Topic: 1968 GT 390 Drive Train Advice Verification

[bullitt68]

C5 main cases it's following the engineering number on the drivers side of the main case right side up and on C8 cases its on the passenger side up side down

Will be moving the discussion/thread since we're moving away from general (applies to many years and models) to more year and application focused details.

My trans looks like it has a date of C11

My starter has Amercian Motors stamped on it, not Autolite. Should I assume that it is from and AMC

On the Third Member there appears to be a cymbal before the date. Does that mean anything?

Thanks Jeff. So much to learn and do, so little time lol!

[67gt390fb]

the "symbol" you are looking at on the third member is a zero with two dots under it, used in 1970.  third member housing was cast  1970 Oct  21

[bullitt68]

the "symbol" you are looking at on the third member is a zero with two dots under it, used in 1970.  third member housing was cast  1970 Oct  21

Great thanks. So would this be the same diff with an incorrect date code. I guess that the only way I will know what gears I have is to open it up. I assumed it was the original diff which came with 3:25 gears. I am going to assume that is no longer the case. Cant wait to open it up and find out. What should I be looking for to confirm what gearing I have.

[Bob Gaines]

Great thanks. So would this be the same diff with an incorrect date code. I guess that the only way I will know what gears I have is to open it up. I assumed it was the original diff which came with 3:25 gears. I am going to assume that is no longer the case. Cant wait to open it up and find out. What should I be looking for to confirm what gearing I have.

The shape/design of the center section and if it is correct for the application is more important then the date code in most concours judging class's. Thoroughbred concours judging is where both design and date codes are equally important.   You can determine what gears you have by jacking the car up and seeing how many times the drive shaft rotates compared to how many times the tires rotate.

[bullitt68]

The shape/design of the center section and if it is correct for the application is more important then the date code in most concours judging class's. Thoroughbred concours judging is where both design and date codes are equally important.   You can determine what gears you have by jacking the car up and seeing how many times the drive shaft rotates compared to how many times the tires rotate.

Thanks Bob. So how would I determine if the centre section of the diff I currently have would be the correct design for my car. What do the numbers on the centre section of my diff decode as.

[67gt390fb]

a little good news,  the C5AW-4668-C  pinion carrier is the heavy duty Daytona pinion carrier (http://www.kevinstang.com/Ninecase.htm) , though probably not o.e. for your car.    From what little i can see in your third member pictures, it looks like you have the higher strength nodular housing.  good to have but again probably  not what would have come on car.   as for gear ratio, spin axles,  you will get 3.25 turns of the pinion to 1 turn of the axles for 3.25 gears. check out the Kevinstang site

[bullitt68]

a little good news,  the C5AW-4668-C  pinion carrier is the heavy duty Daytona pinion carrier (http://www.kevinstang.com/Ninecase.htm) , though probably not o.e. for your car.    From what little i can see in your third member pictures, it looks like you have the higher strength nodular housing.  good to have but again probably  not what would have come on car.   as for gear ratio, spin axles,  you will get 3.25 turns of the pinion to 1 turn of the axles for 3.25 gears. check out the Kevinstang site

Thanks for that info, its very helpful.

[Bob Gaines]

Thanks Bob. So how would I determine if the centre section of the diff I currently have would be the correct design for my car. What do the numbers on the centre section of my diff decode as.

Best to post a picture of the front side of the center section/ pumpkin . Pictures posted do not show the front. If drive shaft is not installed take a straight back picture otherwise one picture from each side of the front.

[bullitt68]

Best to post a picture of the front side of the center section/ pumpkin . Pictures posted do not show the front. If drive shaft is not installed take a straight back picture otherwise one picture from each side of the front.

This is the only shot I have from the back. Will have to take another shot when I get back to my shop

[J_Speegle]

This is the only shot I have from the back. Will have to take another shot when I get back to my shop

The rear shot shows some of the expected features from that angle for the carrier/3rd member we're looking for things like the drain location and the arrangement of the reinforcing ribs to name just a couple. At the same time I think you already posted the date code in your original posting so we know it's been replaced

[bullitt68]

The rear shot shows some of the expected features from that angle for the carrier/3rd member we're looking for things like the drain location and the arrangement of the reinforcing ribs to name just a couple. At the same time I think you already posted the date code in your original posting so we know it's been replaced

Here is another shot

[Bob Gaines]

Here is another shot

basically need to see the casting ribs above the pinion shield. That will determine which design you have and whether it could possibly be original or not . That area is currently in the shadows .

[J_Speegle]

basically need to see the casting ribs above the pinion shield. That will determine which design you have and whether it could possibly be original or not . That area is currently in the shadows .

Better shot he provided earlier. Looks to be cast on  Oct 21 (1970 possibly)  with single vertical rib two horizontal rib design. So it appears wrong design and wrong date at least comparing it to other 68 S code San  Jose examples I have pictures of. The two horizontal ribs are interesting even for a 70 date case

[bullitt68]

Better shot he provided earlier. Looks to be cast on  Oct 21 (1970 possibly)  with single vertical rib two horizontal rib design. So it appears wrong design and wrong date at least comparing it to other 68 S code San  Jose examples I have pictures of. The two horizontal ribs are interesting even for a 70 date case

Thanks Jeff do you need to see any other photos to help ID this diff

[Bob Gaines]

Which type do you have the "N" case or the single vertical rib?