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I plan to check the differential fluid level in my 67 and am trying to determine what to use in my differential if low. I also considered changing the fluid. I was reading a previous thread ‘Replacing rear differential cover gasket’ in which GTR1999 stated to use 2 bottles of GM additive in addition to the oil. What is this additive name or how/where do I find it?
I have had the car less than a year so I don’t know what I may find.
Sam - If you're not planning on dropping the differential out of the car, you won't have to deal with the rear cover gasket.
I just replaced the diff oil in my 66 Roadster and used two bottles of the AC-Delco Lube. The old lube from AC has been reformulated and still works well but
most of us have gone to using 2 bottles instead of just one.
Since the Midyear diffs did not have a drain plug, you'll have to siphon out the old fluid. Some guys use a pump/suction device but I rigged up a simple siphon out of '
copper tubing and plastic hose. Slow process meaning I get it into place with a drain pan underneath, start the siphon and come back in the morning when it, or most of it, has drained out.
Mike T - Prescott AZ
Google GM Limited slip additive. If you wish to change the fluid, remove the plug at the top of the differential and use a suction type gun to suck the old fluid out. Put a quart of fluid in, followed by two bottles of the additive and use the other bottle of oil to top off the differential. Fill until the oil just dribbles out. Put the plug in and you are done. Now would be a good time to go to an empty parking lot and do some tight figure eights in forward and reverse. That should be it. If the rear end cover is not leaking, I would not change the gasket as the removal of the differential is a colossal pain in the butt. Jerry
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One more vote for suction.
I was lucky enough to borrow a homemade device made from an old Nissan oil pump.
It had a chuck welded to the shaft that you just hooked up to an electric drill.
I had all of the oil out in a couple of minutes plus it sounded like a straw at the bottom of a chocolate shake at the end..
Here is what has to come out if you need to change the gasket.
Start here. It is easier to remove the spare tire carrier. You can leave the half shafts installed, but the only way to remove the cover is to drop the crossmember as the four bolts connecting into the cover come in from the top and it is a lot easier to maneuver with the half shafts removed. As stated above, it is a pain in the butt to change the gasket!
You can use the GM additive, it is pictures. Available from any dealer or Summit. Use 2 bottles of it since it has changed over the years. One may work, but two will not harm anything. I use Lucas gear oil, the best oil I have seen. I say that because I opened a diff I built 10+ years earlier, it was never installed or used. It was filled with Lucas 85-140 and turned once in a while and until it was just left in the customer garage and the project car sold. I bought it back, opened it up and the Lucas gear oil was still on the bare metal parts after sitting for years, there was no surface flashing, it looked like the day I finished it 10 years earlier. I did drain it out and put in fresh oil and the diff was installed and is in use today, no issues.
You can pay more for fancy brand name super HD synthetic oil if you like, it won't work any better. If the diff is otherwise stock built or original, you may have other issues to deal with but that's another subject for another day.
You can use 90wt but I have the 85-140 in 100's of diff's I built, no issues.
Here is what has to come out if you need to change the gasket.
Start here. It is easier to remove the spare tire carrier. You can leave the half shafts installed, but the only way to remove the cover is to drop the crossmember as the four bolts connecting into the cover come in from the top and it is a lot easier to maneuver with the half shafts removed. As stated above, it is a pain in the butt to change the gasket!
Beautiful garage and setup. Car likewise. The diff is a narrow and stone stock, if you don't hammer it that will be ok but if you do.
You can buy the additive from NAPA, NCB 4285V, or any other parts store can cross this number to whatever "brand" they sell. All clutch type LSDs use this same additive.
For oil any SAE 80W-90 GL-5, Brand makes no difference. Just look for the API donut and the underlined spec. It's a generic product.
My intent is not to remove the differential cover, especially after seeing Factoids pictures! I would like to first open the plug and check level. If low, I may then remove fluid by some method mention here or in other threads and refill.
If I read the service manual correctly it states, if cold, the fluid level should be a 1/2” below the fill plug. This was from the 1967 Chassis Service Manual in Lubrication section page 0-15. I found this interesting as I would have otherwise added fluid until it came out of the hole. It does recommend filling to the hole if the unit is at operating temperature.
Differential additive
