I was browsing through the BBC info thread earlier tonight and thought why not make one for differentials. I took all of these pictures from Ebay and Superchevy.com, so I'm just giving credit where credit is due. I took none of these pictures.

Chevrolet made a 10 bolt and a 12 bolt. The 10 and 12 bolt name refers to the number of bolts that hold the ring gear to the carrier. This just so happens to coincide with the number of bolts that hold the inspection cover to the housing. This applies to Chevrolet 12 bolts, but not "BOP" differentials.

12 bolt Chevrolet. The big give away that tells you that this rear end is a Chevrolet 12 bolt and not a BOP (Buick, Oldsmobile, Pontiac) "12" bolt (0ther than the 12 bolts holding the cover on) is the "eyebrow" or upside down V on the cover. The upside down V is for oiling purposes. It helps direct oil into the axle tubes. These Differentials have an 8.75 inch ring gear with axles that are retained by C Clips. This is the strongest era correct Chevrolet rear end to put under a Chevelle.
http://img.photobucket.com/albums/v625/YenkoChevelle69/Dan_s_Pics_036.jpg

8.2 10 Bolt Chevrolet. This rear end will have 10 bolts holding the cover on and only half of the eyebrow or V on the pass. side of the cover. I have seen 10 bolt covers with the entire eybrow too. You can kind of see the half V that is used for oiling purposes in this picture. This particular rear end is an 8.2 10 bolt rear end. It's called an "8.2 10 bolt" because it has an 8.2 inch ring gear. This rear end is the weakest rear end of the era that you could put under a Chevelle.
http://img.photobucket.com/albums/v625/YenkoChevelle69/sucp_0812_25_zchevy_10_bolt_rearend.jpg

Here is a picture of an 8.2 rear end with a complete "eyebrow"
http://img.photobucket.com/albums/v625/YenkoChevelle69/ccrp_0603_17_zchevy_8_2_10_bolt.jpg

8.5 10 bolt Chevrolet. This rear end will also have 10 bolts holding the cover on, but it has a stronger 8.5 inch ring gear. It will have a smooth cover and also have the 2 squares on the bottom of the housing. Thats the easiest way to tell an 8.5, short of pulling the cover and measuring the ring gear. This rear end is pretty strong. Not quite as strong as a 12 bolt car rear, but stronger than an 8.2 12 bolt. This rear end replaced the 8.2 10 bolt in the 71 or 72 Chevelle model year. Maybe someone can clear that up. This rear end will have C clip axles.

http://img.photobucket.com/albums/v625/YenkoChevelle69/ccrp_0603_16_zgm_8_2_10_bolt.jpg

Rear end casting numbers are located here in the red circle. As far as I know, this goes for all rear ends be it a 10 or a 12 bolt. This is simple when the rear was cast, no when it was assembled.
http://img.photobucket.com/albums/v625/YenkoChevelle69/rearendcastingdate-1.jpg

The rear end assembly stamping will be located on the front of the pass. side axle tube. This will tell the day of the year, shift, and axle gear ratio that the axle was originally born with.
http://img.photobucket.com/albums/v625/YenkoChevelle69/rearendstamping.jpg


I hope this helps. Feel free to add to it with plenty of pictures and explanations! I'll try and add more tomorrow

any idea what makes the 10 bolts with or without the inverted V? was it a specific year thing or plant that built the rearend? jim

The 8.2 and 8.5 covers will interchange, so don't judge a rear end by it's cover. I have seen many 8.2 10 bolt rear ends with the 8.5 cover on them. The early A body 8.5 rear end doesn't have c clip axles. This is the only 8.5 that had bolt in axles. It is the same as all of the other 8.5 rear ends except for this. The "O" rear end is a 10 bolt with 12 bolts in the cover. It has an 8 1/2 inch ring gear, but is not the same as the 8.5 10 bolt. It isn't a BOP rear end, it's called the "O" rear end. It was used in the Olds Cutlass starting around '68 and was discontinued around '70. It has been seen in some Monte Carlos and other GM cars, but I don't know if it was factory installed in any of them.

The Chevrolet 12 bolt has a 8.875" ring gear, not 8.75".

I've also found the casting number on different sides & web of the housing, not always in the same place. Might be tied to the year of the housing.

You may want to add the axle spline count.
12 bolt 30
10 bolt 28

There is also a rear end with a 10 bolt cover used on later Malibu's (78-up) that has a 7.5" ring gear. The housing has similar mounting points to the 64-72 Chevelle so could possibly fool someone.........I think the spline count can be either 26 or 28.

64 to 67 rear ends are about 1" narrower than 68 to 72. If you go by Moser replacement axle length they are exactly 1.125" narrower overall......

When shopping for a 12 bolt there are a few indicators for the year but the best are the casting number and casting date.

If it doesn't have a casting number (as listed in Chevellestuff) it isn't a Chevelle housing.