Hi I own a 1984 montess and its been a father son project for goin on 3,4 years. Now and is close to being finished up. Were in the process of replacing the original non posi rearend with a 1985 grandnational that I found on craigslist for a great deal. Can anyone help with the facts and knowledge about it. Would it be a good fit. Will I have to shorten my drveline driveshaft. And is there anyway to tell that its really a posi grand national rearend before I purchase it.
85 gn rearend? hellllp
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It should have the 8.5 diameter ring gear if it is a true GN diff. The Hurst/Olds of 84 and the 442's of 85 and up have the 8.5" but the Olds will have 3.73's. The 7.5 will have "fangs" at the bottom of the case and the 8.5 will have to "squares" at the bottom. You should be able to hold one axle still and try and turn the pinion flange and not be too successfull. If it has a GovenorLok diff, you will be able to turn the axles in different directions but if you spin one axle real fast, the clutches will "ramp up" and it will become a "posi" at that point. You may just have to pull the cover to make sure you are getting what you pay for. 
It could still be a Gov/Lok posi. It will spin the opposite direction until you spin one real fast and it will fling out one weight and lock making it a "posi". Some say they are not strong at all but a Dave Henninger in LA (I think) runs one in his 8sec 74ish Camaro. I have never run one but I can see how they would fracture as they lock up and they are unaffective above 25mph if I remember correctly.
turbonut85
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Yes. Believe it or not, the only differences between the 85's and the 86-7's is the motor, from the bellhousing forward. Hard to believe that GM would've put ANY good performance parts in the hot air cars, huh?
Hard to believe that GM would've put ANY good performance parts in the hot air cars, huh?
Well, if they used the 8.5 rear, they at least used ONE decent part.
gn
Hey GNeric this guy agreed to pull the cover off and measure it.To be from a GN it must be a 8.5 rigth. how do I tell him how to mesure and what to look for. And if it is a gov lock posi should I just wait and find a posi or purchase this and what would u says a good price. I'm not familiar with rear differential s and prices
Hey GNeric this guy agreed to pull the cover off and measure it.To be from a GN it must be a 8.5 rigth. how do I tell him how to mesure and what to look for. And if it is a gov lock posi should I just wait and find a posi or purchase this and what would u says a good price. I'm not familiar with rear differential s and prices
If it is the 8.5, it will have the "square" lugs at the bottom of the case with where the cover mounts as apposed to the 7.5 having "fangs". You can purchase as is if it is the Gov/Lok and just put that on Ebay and get a good unit at a later date. The G/L units will usually go for about $100 so you can put that towards your purchase of a "real" posi.
