How much Posi additive for fluid change?
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1,268 Posts
I was told to never go synthetic on these rear ends. It will always leak is what I was told. Something about the old type seals. I just built my rear end and put one bottle of additive and used 80w90 gear oil. Hope this helps.
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589 Posts
I never buy that stuff but use a heavier gear oil ... 140 I think? Haven't had any noise or issues in my 8.5 posis. Doesn't that additive just come in very small bottles? Or at least it used to. :dontknow: IF so, I'd think the amount to add would be the whole thing.
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705 Posts
You can always add more friction modifier if you need it, but you can't take it back out. Start out with 2-3 ounces, then go find a big piece of pavement. Do a series of figure 8s...add more modifier until the posi chattering stops.
And if you're going synthetic, you can't beat Amsoil. Amsoil is pricey, but it's worth it to protect your toys.
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2,253 Posts
I filled mine with Royal Purple 75w-90 with limited slip additive in it. Takes over 2 bottles, but takes out the guess work.
Tim
Tim
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2,807 Posts
1 bottle is meant for an average size diff. Yours is in that category. Just add the whole bottle. I use Coast Oil Co "Super Duty" gear lube. It's an 85-90 weight oil that's clear red and superior to the standard non-synthetic gear lube. I've never heard of using 140 weight gear oil on any diff as mentioned. :beer:
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2,102 Posts
if you have the old s spring type i think they are an eaton unit. very good and serviceable unit. being in the snow belt you might want to use a lighter fluid. my 2000 duelly didnt want to lock up solid this winter with the heavy fluid. probably 85/90 would be best for you, but do use the additive also. it keeps the clutches from chattering and breaking parts like spider gears and axles. it should be changed every year.you can run it longer or with the additive but it could cost you a lot more in repairs. a new posi unit is about 500$. you can do a great deal of maintainance for that
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589 Posts
I couldn't remember if it was 90w140 or 85w140. It's available at Advance or Autozone right along with the lighter weight gear oil.
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578 Posts
I don't intend to drive the car during the winter...salt destroys them.
All my projects are pretty low mileage,with very little rust for their age...I'd like to keep it that way.
I can't remember the last time I drove rear wheel drive in the snow.:yell:
All my projects are pretty low mileage,with very little rust for their age...I'd like to keep it that way.
I can't remember the last time I drove rear wheel drive in the snow.:yell:
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578 Posts
The rear end I'm working with is not broken.
I bought another project last winter,right before the first snowfall.
I didn't get to really inspect the car until spring.
The body work was done,car was solid and primed,and it was mostly complete.
When I inspected the car I noticed the rear spring geometry was off.
It took me a while to figure it out.
Once I ran the rear end code I knew part of the problem...he tried to graft in a 77 rear end to a 71 A body...easy fix...bolt it to the new Elky.:nanawrench:
Everything fit well,except the spring perches were welded in front of the axle tubes,rather than on top,centered by the axle.
I'm now looking for a correct posi for the LeMans.
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I bought another project last winter,right before the first snowfall.
I didn't get to really inspect the car until spring.
The body work was done,car was solid and primed,and it was mostly complete.
When I inspected the car I noticed the rear spring geometry was off.
It took me a while to figure it out.
Once I ran the rear end code I knew part of the problem...he tried to graft in a 77 rear end to a 71 A body...easy fix...bolt it to the new Elky.:nanawrench:
Everything fit well,except the spring perches were welded in front of the axle tubes,rather than on top,centered by the axle.
I'm now looking for a correct posi for the LeMans.
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2,807 Posts
Well that's what I've seen and used. I thought you were saying strait 140wt.
oke:
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439 Posts
GM has always spec'd 80W-90 but 75W-90 is perfectly fine to use.
PS - I have used synthetic oil in GM's posi dating back to my brand new 1977 Monte Carlo 350 V8. I experienced rear end chatter back then and the dealer added that additive on 2 different occasions and it lasted a few thousand miles at most. Thats when I sought out some AMSOIL synthetic 80W-90 dear lube and put it in while still under warranty and the dealer did it although they chastied me for using it. From that point on I never again had rear end chatter from the posi rear and I went on to drive that car for over 100,000 miles in 4 yrs never once changing the gear lube again. I sold the car to a co-workers friend (bought new cars every 4 yrs) and he went on to log over 200,000 miles on the car till the wrecked it. So any talk of synthetic gear oil being no good or causing leaks is myth. Bad seals cause leaks.
PS - I have used synthetic oil in GM's posi dating back to my brand new 1977 Monte Carlo 350 V8. I experienced rear end chatter back then and the dealer added that additive on 2 different occasions and it lasted a few thousand miles at most. Thats when I sought out some AMSOIL synthetic 80W-90 dear lube and put it in while still under warranty and the dealer did it although they chastied me for using it. From that point on I never again had rear end chatter from the posi rear and I went on to drive that car for over 100,000 miles in 4 yrs never once changing the gear lube again. I sold the car to a co-workers friend (bought new cars every 4 yrs) and he went on to log over 200,000 miles on the car till the wrecked it. So any talk of synthetic gear oil being no good or causing leaks is myth. Bad seals cause leaks.
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2,536 Posts
Actually that "myth" is partially true. As seals age they start to harden. If you switch to a synthetic oil, it tends to soften the seals. That can make them leak. Fear not. The seals will eventually expand back to their original size and stop leaking.
If you don't believe that, just ask Amsoil, or read about it on their web site.
Jack
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2,102 Posts
if the seal is hard enough to leak replace it .i dont want the mess under my car or on my driveway.for big trucks the synthetic is the way to go.when we bought new units from eaton they sent a disclaimer with each unit stating the warranty was 250,000 miles with conventional oil and 750,000 miles with synthetic on the same part. the reality of it is that the diff seals are a high wear seal and require service and not many people look under a car enough to tell exactly when a leak first occurs.the pinion seal on the average turns 3 to 4 times the axle seals.
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