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Father in law has restored a 72 Pickup. It has a chevy rearend in it with a 12 bolt pattern. What type of rearend is this? He is looking for a 3.73 or 4.11 posi gear to put in it. It's a 7.5, or just called 12 bolt, etc. What spline is usually stock? I'm clueless on chevy rearends...Thanks in advance!
If the rear has a bolt at each bottom "corner" of the cover and all the bolts are symmetrically spaced around the housing, that sounds like the classic (and coveted) Chevy 12 bolt rear. I believe they have a 30-spline count and the ring gear should be 8.875 inches.
I think there was another GM rear that had 12 bolts in the diff cover, but that isn't the one people refer to where they say "It's a 12-bolt rear".
You can look in a Jegs catalog at the different bolt patterns to make sure which one you have and what gears are available.