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so I got my nova a little while ago and im looking at the rear axle in this terrible cold snow and i cant find the I.D code the car is a 1973 was the V6 but now has a 1965 283 in it so im not sure if the rear was changed since the engine was i cant find a stamped code in the passenger side but on the diff plate on the front i did find this. it said on the driver side N 1285542 and on the passenger side front GM1 it has a 2-speed powerglide in it if that helps at all please help me find this out because i want to get a turbo 350 trans and 3.73 gears but i want to know if its possi already
 

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Go do a burn out and and that will tell you if it's a posi or not. Or jack up the rear and put the car in neutral and spin a back tire. If they both rotate in the same direction it should be posi, if they go opposite it's an open diff. You might be better off pulling the rear cover and checking it out if you want new gears as well. It could be an 8.2" or 8.5" carrier.
 

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using the link above you'll be able to tell which rear you by comparing the pics, posi or not can be found with info above and while you have it safely in the air check the ratio.
Differential Gear Ratio determines the number of times the drive shaft (or pinion) will rotate for each turn of the wheels (or ring gear). So if you have a 3.73:1 gear ratio the drive shaft turns 3.73 times for every turn of the wheel.
 

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72 Frame off, bare metal resto-mod. 383, TH350, Eaton Posi, Complete new suspension, disk brakes
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Old cars

They are a challenge. You should be able to find out what kind of housing you have, 8.2 or 8.5, from the casting numbers and the link above. However, I agree that the only way you are really going to find out what gears you have will be to pull the rear cover. You probably have 2.9's and it was never changed. Be sure to wear some clothes you can throw away 'cause that gear oil stinks and I have never been able to get the smell out after getting on my clothes.
 
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