C1 (1960) rear end/axle replacement with Positraction

[Donald H.]

I believe the rear axle on my 1960 project originally had a posi unit. It is still in the car, so somewhat difficult to inspect closely, but I can't see any casting or stamped numbers on the axle housing itself, however, it does have the posi vent line.

The case has the casting number 3707306, which from my research on this forum should be correct vette case, but non-posi.

What I would like to end up with is a posi unit and I think 3.70 ratio. I believe the 3.70 was the "standard" ratio. I want something that will cruise comfortably, i.e. reasonable RPM.

Is the 3.70 what I should be looking for? and what should I expect to pay for a posi unit?

Thanks,
Don Harris

[John F.]

Re: C1 (1960) rear end/axle replacement with Positraction

I run a 3.55 posi with stock 250hp-327 and you are turning 3000 rpm around 60 mph. For pure cruising I would drop to a 3.08 or 3.36. JMHO
John F

[Gene M.]

Re: C1 (1960) rear end/axle replacement with Positraction

My '65 has 3.36 and runs well at highway speeds when it's not real hot days. Highway speed is 65-70 mph. I prefer to run more like 75 mph but the rpm's start going up as well as the temperature of the engine.

We ran the road tour averaging 15.5 to 17.5 mpg depending on the "fun" level of spirited driving. I think the 327 could handle a 3.08 gear with out any added strain on the drive train. Sure to say comfort level at 75 mph and less gas consumption would be benefits. Acceleration might suffer a bit but it's pretty strong with the wide ratio tranny.

[Tom P.]

Re: C1 (1960) rear end/axle replacement with Positraction

I believe the rear axle on my 1960 project originally had a posi unit. It is still in the car, so somewhat difficult to inspect closely, but I can't see any casting or stamped numbers on the axle housing itself, however, it does have the posi vent line.

The case has the casting number 3707306, which from my research on this forum should be correct vette case, but non-posi.

What I would like to end up with is a posi unit and I think 3.70 ratio. I believe the 3.70 was the "standard" ratio. I want something that will cruise comfortably, i.e. reasonable RPM.

Is the 3.70 what I should be looking for? and what should I expect to pay for a posi unit?

Thanks,
Don Harris

Don,
The 62-earlier axle housings have ZERO casting or stamping marks on them.
The ONLY casting numbers-letters and stamped numbers-letters are on the CAST IRON CENTER CASE.
A 60 posi case will have the 3743833 casting number (and a cast date) as well as the large, THIN P for posi. It will be cast on the DRIVER side of the case.


The PASSENGER side of the case will (or SHOULD have) stamped letters-numbers for the gear ratio and assembly (mo-day) date code.
SOMETIMES, the stampings are extremely faint, and can be very difficult or impossible to discern (or there may not be any stampings).


If the axle housing has the posi vent line (which terminates at the top-center of the axle housing), it is highly probable that it was originally equiped with a posi rear (it's not likely that someone would have added a posi vent line to a non-posi rear).

[Terry D.]

Re: C1 (1960) rear end/axle replacement with Positraction

Donald
You don't say how fast you want to cruise so I will just throw this out. At 70mph with 27 1/2 inch tall tires (225x70x15) you will be up to 3200rpm. Go to www.keislerauto.com, at the bottom right under "tool kit" click on "Speed Analyzer". Then click on GM analyzer. You can plug in your tire size, then the rpm you want to turn and it will give you the mph. This is much easier and cheaper than the old trail and error method.
Terry

[Tom P.]

Re: C1 (1960) rear end/axle replacement with Positraction

And in addition to Terry's input, my 56 has a 3.08 posi with 225R75/15 tires and at 76mph, the tach shows 3000.

[Donald H.]

Re: C1 (1960) rear end/axle replacement with Positraction

Thanks to all,

How much can I expect to pay to get a posi unit? This 1960 project car is my first corvette restoration, for that matter, my first car restoration period.

I am going to Carlisle and will be on the look out for a posi unit. Preferrable a 3.08 or 3.36.

Tom,

If I find a unit, how do I decipher the gear ration stamping? In your picture of the passenger side I see an AB 9 and 27. I understand that someone could have changed the gears. Can I count the teeth on the Ring Gear to make sure the stamping and actual gears match.

Again, thanks,

Don Harris

[Tom P.]

Re: C1 (1960) rear end/axle replacement with Positraction

Thanks to all,

How much can I expect to pay to get a posi unit? This 1960 project car is my first corvette restoration, for that matter, my first car restoration period.

I am going to Carlisle and will be on the look out for a posi unit. Preferrable a 3.08 or 3.36.

Tom,

If I find a unit, how do I decipher the gear ration stamping? In your picture of the passenger side I see an AB 9 and 27. I understand that someone could have changed the gears. Can I count the teeth on the Ring Gear to make sure the stamping and actual gears match.

Again, thanks,

Don Harris

Don,
The 56-62 Vette and 55-64 passenger car center sections are totally 100% interchangeable.
The Corvette community swap meet vendors ALWAYS price the parts, which are interchangeable between pass cars and Vettes, at unreasonable prices. If you ask for a part (that is the same as for a pass car, such as a posi rear) and say that you want it for a 59 Chevy instead of for a 59 Vette (or whatever), it will invariably be cheaper. A complete, ready to go posi center section if worth $700-900, but the Corvette vendors will be asking $1500-2000 for it. If you MUST have a correctly dated and ratio coded rear for a Corvette, then get ready for a $1000-higher price tag. Also, the gear ratio will affect the price (some ratios-date codes have a higher demand).
Now, with this said, there IS an option. Eaton now makes a VERY NICE posi unit to fit these early Chevy/Vette rears for about $500. Once installed, there is no way to know what's inside your rear.
Here is an article I put together on the Chevytalk forum many months ago about how to convert a non-posi to a posi rear.

http://www.chevytalk.org/fusionbb/showtopic.php?tid/223912/

If you should find a 3.08 or 3.36 posi rear from a 61-64 full size Chevy pass car, AND YOU'RE NOT HUNG UP ON PROPER DATE/RATIO CODING, it will be a 100% drop-in bolt-in swap!

If the rearend center section is out of the axle housing, then all you have to do is look at the end of the pinion gear. There will be a 7digit part number, tooth numbers for both the pinion gear and the ring gear such as 10-37 (below) and a date code. Simply divide the number of ring gear teeth by the number of pinion teeth and you got the ratio (37/9=4.11, 37/11=3.36, etc, etc).

[Donald H.]

Re: C1 (1960) rear end/axle replacement with Positraction

Tom,

Thanks very much for all this info. I have bookmarked all if it and am sure I will be using when I get to the chassis and drive train restoration of my project.

I want to do as much of the restoration work myself that I am capable of doing. Without people like you who are very generous to share your knowledge, people like me would not be able to do much.

Again,

Thanks very much