How can I tell the difference between a 2 series and a 3 series 10 bolt 8.5 diff. when looking at a whole pile of them? Just the spools, I think thats what you would call them?

The 2 series sits 1/4" closer to the bearing on the side where the ring gear bolts on. If you were to line up the bearings of the two the one that has the ring gear land sitting furthest to the center is the 3 series. If all else fails you can use part numbers.

I would look up the part number or decode the gear ratio. a 2.73 and numrical higher will be a 3 series carrier. Watch for a spacer of sometype they have those to swap gears from one series to another. like a 3.73 on a 2 series

http://community.webshots.com/photo/55410292/55410552ePqBsq
If you set the differential on a flat surface, like this one, you can measure from the flat surface to the surface of the ring gear flange to tell the difference. The 3 series will measure about 1.730 inches. I haven't measured a 2 series. If you are looking for a posi the 2 series has the casting number 410409 and the 3 series is number 410408.

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'69 RS/SS396 Pro Street
427 4 speed 9"
Byars Performance LLC
High Performance Drive Train Parts And Service
www.lubedealer.com/biggearhead (http://www.lubedealer.com/biggearhead)

So if I have a 2 series I could put 3.73 gears in it if I use a spacer? How strong would that be?

<font face="Arial,Verdana" size="2">Originally posted by sandboxbob:
So if I have a 2 series I could put 3.73 gears in it if I use a spacer? How strong would that be?</font>

yes but I have no idea if it sacrafices strenght.

It won't be as strong. There are other problems with using a spacer, such as ring gear alignment.

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'69 RS/SS396 Pro Street
427 4 speed 9"
Byars Performance LLC
High Performance Drive Train Parts And Service
www.lubedealer.com/biggearhead (http://www.lubedealer.com/biggearhead)