Tom, I'm told that the '62 to '64 Chevy II had a different case than the other '55 to '64 Chevy rear ends. I'm told that the big car case will bolt into the Chevy II housing. I'm told that the bearing sizes are different in the Chevy II and that the full size car differentials won't fit these cases. I'm told that to put a new Eaton Posi in a Chevy II that you will have to use the case from the full size car. What do you know about it?

Freddie,
Every bit of that is EXACTLY correct!
The 62-63 (not 64) ChevyII rearend center section------------AS AN ASSEMBLED DROP OUT UNIT----------is in fact interchangeable with the 55-64 pass car and 56-62 Vette. Pull the axles, pull the center section, bolt in a 55-64 center section, shove the axles back in, you're done.
And that's where it ends.
NOTHING whatsoever between the ChevyII center sections (not even the empty case) and pass car/Vette center sections is the same. The pinion and side carrier bearing bores are different sizes. The R&P gears are a different size, the differential unit is different, the U-joint yoke is even different. ONLY the bolt pattern of the case and the 17 axle splines are the same.
Thus, you CANNOT stick a posi (such as the new Eaton) into a ChevyII rear center case.
And, also, as I recall, the ChevyII axles have 4 lugs instead of 5 like the pass car axles.
Sooooooooooo, to beef up a ChevyII rear, swap in a 57-64 (not a 55-56) center section and have the axles redrilled for the normal 4 3/4in 5hole bolt pattern. Nope, pass car nor Vette axles will fit. :thumbsup:

I tried to cover it all in this link for the early Chevy crowd. The pictures are on the later pages.
http://www.chevytalk.org/fusionbb/showtopic.php?tid/223912/

Yea, the Chevy II axles are 4 lug. So why is a '57 to '64 better than a '55 or '56? I've seen a few P cases with 3.55 gears in them at swap meets for $500. Is that a pretty good price? I figured that if I bought one I'd never be able to sell it.

Freddie,
Back "in the day", you could buy complete posi center sections in good shape for $100-200, but today a complete, good posi center section is $700-1000, depending on year, condition and gear ratio!!! So, yes, $500 is a good price, but if you have to go completely through it with new bearings, clutches, crush sleeve and seal, you will then have an additional $200-250 in it. So, now you're up to $700-750 out of pocket. That means you have to sell it for $800-1000 to come out ahead.

Unlike the ChevyII centers, everything in the 55-56 centers is interchangeable up through the 64 centers---------------buuuuuuuuuut, EVERYTHING in the 55-56 centers, including the case itself, is thinner/weaker. Below are pictures of 55-6 parts compared to 57-64 parts.

Here is a good comparrison between a 55 case (someone has added a posi unit) and a 57-64. Notice the webbing around the area of the 57-64 carrier bearing is much beefier.
http://i65.photobucket.com/albums/h219/DZAUTO/Rearends/55-6Rearendoutlined.jpg

http://i65.photobucket.com/albums/h219/DZAUTO/Rearends/strongerrear.jpg

Here are comparrisons of the 55-56 and 57-64 ring gear carriers. It is obvious that the 57-64 is much heavier/stronger. If a person wanted to add a posi unit to a 55-56 rear (like the top picture), then 57-64 gears must be used (55-56 ring gears are thinner) for a 3-series, or a 4-series posi unit can be used if the 55-56 ring gear is to be retained.
http://i65.photobucket.com/albums/h219/DZAUTO/Rearends/100_0134.jpg

http://i65.photobucket.com/albums/h219/DZAUTO/Rearends/100_0135.jpg

http://i65.photobucket.com/albums/h219/DZAUTO/Rearends/100_0136.jpg

Here is a comparrison of a 57-64(left) ring gear and a 55-56 ring gear. BOTH are 3.70 gears and both are on a 4-series posi unit.
You can clearly see that the 55-6 ring gear(right) is thinner than the 57-64 ring gear.
http://i65.photobucket.com/albums/h219/DZAUTO/Rearends/100_1924.jpg

Hope this helps.

Wow, I didn't know that there were so many differences in the old rear ends. Thanks for the information!