Rear Diff. ID

[Mark]

Please help me ID the following rear differential: casting # = 3871375; Date code F135; and code GM 10 (IO?). Any help would be appreciated. Thanks.

[Trebor M #25911]

Re: Rear Diff. ID

Mark

I believe the casting you see is 3818753. The code # is probably AO and it stands for 327cid , 4speed, posi 3.70:1. This is the closest I see in Colvin's book "BY THE NUMBERS".

[John Rohe]

Re: Rear Diff. ID

Mark,

You've definitely got the casting # wrong. Here are the 3 castings that I know of:

3818753N: thru Feb '65 3876476N: March '65 thru model year '66 3899143: model years '67 thru (at least) '75

If you're reading the date correctly it would be either June 13, 1965 or June 13, 1975. It's more likely that your casting # is really 3876476 than 3899143 (based on what you think it says) so I am guessing that this is a '65 rear with a 6/13/65 casting date.

But what you really need to do now is check the bottom of the rear (just in front of where the cover goes) to get the two digit STAMPED IN (not cast) ratio code. There should be a stamped in date there as well. Post what you find on the bottom and I'll check the code. Of course, what ratio originally came in that rear may not be there today...

John R.

[Mark]

Re: Rear Diff. ID

Now I have to show my ignorance. The numbers (other than the date code) I posted are visible from the rear of the car, if you put your head where the spare tire would go and look forward at the rear end of the rear end. It has dawned on me that the casting number to match those posted by others is on the front half of the rear differential (the half that accepts the driveshaft). Unfortunately, I can't find it. Where do I look?

Also, the ratio code is so faint that I can't make it out without putting the car up on ramps, which I will do this weekend.

Thanks for the help.

Mark ('64 convertible 327/300)

[John Rohe]

Re: Rear Diff. ID

Mark,

The casting #'s on the rear are very hard to see when the rear is on the car, but again the casting date you gave (which should have been on the drivers side) signifies the rear as produced in either '65 or '75 for the same model years. The "stamped in" codes should tell us the rest.

If the "stamped in" date is a julian date (e.g. 182 = July 1st) then it is a '75 rear (julian dating started in '71). If the "stamped in" date is something like "07 01 65" then obviously it is a '65 rear. Post the axle code as well (the first two digits - both letters) and then we'll know what this rear is (or at least what it originally was).

Good luck, John R.