Thread: how to tell if my 12 bolt is a posi

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I have a gm 12 bolt that i purchased from a guy who didn't know if it was a posi or not i have it sitting on stands in my shop with the cover off, what do I look for? when i turn the input shaft both axles turn, but they also both slip when I grab them while turning the input shaft,
thanks for the help
Rob

there should be numbers on the axle and the cover. you could run them and it should tell you what you have if everything is stock.

Turn one axle, if the other one turns the same way its a posi, if it goes the other way its not.(Or it was but its shot!)
With the cover off, the carrier is a bigger, bulkier piece, held together with a ring of bolts.

I think I figured it out, when I hold just one axle it slips which I assume means it's a limited slip, and when both are held they both turn, so i guess its a limited slip posi. I also counted the rotations of the axle vs the input shaft and the input shaft turns about 3.75 turns for one turn of the axle, so I'm guessing the gear ratio is 3.73, sound about right?

That pic looks similiar to the open ,...other than the springs and tie bars ,...what else is hidden under those pieces...?
What would be involded in converting a open to posi ,could the existing spiders gears and pin be used or is there something else hidden under that mess ?

All the GM Posi rear end that I have had when you lift up the car on the passenger side . and let the drivers side on the ground . the R/R passengers tire would not turn or be hard to turn . I did see in a shop manual they test posi clutchs this way for slip with a torque wrench I think it was 120 ft lbs .

put it in your car and burn rubber .. one blackmark is possitively no posi -- two blackmarks is possitivley posi ..