1960 Gear Ratio Identificationbt

[Kirk M.]

Helloooo Corvette Experts!
I have an early '60 (build date 10/26/59). I want to find out more about how it was equipped on its delivery date. Car is a "driver," no known history. Tach is a 6,500 rpm redline. The casting date code of the rear gear unit seems likely to be original to car.
My focus is the rear diff. Thin "P," 833 casting #, cast 7/22/59, assembled 9/11/59. Has vent tube. Per Chevy by the ## the drain plug that is present should not be there if it is a posi.
3 characteristics that a posi should have that are missing on this rear end are: 1) fill hole "posi lube requirements" metal ID tag; 2) posi notice on spare tire; and 3) dog tag at 2 o' clock stud that gives ratio.
I followed Allan Colvin's instructions in vain, hunting for the 2-letter ratio code on the axle tube. I stripped and sanded the axle/gear housing from end to end, top to bottom, no 2-letter stamp. No stamp of ANY kind.
I marked pinion and driveline and tire and wheelwell, did the math and gears are a 3.08.
Questions:
1. Why wasn't the axle carrier stamped or tagged?
2. Why is there a drain plug?
3. Why are 3.08's listed only under "Passenger Car" in Chevy by the ## and not listed as a possible Corvette gear ratio?
4. Are 3.08's common in '58-'62?
5. Would 3.75's be more "Corvette-like?"
6. Does that 6,500 tach go along with 3.08's?

I will be grateful for ANY assistance you can give me.
Thanks!

Kirk

[Mike E.]

Re: 1960 Gear Ratio Identificationbt

Kirk:
The two-digit stamping, along with the date stamping are very shallowly stamped, and are on the posi unit itself, not on the axle housing. It is on the passenger side, in approximately the same place on that side that the date casting is on the opposite side.
All axle housings had drain plugs during that vintage--some time in early 62 they eliminated the plug.
3.08's are not uncommon--and were also not terribly desirable until gas prices went through the roof.
There is no 3.75--3.55 and 3.70 are pretty good gears--3.70 for around town is nice--3.55 and 3.36 are great highway gears.
6500 tach and 3.08--not a common combination, but any means.
It really, from what you have said, sounds like your car may not originally have been a posi car. On the top of the axle housing there should be either a vent or a brass elbow that has a vent tube attached. The presence or absence of the brass elbow gives a better hint--if there's a vent, it's almost surely not a posi-equipped car from the factory.
If you REALLY want to check it out--take the posi unit out. If there are baffles in the rear housing ner the posi unit in each tube, it was a posi unit, if there are no baffles, it was a non-posi car. (Unless the entire axle housing unit was changed, and then we don't have a clue what was originally there.)

[Edward B.]

Re: 1960 Gear Ratio Identificationbt

Kirk: In 1960, if I remember correctly, 3.70:1, 4.11:1 and 4.56:1 were the only ratios offered for the Corvette.

[Bill M.]

Re: 1960 Gear Ratio Identificationbt

Questions:
3. Why are 3.08's listed only under "Passenger Car" in Chevy by the ## and not listed as a possible Corvette gear ratio?
4. Are 3.08's common in '58-'62?

Kirk

According to Best of Corvette News, 3.08 was introduced for 1962...

[Kirk M.]

Re: 1960 Gear Ratio Identificationbt

RE: "If you REALLY want to check it out--take the posi unit out..."

Mike,
Thank you for your thorough, very helpful response. I'll jack it up and take another look per your advice.

Kirk

[Kirk M.]

Re: 1960 Gear Ratio Identificationbt

Thanks, Edward! You guys are better than the books!
Kirk

[Kirk M.]

Re: 1960 Gear Ratio Identificationbt

Thanks for the helpful response, Bill. I'll have to do some more homework...
Kirk

[Kirk M.]

Re: 1960 Gear Ratio Identificationbt

Hi, Mike, Kirk again --

What I thought (per Colvin) was a build date seems to actually be the stamping I was hunting for in the diff carrier which is AM 914. This, in Chevy by the ## indicates a Glide with 3:36 posi passenger car gears. I'll jack up car again and check the second letter -- maybe it wasn't an "M!"
The housing has 90 degree brass and tube which I think you called a "vent tube." I'm not clear about the difference between a "vent tube" and a "vent."
In Chevy by the ## (page 202 in 1955-59 book by Colvin), under passenger cars, year 1959, what does the "348" mean just before the ratio 3:08 code: AW? or AX 3:08 posi?

Thanks for your expertise,
Kirk

[Edward B.]

Re: 1960 Gear Ratio Identificationbt

Looking at my sales brochures it appears that in 1961, 3.36:1 was added as a Positraction ratio and that 3.55:1 was available in a non-limited slip differential. In 1962, 3.55:1 was listed as being available in Positraction and a 3.08:1 gearset was added to the Positraction line-up making a total of six Positraction rear ends available depending on the engine and transmission options selected by the purchaser, to wit: 3.08:1, 3.36:1, 3.55:1, 3.70:1, 4.11:1 and 4.56:1.

[Kirk M.]

Re: 1960 Gear Ratio Identificationbt

Hi, Edward,
Thank you for your sales brochure information. I put the '60 up on a lift at a local muffler shop and removed the diff. carrier. It had NO baffles, but was a posi with a 3.08 (so, not factory posi). The detective work is continuing.
Thanks again,

Kirk