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Hey guys I own a 1978 silver anniversary and I plan on building a new differential. I know I would like to go with either a 3.73 or a 3.70 gear ratio but I currently have a 3.55. Do I need to buy a different case for the diff if I am going with these longer gears? And if so where can I find the new housing? Also is there a way to see what my current spline count is without opening up the cover.
Ill add a couple photos of my baby
Last edited by Nick.Faga; Oct 8, 2024 at 08:40 PM.
2025 c3 ('74-'82) of the Year Finalist - Unmodified
Nice looking 78, welcome aboard!!!!
Think this would be better posted, and receive more assistance, in the C3 tech/performance section, perhaps one of the mods will move it for you.
Is it a stick or auto? If it's a stick, consider changing the transmission to a Tremec. The first gear is very steep and will also give you a 5th gear for overdrive. Jerry
I had a 3:55 and changed to a 3:70 years ago and the difference was negligble. As mentioned, good replacement gears are scarce. My advice is be happy with what you have.
Is it a stick or auto? If it's a stick, consider changing the transmission to a Tremec. The first gear is very steep and will also give you a 5th gear for overdrive. Jerry
I have an automatic it should have overdrive but I’m not 100% sure
To answer your question. You do not need a new posi case to go from 355-370/373. There is little performance difference. Can it be done, yes, is it worth it- only you can make that decision.
However, what you may not know about your 1978 differential is they had issues, as all vette diffs had from 63-82.
78's had bad ring gear bolts, some had poorly tapped RG bolt holes, the axles were case hardened and once they hit about 50k miles started to wear rapidly, the posi clutches were weak and broke. The good things, they had the best posi cases and spiders.
If your car is 100% original you should check the axle endplay and see what it is, since that may dictate if the diff has to come out. There should be no oil leaks, again that can be a serious issue.
Next is what parts to use and who does the work. I got 3 calls today from guys who had C3 diffs built by local shops and all had issues without support from those shops.
To answer your question. You do not need a new posi case to go from 355-370/373. There is little performance difference. Can it be done, yes, is it worth it- only you can make that decision.
However, what you may not know about your 1978 differential is they had issues, as all vette diffs had from 63-82.
78's had bad ring gear bolts, some had poorly tapped RG bolt holes, the axles were case hardened and once they hit about 50k miles started to wear rapidly, the posi clutches were weak and broke. The good things, they had the best posi cases and spiders.
If your car is 100% original you should check the axle endplay and see what it is, since that may dictate if the diff has to come out. There should be no oil leaks, again that can be a serious issue.
Next is what parts to use and who does the work. I got 3 calls today from guys who had C3 diffs built by local shops and all had issues without support from those shops.
I know the diff has to come out bc it has metal in the fluid and it sounds like a Star Wars tie fighter when turning and accelerating. I plan on learning how to rebuild the diff myself. I know the cover is also leaking but the axels are good and sound. When the diff comes out I will be replacing all of the U-Joints and whatever else needs to be done. So yes I plan on doing the work myself, I work as an apprentice tech at a ford dealer. so I can do the work without worrying about anyone else but myself doing something wrong. The only thing that is keeping me from ordering parts is that I don’t know what the spline count is.
I know the diff has to come out bc it has metal in the fluid and it sounds like a Star Wars tie fighter when turning and accelerating. I plan on learning how to rebuild the diff myself. I know the cover is also leaking but the axels are good and sound. When the diff comes out I will be replacing all of the U-Joints and whatever else needs to be done. So yes I plan on doing the work myself, I work as an apprentice tech at a ford dealer. so I can do the work without worrying about anyone else but myself doing something wrong. The only thing that is keeping me from ordering parts is that I don’t know what the spline count is.
What part and where are you seeing more than one spline count for a 78 differential?
What part and where are you seeing more than one spline count for a 78 differential?
well I’m looking at new gearing and I want to make sure I get the right parts I don’t know what the current spline count of the diff is because I’ve seen different gear sets with 28 versus 30. I don’t want to order the wrong parts and get stuck when trying to build my new diff. I will be building the diff outside of the car and just replacing the old one. I will be rebuilding the old differential to save as a backup but with my situation I cannot let the car sit so I want to get everything done in 2 days.
well I’m looking at new gearing and I want to make sure I get the right parts I don’t know what the current spline count of the diff is because I’ve seen different gear sets with 28 versus 30. I don’t want to order the wrong parts and get stuck when trying to build my new diff. I will be building the diff outside of the car and just replacing the old one. I will be rebuilding the old differential to save as a backup but with my situation I cannot let the car sit so I want to get everything done in 2 days.
That's why I asked where are you seeing two spline counts? You may be looking at the wrong ring and pinion.
I saw a ring and pinion from a 10 bolt Chevy differential that was 28 splines
You are working on a Corvette differential not a 10 bolt Chevy differential, totally different animal. You are looking at the Wrong parts. You need to do further research and listen to what GTR1999 is telling you and search for his posts in this forum.
1978 Vette Diff
