not to rebuild it but if it must be done, then it shall be done.

As said before the Ford 9" is a very strong and reliable rear end. Basically check for ring gear/ pinion play. If it is still with in spec I wouldn't touch it. Now as for the LSD (if applicable) your clutch pack may be a little or completely worn out at this point. You can usually check if it does have one by jacking up the car and holding one of the wheels still while turning the other. Or just get the info off the axle tag if it still exists. Wear is pretty obvious. Metal flakes, shavings or chunks. Burnt smelling oil or silvery color. Rebuilding a Ford 9" is one of the easiest ones to do since all of the axle comes out in the chunk or front section. But be careful! Its VERY heavy!

 

[BigBird28]

As said before the Ford 9" is a very strong and reliable rear end. Basically check for ring gear/ pinion play. If it is still with in spec I wouldn't touch it. Now as for the LSD (if applicable) your clutch pack may be a little or completely worn out at this point. You can usually check if it does have one by jacking up the car and holding one of the wheels still while turning the other. Or just get the info off the axle tag if it still exists. Wear is pretty obvious. Metal flakes, shavings or chunks. Burnt smelling oil or silvery color. Rebuilding a Ford 9" is one of the easiest ones to do since all of the axle comes out in the chunk or front section. But be careful! Its VERY heavy!

Awesome, thank you for that guidance! As far as the axles, should I still look for the obvious signs of wear?
All things considered it's a great unit given it was stored on a street for 30 years. I'm currently planning to "de-smog" it (which I'll save for another thread) but I'm trying to address the front and rear ends and any and all leaks. I figured the rear diff was a good place to start. I've done a ton of electrical work on it and it's finally in a place where I can get into some of the more "down and dirty" work.
I also see I didn't answer your question, coils in front and rear. Not a leaf spring.