I know this has been discussed countless times. But, I did a search and came up with nothing. I just checked my rear and this is what I found. On the left front of the housing, I found 3894939. Can anyone please explain this to me. On the right front, I found GM8. What is that?? On the axel tubes, I found this. Right axel tube, was this, CW1111B, and on the left axel tube was this, J278. Any help would be greatly appreciated. This is on a 67 SS. Does this rear belong on this car??? Or better yet, would it have come on this car?? Thanks again.

Freddy

CW1111B looks like a 3.31:1 non posi built Nov 11 at the Buffalo plant.

http://chevelles.com/years/67/67rearcodes.htm

That casting number was used in 1967 and it may have also been used in 1968 and into 1969 although I have seen a different casting number used in 1969. The casting date (J278) certainly is October 27, 1968 which would be in the 1969 model year. Could it be a service replacement for 1967? You will have to measure it to be sure of whether it is the narrower 67 or the wider 68 and later. The different years could have used the same casting number but different width axle tubes. Measure the housing, 67 should be 60 inches, drum to drum, and 68 and later would be 61 inches, drum to drum.

Alan Colvin, in his book Chevrolet By The Numbers, says the 939 casting was used into 1969. Perhaps that casting was used until mid model year 1969, when the 3959038 started to be used. I've seen a couple of 3959038 housings dated in late October, 1968.

Freddy, is there any chance that the casting date is J276 and not J278? Sometimes 6s and 8s are hard to distinguish. Would J276 and November 11, 1966 be before the build date of your car?

Just to add a bit more confusion, CW was used for a 3.31 non-posi in 65, 66, 67, 68, 69, 71 and 72 (1970 used CCW). And, from the FWIW department, an old article "Inside '64 -- '72 A-Bodies, (Car Craft Magazine)" said:

* 1964-67 Chevelle rear ends were manufactured by the Chevrolet Gear and Axle Division. The housing measures 56.5 inches from flange to flange, and with the brake drums in place, the total width is 60.5 inches.

* 1968-72 Chevelle rear end housings measure 58.5 inches from flange to flange, and with the brake drums in place, the total width is 62.5 inches.
The 10 bolt Chevelle rear end has the 8.2" ring gear from 1964 to 1972. It wasn't until late 1972 that the 10 bolt 8.5" ring gear was used in Chevelles.

Rear spring mounting pads differed in that 1964-66 rear ends used a flat pad with a hole drilled in the center. 1968-72 rear ends have circular spring mounting pads, which are 3/4" higher that the early flat pad.

1967 was a transition year in that A-cars might have a 1964-66 style rear end, a 1968-72 rear end (which is wider, and commonly available), or a "hybrid" rear end, which will have the 58.5" width, but with the early spring mounting pad and trailing arm brackets.Although I've never seen one of the mentioned "hybrid" rear ends in a 67.

Here are the numbers I have found to be the most prevalent in Chevelle 12-bolts:

1965 Chevelle 3859140N
1966 Chevelle 3875745N
1967 Chevelle 3894939NF
68-69 Chevelle 3917124NF
1969 Chevelle 3959038NF
70-72 Chevelle 3969278NF

As always, there will most likely be some rumor of overlap in numbers but unless you have a build sheet/protecto plate and can verify the stamped codes as original, who knows. It should also be noted that the 1965 12-bolt is the only one with the diagonal bracing on the center section casting. Maybe Dale can add this to his web site!

This is on a related subject. In another thread I've been trying to determine what stamped date codes would be expected on the transmission and rear end of a '70 454 built in Baltimore in June of 70 with an M22 and 4:10 posi. I've gotten some good responses, but haven't closely enough nailed down the code for the rear (ie. CRW*****). Does anyone have or know of a June Baltimore car that could help as an example?

Erik.