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Morning,

I recently gutted the 12 bolt from my Chevelle and sent it off for blasting and powder coating. Before doing so, I noted that it was stamped with CX, which from my understanding translates to the original gear ration as being 3.07. While the rear end is away I was shopping around for the best price on new R&P, a posi carrier, axles, install kit, etc.

From the CX stamp, this would make it a 3-series, yes? And I should be able to use a 3.42 R&P and I would think that the Eaton TruTrac Carrier would work. However, if on Summit or Jegs, I click their "Check Fitment", it always comes back as this won't fit. Please help me understand what I am missing.

Mfg. #: 913A555
Application: GM 12 Bolt (Car)
Ring Gear Diameter: 8.875"
Carrier: 3- Series (3.07:1 to 3.73:1)
Side Gear Spline: 30 Teeth

Thanks,

D
 

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Good morning, My 12 bolt was also originally a 3.07 R&P gear set. When I had a shop build mine way back in the day, they said I could go as low as 3.90 with that carrier, so that's what I had them put in there. It was a great match with my engine build and torque converter and a helluva lot of fun! Eventually many years later I went back to a new 3.07 set because of the high rpm's on the interstate. I currently have a 3.55 R&P set in there and am happy with it for the most part. I don't know too much about the Eaton TruTrac carrier, but I would call the folks at Eaton to find out.

Steve
 

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All of the housings are the same. You can put any gear ratio in any housing. The differential is where the difference comes in. There are 2,3 and 4 series differentials for the Chevy car 12 bolt. As long as you get the correct differential for the gear ratio that you are going to use it will work fine.

Check with Tom's Differentials for Eaton parts, US Gear and the best installation kits that you can get. Tom's was the largest Eaton distributor in the country back when I was buying from him. Tom's daughter is running the place now and would appreciate the business.
 
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