I would like to put the stock P case posi in my '61 283/FI. What casting number should I be looking for. What are the terms "third member" and "3 series" refering to and which should I be looking for? I would like to put 370's in the car. Thanks All.

 

Here is a pic of my 61 FI rear housing.
'3' series refers to the carrier that the ring gear bolts to.
'3' series is 3.73 and lower ratio gear and '4' series is 4.10 and higher ratio. You can put a 4.10 and higher ring gear on a '3' series carrier but you must use a spacer because the pinion on the higher numerical gear sets are smaller in diameter and the ring gear must be moved closer to the center.
You cannot use a 3.73 and lower numerical gear set on a '4' series carrier.
I believe the '3rd member' is the whole center section that the drive shaft attaches to which includes the housing, ring and pinion, carrier/posi unit and companion flange. This unit bolts from the from to rear of the rear end housing.

 

Most of tha above it correct.
Positraction was available from mid-57 thru 62 in Vettes, although that same rearend was also used through 64 in passenger cars.
Within the 57-64 posi family, there were some variations. There was the 'early" posi (57-59) and the late version (60-64). Both early and late are TOTALLY, 100% interchangeable into ANY 55-64 axle housing. BUUUUUUUUUUUUUUUUT, the internals of the early-late units have some variations and not everything is interchangeable between them.

YOU WANT THE LATEr 60-64 STYLE (if you can locate one).

Now, there were even more variations in the 57-64 posi rears (as well as NON-posi rearw).
There was a Detroit Gear and Axle plant rear (Corvettes ONLY got this one) and there was the Buffalo (NY) rear. The rear in the above picture is a Buffalo rear (pass car ONLY).
The Detroit rears (posi or NON-posi) had a cast date code that had ONLY ONE number at the end of the date code for the year. For example, if the rear was a 1961, the date code ended in 1. But if it was made in Buffalo, the date code ended in 2 numbers for the year, ie 61 for 1961.
The Buffalo posi rears got the THICK P, as in the picture above, and the Detroit posi rears got the Thin P, as in the below picture (notice that BOTH the above and below posi rears are 61).


ANY Detroit or Buffalo posi rear center section from ANY 57-64 car or Vette will bolt right up to your existing 61 axle housing with ZERO problems, again, they are 100% interchangeable.
If you need some additional info on early, late or Buffalo, Detroit posi rears, let me know.

Oh yes, one more thing. Eaton is now manufacturing a posi unit SPECIFICALLY for the early Chevy rears (55-64), which will allow your NON-posi rear to be converted to posi (in my opinion, this is a considerably more desireable way to go).

 

Thanks Gentleman...great information!!

 

I just put a EATON posi unit in my 61 3 weeks ago. I destroyed the original Posi 30+ years ago. I just went to the junk yard and put in an open (non-posi) which I have driven around all these years.

The Eaton and new bearings went in easily...I had an OLD mechanic adjust the final assembly for me.

Joe

 

Hi!
I am in process of rebuilding the positraction differential for my 315-HP 1961 FI car. I purchased aftermarket internals from Tom's Differential to include the 3.70-1 gears, posi unit and installation kit. I secured the correct last iron housing on Ebay. Insure that the housing has the thin "P" symbol that was used in Corvettes.

You can view information that I have collect on the 1961 at:
http://www.rayscorvette.com/Fawn%20Chassis.htm

Is there a chance you may have a close-up picture of the back of the distributor ignition shielding? I would like to see the mounting slots.

Ray

 

I believe I have a thin P unit with a 4 date code.
Is that a Detroit unit from a 64 Impala ?
Obviously after 62 it can't be a Corvette solid axle 3rd member.

 

Is it possible to build a posi on a carrier without the P marking?

Stan...

 

Joe,
You are correct on both issues.

Stan,
ABSOLUTELY, you CAN convert a NON-posi to posi. It involves some MINOR removal of a gusset that is cast into the NON-posi case which a posi case does not have. Tooooooooooooooooo easy to do this.
Below is a photo of a NON-posi case which has had the required removal of the above mentioned gussed. The partial removal of this gusset MUST be done all the way to the inside wall of the case because the clearance for the posi unit is TIGHT. In the second picture, you can see how close the posi unit is to the inside wall of the case.





FACTORY posi units (originally made by DANA) for the early Vette are becoming scarce and expensive. But there is an alternative. Eaton is now making posi units for the 55-64 pass car/56-62 Vette rears. As you know, Eaton has been making posi units for GM for a loooooooooooooong time, and they are excellent posi units!!!
My recommendation would be to buy a new Eaton unit to convert a NON-posi rear, or, even to replace a factory posi unit that is broken or junk.
I've done several of these eaton installations and they are an EXCELLENT option.
Below are photos of factory (top) and Eaton (2nd-3rd) posi units.

 

Thanks DZ..

I picked up a 3:36 open third member at the Pate Swap meet for $50. Thought it would be good to have a spare ..even if it wasn't a posi. My posi carrier is a 3:54..

Do you happen to have a pic of a case that still has teh usset you mentioned..I'm not sure I see the portion that has been removed. (I can't look at one of mine either since I'm out of town.)

How much is a Eaton posi..?

Thanks...Stan

 

Stan,
I do not have an empty case to show you.
But, if you will take a close looke at this picture, you will see an angled protrusion that is just above a squared off protrusion. That angled protrusion originally continued on up to the end of the case where the bearing cap mates to the case. There is a portion of that angled protrusion which has been cut away and ground down smooth. That angled protrusion is ONLY in a NON-posi case and a portion has to be removed to install a posi unit.

Recently, about the best price I've seen on the Eaton posi unit is around $450.

 

All rightey then, Tom..

Now I see where you are talking about. I think my $50 case has been ground down already...I think I remember seeing that protusion and it looking like that.

The guy I bought it from said all posi cases have that protusion. He was wrong I guess, unless he meant that the protusion looked like that if converted for posi use, or maybe he's just gettin' forgetful like me...

Thanks for all the information..

Best regards...Stan...

 

Stan,
TRUST ME, the posi (P) cases DO NOT have that angled gusset. That angled gusset was cast ONLY into the non-posi cases. Although, all of them do have that squared off protrusion--------------that helps to deflect oil down into the gears. Works kind of like the oil drippers on the underneath side of later model Vette valve covers.
And, additionally, the 55-56 cases (they're thinner, weaker) also do not have that gusset.
I've been building these rears for over 40yrs.

 

Right Tom...

I understand, and surely know that your information is correct...I wasn't questioning your knowledge.... especially compared to Bubba at the swap meet trying to sell something..

What I was saying is that I bought a non posi third member, that I think has been altered already. (like your pics), it however does not have the posi assembly.

Thanks again..Stan..

 

[QUOTE=DZAUTO
I've been building these rears for over 40yrs.[/QUOTE]
I'm hoping with your experience that you can ID this 3rd member for me, please.
It's definitely a 3.55 posi but the housing doesn't have a "P" of either style. The casting # (on the LR/ driver's side) is 3707306 with GM 3 nearby.
On the right side ther is a stamping that appears to be ACJ28.
Can you tell if this is Corvette and possibly the year ?

Thanks, Joe

 

3707306 is the '56-only center section. "AC" is a 4.11 open passenger car ratio code, and the J28 is October, 1958 - probably a service replacement.

 

The CASTING date on the driver side will (or should) have the YEAR.
The STAMPED code on the passenger side only contains the Mo-Day. Thus the J28 is Oct 28 (thererore, it is most likely that this case was cast in Oct 55 for an early production 56 pass car).
Take another look and see if you can find a casting date near the 3707306.

Joe,
You got a picture

 

Thanks JohnZ & DZ. I think that J might be a 9 in front of the 28. I won't get another look at it for at least 10 days - it's at my shop and I'm away. I was really curious to see if it was an original Corvette and apparently it's not. At some point I'll see if there are any stamped #s on the left side. Thanks again. Joe

 

Numbers are STAMPED only on the pass side. Any and all numbers on the driver side are cast.

 

Oops - brain fart.