I found what a guy claims is a “good” 3.70 out of a C3 for sale. He’s got some other C3 parts so I’m inclined to believe that’s what it is. But without cracking it open and seeing what kind of shape it’s in, not that I would know what I was looking at anyway, what is a reasonable price on one of these units? Any obvious things I could check to get an idea of the condition? He’s asking $750. Thanks!

 

Rebuilders buy them for $250. Market price for an out of the car, unseen diff- $300-$400 tops. Buying any 63-79 diff without opening it up is risky. Many don't even know what to look for even if they opened it up before selling it.

 

Thanks GTR1999! That's exactly what I was looking for!

 

The only thing I would add is if you are looking for a particular date code or stamping. In that case some will jack the price up knowing you want that one housing-regardless of if it's worth it.

I have bought sight unseen diff's and "spun the wheel" only to find a cracked or broken posi or other parts. I would never buy a used one and expect to bolt it in and drive it. You may, but I never would and if you saw some of the ones I opened up you would see what I mean. I buy them only as a core and toss out most of the parts.

 

so basically buying a diff for dummies means buy it from Gary. oh, that's right. then you aren't a dummy...

 

Anything wrong with your current differential? If you are just looking to change the ratio, you should consider rebuilding what you have. If yours is original to the car, then you should have the 3rd gen posi carrier (which is preferable). As mentioned by gtr1999, buying a used unit is pretty much a crap shoot.

 

Here is a good example, one I opened last week. 1967 original diff, no issues with it in the car. Sent for upgrading. However, it was indeed a time bomb as the posi is cracked. The diff otherwise would have operated fine up till a few clicking sounds and then complete failure.

 

There's nothing wrong with my original 3.08 diff. I just want to switch to 3.70s and figured I would keep the original intact in case I or someone else wanted to revert to factory down the road. I wasn't planning on throwing it in until I have a shop give it a once over.

I was quoted $1000 to swap the gears out in mine. I know I'm rolling the dice here but if it's serviceable or doesn't need a ton of work I could potentially get out the door under the $1k it would cost to just swap the gears in my original unit. If not, I guess I've got a core for a rebuild. Of course this all hinges on whether or not I can talk the guy down to something more reasonable. Am I way off here? Should I just pay to have the gears swapped and not mess with this thing?
Thanks again!

 

All depends on the ability of your shop. $1000- what does that include? what brand of gears are used; do they know there are no more USA made gears left? Look at gear prices from vendors or summit, Jeggs, etc- $400 or more most times, so is the other $600 labor or are you going to get some up charges when you go get it?

I would expect most competent rebuilders would be replacing the bearings and clutches if going into a 50-year-old diff but then what I see with some of them is clearly not the case.

There is cheap, there is good, then there is the best. None of them go together.

A cheap gear change is just that, install the new gears, hopefully the lash and preload are correct. but the work is done against a flat rate book or the clock.

A good job is one where the bearings and clutches are replaced, the gears setup correctly and done. This usually costs more.

The best job addresses all the issues these diff have and corrects them. They are built to the application by experienced and knowledgeable techs, they are not a race against the clock. They are not cheap in cost or quality.

There are a lot of vette diff's out there. Many guys make more money parting out mid to later C3's than restoring them. Most of the time the parts are worn out or have bad original parts in them. I looked online today and found used, out of the car diff's selling between $300-$800. The ones I was able to see inside clearly were nothing but a stock diff and nothing "I" would pay more than $300 for. They looked just like the one I pictured above.

Here is a test for you gear heads out there, do you see anything in the picture above - other than the crack in the posi, that is not right?

There is also the myth that BB differentials were stronger and thus better than SB diff's. They were not. They had so called HD axles but how many broken axle U-bolts have you seen? The posi's on some BB diffs were said to be shoot peened, yet I can tell you plenty of original BB posi cases have broken.. To those looking to get a housing with the correct date code and stamping, those are valuable to that person for correctness not because of strength.

My personal opinion, if a seller is honest and has the knowledge on these diff's he will show you pictures of the inside. If you are able to pick it up, vs shipping it, you should remove the cover for inspection. If that is not offered, how do you feel about trusting it?

 

How much time and effort are you willing to put in to educate yourself on the topic of C3 differentials? There is a TON of information on this forum on what to look for, how to build them, etc. Gary Ramadei (gtr1999) is the resident guru on them and I studied his posts for a long time before I dove into building mine.

Being informed is key. Absent that base of knowledge, you’re unnecessarily exposing yourself to disappointment.

I’d first start with the basics. Get the stamping numbers and cast date of the housing. At a minimum, have the seller remove the cover and shoot some pictures of the ring gear to prove it is a 3:70 (could be a 3:73). As Gary’s pictures show, a cracked posi case is junk. Know also that there are 3 versions of the posi case so you want to be sure you have a “large window” 2nd or 3rd design case. Have him measure the axle play. Get a picture of the pinion flange to see if it is a 1310 or 1330.

If he’s unwilling to do a any of the above, then the most I’d pay is $300.

 

Thanks again gentlemen. You've given me a lot to think about. At a minimum I'll try and get some pictures of the internals and casting #'s. These are the only pictures on the listing:

 

I'm also dying to know what the other problem is that GTR1999 hinted at on the example he provided.

 

Rowdy, it sounds like you might be much better off taking/shipping your original diff to Gary (GTR1999) and get it done correctly rather than spend a whole bunch of $$ on an unknown diff from an unknown person, and then spend a whole bunch more $$$ on top of that to rebuild the unknown diff anyway. I shipped mine out to be rebuilt last year - to a quality shop - because it was leaking and it was 53 years old. They did a great job and I'm very happy with it. I've since found out that Gary has a great reputation on this forum.

My parents taught me something that I've found to be very true: "when you buy true quality you only wince once". Best, Paul

 

Right bearing cap installed backwards...
Kevin

 

Bingo, sharp eye Kevin. I posed that same question on FB a few days ago and no one responded.

 

For the rest of us, how to tell?

 

For reference, I found my 3.55 rear end last summer rebuilt and open for inspection, on facebook for $500 it came with new bolts and gasket for the heavy duty rear cover it came with. I consider myself lucky to find it but deals like this pop up every now and again. the seller had it saved for a project he decided against finishing.

 

Thanks Gary! I had the best teacher, you know him very well @GTR1999

 

Not the expert, but smart enough to have had Gary rebuild mine!!!

It looks like the bearing caps have a straight edge side and a beveled edge side near the bolt holes. I believe putting the beveled edges toward the diff case is the proper way. If you note in the picture of post #7, the right bearing cap has the beveled edge towards the posi, not the case. The left cap appears correct.

Remember, this is only my guess...

Paul

 

unlike my 79 which had tabs..this one i didn’t see until pointed out..and assume the rounded part goes outside to rounded case.