Corvette Differential Question(s)

**Mike T.** · February 26, 2015, 08:34 AM

Re: Corvette Differential Question(s)

Rob - Does the seller have any paperwork to back up the claims on the differential? My 65 300 horse Coupe most likely originally had a 3:36 rear but when I bought it in '85 I found it had been replaced with a '68 3:08 posi, with the 'AT' code listed as H.D. just like the AW. Not sure why they'd have two different codes for the same ratio listed as H.D. Anyway, curious as to what the story is with your original differential. Is the original ratio too high numerically and that's why you're interested in the 3:08?

**Joe R.** · February 26, 2015, 09:50 AM

Re: Corvette Differential Question(s)

Rob,

IF you want an original or close to original rear why don't you contact Pat Ikerd at 1 812 279-4828. Tell Pat what you want, year, gears etc. and he will check his inventory. The rear end you get from Pat will have a guarantee unlike the rear end in California.
This just might save you a lot of money and heartache!

JR

**Gary R.** · February 26, 2015, 01:06 PM

Re: Corvette Differential Question(s)

That unit will bolt right in no problem. As you know it is not a '67 housing as there would be a 67 stamped in it plus you mention it is from a 454 car( 70-74). It would have the better posi case and spiders, soft axles if original and lousy clutches if not replaced.

All the axles from '63-79 were 1350 in size so that sounds like a bit of sales talk to me. The "HD" axles are the ones with the bolts and caps but in all reality I doubt there is would be any advantage for a street car, SB or BB. Given a choice I would use the "HD" but it shouldn't be a deal maker or breaker.

There is no difference between BB or SB differentials other then the stamping and those axle types.

Value- if it was rebuilt and I would really ask for some pictures or proof, then $700 is a fair price for a common vendor type rebuild. I buy "out of the car" diff's for what is considered "market price" which really means between $100-$300 depending on condition. I find them at local swap meets every year in that range and a local parts guy sells them for that when ever he rips apart a c3. These are sold "as is" and they all need to be rebuilt. The "retail" price on a housing is about $300, unless you ask for a specific number set then some suppliers prices inflate to $400 or more.

Before you spend any cash decide on just how much power you are going to build and how you're going to drive the car. A lot of guys have been building engines with 2-3x the original power/torque levels and expecting a drivetrain designed around 300-435 hp to hold up. They will not if you hammer the car. The highest powered vette I built a IRS for was dyno tested to 975hp at the flywheel. Once you get into the 475-500hp range the 10 bolt is not going to hold up to abuse for long. You might be better served to rebuild yours- if it needs work. Then you will have the original housing and can build it to application.

Good luck, if you get picture and what me to take a look let me know.

**Rob V.** · February 26, 2015, 03:16 PM

Re: Corvette Differential Question(s)

Mike, Joe, and Gary--

As I have come to expect, I have once again received the usual, excellent responses from you three--"thank you!"

Just for discussion sake, I, unlike many individuals, prefer the 3:70.1 gear ratio, as most of my driving is on the city streets in the Denver metro area, with short blasts of 10 to 15 miles on the freeway. It is believed that my '67 Coupe was originally equipped with a 390 HP 427; but now has some crappy, over-cammed, scabbed together, 327 cubic inch engine in it; and is a real dog when initially pulling away from a stop. We cannot confirm that the current differential is original to the car, but it appears to be a 3:36.1 ratio, and does have the "big block" rear axle yokes with threaded caps. I wanted to have the 3:70.1 differential installed now, to have a little more streetability, until my big block engine is completed. The same friend that found the differential, (that you have just helped me evaluate), has also helped me plan the big block engine build, and realistically, with all the "hot-rod" parts should displace 400 to 450 HP. (Again, in the past, I have had 3:36.1 gears replaced with 3:70.1 gears in two big block Chevelles that I have owned, and I have loved them). To answer your question, the seller does not have any paperwork to document the differential rebuild. Predicated on your comments and recommendations to me, I think I will pass on this differential. Additionally, I will contact Pat Ikerd as Joe suggests; and if my friend has received pictures of the differential from the seller (and I can figure out how to pass them on to Gary, I will). Once again, many thanks to the three of you for helping me with this issue.

Best Regards,
Rob

**Mike T.** · February 26, 2015, 03:42 PM

Re: Corvette Differential Question(s)

Rob - Gotta say, I like the way you describe the current smallblock..."crappy, over-cammed, scabbed together, 327 cubic inch engine". Direct and to the point ;-). On the subject of compatible rear gear ratios, I agree that the 3:70 is quite decent if you're not doing serious highway driving. I have two Coupes, the 65 has the stock 300 horse 327, wide ratio trans and the aforementioned 3:08's. My 66 is a slightly detuned L79 (lower compression with flattops and a 214/214 cam in lieu of the original 222/222 duration cam. That 66 also has a non-original wide ratio box with 3:70s and it's a lot more fun to drive than the 65. I do have plans to add a mild 427 to the 65 Coupe and will probably leave the 3:08's in place but suspect I'll eventually look around for a 3:70 rear for it. Does your 66 have a wide ratio or close ratio trans?

**Rob V.** · February 26, 2015, 05:54 PM

Re: Corvette Differential Question(s)

Hi Mike--

You always ask good, thought provoking questions. I have read on the TDB how to tell a wide ratio from a close ratio if the transmission is in the car--which it is; I just cannot remember how to tell the difference. The transmission is original to the car, VIN stamped on it; and I don't think it was until 1969, that the factory stamped the case with the code for wide or close ratio. Of course even looking at the input shaft (if the trans is out of the car), is not conclusive, as some previous owner could have changed the gears. Guess I should have succinctly stated, "I don't know," but I did not want to sound rude.

I did check with Ikerd's on the cost of one of their rebuilt differentials. If I wanted a date code correct case, it would be $1,000 for the bare case. If I just wanted a suitable substitution/replacement differential, then it would be $1050 plus core for a rebuilt unit. On a different note, (and I am sorry that I cannot remember), do you already have a 427 cubic inch block? When I was trying to locate a suitable 427 block, they were out of my price range. Fortunately, Mr. Joe Lucia prevailed, and guided me toward a 454 block. My friend located a two bolt standard bore 454 block that is out of tolerance for a standard bore, and needs to be bored and honed. So that is what I will build to put in my car.

Rob

**Mike T.** · February 27, 2015, 09:53 AM

Re: Corvette Differential Question(s)

Hi Rob - You mention you can't verify if the current differential is original to your 66, are the codes ground off? Any pics of the stamped characters on the bottom or raised casting figures on the side? If they're not prominent, maybe we can figure out the current rear gears by plugging in some other known values like rpm at 60 with your tire diameter. Could get us close at least. To figure out whether you have a wide or close ratio box, there's probably another formula that might help, but probably have to run it in first gear for that one compared to 4th gear for the rear gear ratio.
Re: my 427 installation project - yep, I have the engine already - I sent you an email.

**Rob V.** · February 27, 2015, 01:06 PM

Re: Corvette Differential Question(s)

Hi Mike--

No email received. This is the email address: robdianev@yahoo.com

Thanks,
Rob

Corvette Differential Question(s)

Corvette Differential Question(s)

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