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Lat month I got a killer deal on 12 68-72 12 bolt posi rear ends. One was a 4.88 geared on and the other was a stock 3.31 all GM rear end. I decided I would use the 4.88 one (after a gear change) in my Camaro with some 3.08's that Glenn gave me that came on an old GM posi unit. I ended up taking out the 4.88's and noticed it was setup kind of strange, a lot more shimming on one side of the carrier then the other side. So I pulled out everything cleaned up the 3.08's and did a test fit, WOW! the shimming was way worse, WTF? and it looked like from what people had told me (even though most were wrong) that I had a 12 bolt truck posi. How can that be? well I guess as someone said it has two large opening and that's what truck 12 bolts have. I went on Amazon and ordered a new posi and wait for it to come. Got it and the new one look super good, so I put the ring gear on this and BAM it's still phuked up, again WTF. I posted this problem on several sites and several people quoted that a car 12 bolt 3 series posi has 1.82 spacing from the ring gear mount flange to the outside of that part of the carrier. I also was told that 12 bolt 3 series truck posi's are 1.70 so great that my problem, all of mine are truck posi's, or so I though. On one site I found a big old error on this and reall got into it with some goon who said he and the article was right and I am dead wrong. Well as luck would have it I had that 2nd 12 bolt, and yesterday I took the rear cover off to take a peak. Well what I found was eye opening, after pulling the cap off the measurement was the same as all my other posi units, you guessed it 1.70 no with this info I went back and told that bum he was really wrong and there is a different problem. So now I am going to do some more testing when it warms up and the 2nd 12 bolt really helps here as I will measure from the pinion heel to the rear cover case opening. Take that number and check it against the other one to see the difference. I suspect the rear pinion bearing race is the problem here as it has been all along. I have setup many a rear axle in my day and this is the 1st one that has presented so many problems. Good news Joel, I also have for my Son's car an 8.5 ten bolt out of a GM that's going in his Biscayne if the 12 bolt does not work out. Some are telling me it worth some are telling me it's worth some bucks now.
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I still have a 12 bolt posi somewhere that I ran for a few years in the Chevelle. It had a ring gear spacer, so I thought I'd get a posi where I didn't have to run the spacer. I was told it was strong enough since I was running an automatic but I didn't trust it. So...I borrowed the posi from my brother's Showdown Camaro. It was built by G&G Specialities. I remember it had springs that looked like valve springs. A guy convinced me to run Amzoil diff lube because he saw me at BIR with the car on a jack in the pits heating up the 90 weight. He assured me I would no longer need to do that if I used Amzoil. Well, after running Amzoil, I tried pulling it out to give it back to my brother but couldn't get the pin out. The spider gears had practically welded to the pin and hollowed out the hole where it went through the casting. I had to bang the pin and then grind it off so I could turn the posi, then bang it back the other way to get it out. In other words, it destroyed the posi that I had borrowed. I have no proof the Amzoil caused the prob, but I had no issues running the conventional lube before. -
My 12 bolt is all apart again and the guts are out of it. Doing measurements now and I found that the difference between the screwed up 12 bolt pinion depth and the 3.31 12 bolt is like .100 and that's a lot. Not sure what screwed up here, but something is a skew.Comment
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Fixt it, turns out all I needed to use was the 3.08's and the Posi unit Glenn gave me. Damn, I sure wasted a lot of time and money for nothing on this whole deal.Comment
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She's done and the 3.08 12 bolt is sitting on the spring perches in the Camaro. Dang this was a lot of work, and I over analyzed this. Shoulda just put the 3.08's on the stock posi in the 1st place. Today it was 55° so I tore the GM posi apart and cleaned it all out and reset up the whole assy. Turned out great with a .008-9 backlash, good enough for a street car. This will be great going to car shows later this year. Will take the Camaro to Car Craft Summer Cruise if JJ has not sold Brad's old Nova.Comment
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It's done, woo hoo.Comment
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