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HybridZ

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So I decided to remove my rear diff which is a 3.7 and install a 3.9 I had laying around. When I romoved the back cover on the one I removed from the car I noticed a little piece in the centre (first pic) that the 3.9 I planned on installing did not have(second pic). Does anyone know what it is? Some kind of locker?

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Not positive but I think that the piece is part of a a four pinion diff.  Your 3.9 is a two pinion.  They're both open diffs, but the 4 pinion is stronger, I believe, or at least, more durable.

 

Edit - #7 here shows a good comparison picture.  http://www.gmh-torana.com.au/forums/topic/66959-mystery-diff-conversion/

Edited by NewZed
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It might just be a different style of shaft, instead of a length of machined rod.  Could be that the center piece locates and retains the shaft ends in the center of the diff.  The sides of the piece also support the spider gear, and act as another bearing surface, probably reducing wear.  It looks like just a more robust two pinion diff.  Probably little benefit to swapping, unless the other diff is worn.  Spend some time measuring wear on the other diff.

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I showed the picture to mike at whitehead performance here in Ontario and he said it looks like some kind of cheap locking device but not completely sure and that he is used to seeing them open like the pic on the right (3.9). Still going to put the 3.9 in and yes for sure i will check for wear before swapping. Thanks for the suggestions guys. 

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