77 Rear Diff fluid change

Jr. Corvette

Jr. Corvette

Regular
2 You're 10
Jan 1, 2014
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578
Calgary AB
VetteCoins
7,165
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77 Corvette L82
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AB
Hey Guys!

I've been trying to find some information on this subject but haven't found anything that really answers my questions.

1) I've got Royal Purple fluid that I plan to add. It's synthetic and has the limited slip additive already. I picked up a GM positraction additive as well a while back. Do I need to put in the additive if the fluid already has it in it?

2) How full do you fill it? Do you fill it from the drain plug?

3) Is a cheap pump the best way to remove the old fluid? I've heard of guys using some sort of gun to remove the liquid. Does anyone know what this is?

4) What are the torque specs for the plug? Do I need to put something on the treads of the plug during reinstall to avoid leaks?

Thanks!!
 
1) Not sure about the additive. I would assume you don't need to add more if it is already in the oil.

2) There is no drain plug. The plug on the passenger lower side is the fill plug. Square head plugs take a 5/8" wrench, 5/8" 8pt socket, or 3/4" 12pt socket. If it is a female square (Later years I think) then a 3/8" drive ratchet/extension. Fill until the fluid is level with the bottom of the plug (starts coming out). I think this is about 2 qts. If putting additive in put it in first so you can fit it all.

3) I have heard of guys drilling and tapping a small hole in the bottom of the case for draining purposes. I would do this only if you are able to flush out the fillings. Have heard of guys using a turkey baster with a tube attached too. I am sure a small transfer pump would do the trick too. And you could removed the cover but that is a lot of work. I would only remove the cover if you wanted to inspect inside or knew something was wrong anyway

4) I believe the proper torque is 20 ft-lb. Someone correct me if i am wrong. I am sure most people just put it in "snug". It is a pipe thread and could have sealing compound or tape added but should not be required.

T
 
i use a small hand operated pump for draining and re filling, they are sold for filling outboard motor lower units. marinas and canadian tire sell them, i think they are about 12 bucks. use some thread sealer on the pipe plug and there may be a small gasket too. i don't use the gasket and have not had any leaks. like taylor i don't know about the synthetic and the additive.
 
Like Doug I used a marine lower unit pump and they do work well. Just make sure that the tube cannot come off and fall into the rear end. Diff fluid is heavy and it stinks. I found that getting the new oil hot by putting it into a pan of hot water made the pumping a whole lot quicker.

Good luck just make sure the car is well supported.
 
Great! Thanks for help guys. Some great answers! I won't put in the additive then. I would like to remove the cover but that'll have to wait for next year as I got too wrapped up in other projects (Stupid brake calipers keep leaking:/) and it's almost driving season. I'll probably drop the whole diff though a rebuild it when that time comes (along with the rest of the rear end).
 
Last thing. What are the torque specs for when I reinstall my leaf spring?

1) Center bolt (holds leafs together)
2) mounting bolts (4 blots that secure leaf spring to diff cover)
3) Leaf spring ends

For the leaf spring ends you just snug them while it's on jacks and torque them once the spring has the weight on them to prevent diff cover cracking, correct?
 
....Here's some pics of a simple suction gun that can pull out the gear oil and fill new stuff back in. May need a longer hose on it to get to the bottom of the diff but is should work. Can even make one up from a old grease gun as that is what it basically is. Pull the rod out and it sucks, push it back in and it pushes the oil out. The idiot who designed the corvette diff never put a drain plug in it.. I'm a new vette owner and when I seen that in my 82 I couldn't believe it, and no simple cover to take off either . As far as the oil, if it has the additive in it then you shouldn't have to add any more. If after driving it you hearing the clutches chattering making a turn then maybe need to add some but I think the royal purple will be fine.





 
spring mount to cover - 70 lb
cover bolts - 50 lb
crossmember to diff - 60 lb
crossmember to chassis - 55-65 lb
plug - 20 lb

the spring ends don't torque because they are castle nuts with cotter pins, basically line up the holes and put in the cotter pin. why do you have to separate the leaves ? i wouldn't recommend it. do NOT torque the 4 bolts mounting the spring to the cover until the weight of the car is back on the ground - you run the risk of cracking the mounting ears. do NOT torque the crossmember to frame bolts until the crossmember has fully seated - use your jack to press them on. if they are torqued to try and draw them up to seat - you will pull the threads right out of the sombreros.
 
That looks like a good tool! I pulled the leafs apart to restore the leaf. As I'm 17 I can't afford a new spring so I took it to a company my dad does work for (they let me use there sandblaster) and I either powder coat or rust proof the cleaned parts. I did that to the spring and ordered some new leaf liners.

I had to cut the end link bolts for the leaf spring and got some new ones without the holes (I have to drill the holes). How far up the treads are the holes?

Also my Dad made a good point to me. My diff was pretty gunked up so he thought I might have a leak and have to remove the diff cover. He said even if it wasn't leaking I might want to pull the cover, clean, inspect, and new put a new gasket on it so as to prevent leaks from the synthetic.

Once I check the fluid level, if I determine that it lost some fluid and ultimately has a leak. How hard is it to pull the cover and crossmember? What are some recommended steps to accomplish this? How long does it typically take (assuming that my car doesn't hate me)?

I'll post a pic of the spring in resto.
 
Some pictures of the spring before, during and after.
 

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Forget the castle nuts and go with nylocks. Since you have to replace the end bolts anyways, pick up grade 8 9/16" 8" bolts and that will give you a little room to adjust your rear ride height when she goes back together. As stated DON'T torque the spring bolts down until she's sitting on the ground. Once that's done, roll her around a bit to seat the suspension, then make adjustments on your nylocks to set the ride height. Make a slight adjustment, roll the car, make an adjustment, roll the car, etc until your ride height is where you want it.
 
What a great idea!!! I never thought of that. I already had picked up the bolt and castle nut but never drilled the hole for the cotter pin. That'll be good because then I can have an adjustable rear suspension! Thanks!
 
while you are in there check the end play on your yolks for the Drive & 1/2 shafts, no better time. Depending on how many miles (Km) are on the car you may want to replace the bearings and seals also a good time to change out U-joints
 
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I had mine apart and I just gave it the old heave ho to tighten it. If you are concerned about getting the proper torque there are many charts on the internet to show torque on a bolt with a specific diameter. Measure the diameter of the bolt and look it up.

Hope this helps

T
 
There is no specific torque spec in the AIM (Assembly Instruction Manual), as the spring was shipped to St. Louis as a complete assembly. The spring to carrier spec is listed as 33 ft-lbs for my '79 spring.

I then checked my '79 Service Manual to see what the mechanics at the dealership were told to do. Interestingly enough, once the spring was rebuilt with new liners and cushions; technicians were told to "Insert drift into center bolt holes in leaves to align
spring leaves, install center bolt and tighten securely." Go figure. I guess as long as the spring bolt is secure enough to keep the individual leaves aligned until it's mounted in the hole in the carrier; then the four carrier bolts and the centre clamp plate hold everything in place nice and tight.

As an aside, if you plan on keeping your car for any length of time and doing a lot of your own work; get an AIM and Service Manual for your model year. very valuable tool to have handy.
 
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