C1 - Rear Axle Open to Posi Conversion

[Joe P.]

I have an open 4.11 rear end, that came from a 64 passenger car now in my 60. I was thinking about swapping in a 3.7 posi, proper dated Corvette third member.

Being the not so mechaninally inclinded individual, and learning so much thanks to the input from fellow members, and searching this vast knowledge base.

I need some advice, or suggestions.. Is this a change as simple as swapping out the third member?

What else should i be aware of before i undertake the task of hunting down a third member..

Thanks..

Joe

[Don H.]

Re: C1 - Rear Axle Open to Posi Conversion

It should be a simple swap, pull axles, drain and pull. Be aware the axles are different length side to side. Don H.

[Tom P.]

Re: C1 - Rear Axle Open to Posi Conversion

I put this together a few weeks ago for the early Chevy guys.
If I recall, I think I also posted it here.
Here it is again.
http://www.chevytalk.org/fusionbb/sh...hp?tid/212343/

[Joe P.]

Re: C1 - Rear Axle Open to Posi Conversion

I put this together a few weeks ago for the early Chevy guys.
If I recall, I think I also posted it here.
Here it is again.
http://www.chevytalk.org/fusionbb/sh...hp?tid/212343/

Thanks Tom, Great information!!.

How can i tell if the rear axle itself is a passenger or corvette? By swapping out the posi for the non posi, is there any modifications that need to be made the the axle itself.. i read somehere that the axles had to be cut down by 1/2".. and a vent hsa to be installed.. is this true?

I see a difference in pictures where a block and hose comes on the Posi versions that does seem to be on the open rear ends.

Thanks..

Appreciate the CHEV booklet link, great library.. More Old Online Chevy Info.

Joe

[Tom P.]

Re: C1 - Rear Axle Open to Posi Conversion

Joe,
Shortening of the axle shafts ONLY applied to 55-57 pass cars, which ORIGINALLY had non-posi rears and a FACTORY posi was added (originally made by DANA). This also applied to 56-57 Vettes that originally had NON-posi rears and a factory style posi was added.
If ANY 55-57 (car or Vette) NON-posi rear is converted to posi with an Eaton unit, shortening of the axle shafts is NOT required. In 1957, there were two different length axle shafts: posi axles and non-posi axles. The posi axles were 1/8in shorter than the non-posi axles to prevent interference between the end of the axle splines and the cross shafts inside the factory posi unit. That's why when a FACTORY posi is added to a 55-57 car or 56-7 Vette---------------THAT WAS ORIGINALLY BUILT WITHOUT POSI----------------it is necessary to whack off 1/8in from the ends of the axles.
In 1958-later rears (car and Vette) this was no longer an issue and ALLLLLLLLLL axle shafts were the same, regardless of posi or non posi. Sooooooooooo, with 58-later, when adding a posi, there is no issue with the existing housing or axles.
And last, if an Eaton posi unit is added to ANY 55-57 rear, THERE IS ZERO ISSUE WITH AXLES.
In my book, installing an Eaton posi unit in ANY of the 56-62 rears (55-64 for pass cars) is the most practical way to go. Replacement parts are available, they are brand new and not stressed or cracked, they are cheaper, they are a direct fit, axle length is NOT an issue and once installed, there is NO WAY to know if the posi guts are DANA or Eaton!
Yes, yes, yes, if ANY posi unit is installed into a NON-posi case (57-64), a VERY SMALL modification is required of the NON-posi case (partial removal of the gusset as shown above), and that is TOOOOOOOOOO easy!

Oh, and I forgot, yes, the cars that came with a factory posi rear did have an elbow with a length of 3/16 steel tubing installed in the axle housing, in the hole where there was normally a vent installed. Unless it is imperative for a person's peace of mind, there really is no need to replace that vent with the elbow and length of tubing. But if you install a posi, and you JUST GOTTA HAVE that elbow and tubing, then go for it!

[Joe P.]

Re: C1 - Rear Axle Open to Posi Conversion

Joe,
Shortening of the axle shafts ONLY applied to 55-57 pass cars, which ORIGINALLY had non-posi rears and a FACTORY posi was added (originally made by DANA). This also applied to 56-57 Vettes that originally had NON-posi rears and a factory style posi was added.
If ANY 55-57 (car or Vette) NON-posi rear is converted to posi with an Eaton unit, shortening of the axle shafts is NOT required. In 1957, there were two different length axle shafts: posi axles and non-posi axles. The posi axles were 1/8in shorter than the non-posi axles to prevent interference between the end of the axle splines and the cross shafts inside the factory posi unit. That's why when a FACTORY posi is added to a 55-57 car or 56-7 Vette---------------THAT WAS ORIGINALLY BUILT WITHOUT POSI----------------it is necessary to whack off 1/8in from the ends of the axles.
In 1958-later rears (car and Vette) this was no longer an issue and ALLLLLLLLLL axle shafts were the same, regardless of posi or non posi. Sooooooooooo, with 58-later, when adding a posi, there is no issue with the existing housing or axles.
And last, if an Eaton posi unit is added to ANY 55-57 rear, THERE IS ZERO ISSUE WITH AXLES.
In my book, installing an Eaton posi unit in ANY of the 56-62 rears (55-64 for pass cars) is the most practical way to go. Replacement parts are available, they are brand new and not stressed or cracked, they are cheaper, they are a direct fit, axle length is NOT an issue and once installed, there is NO WAY to know if the posi guts are DANA or Eaton!
Yes, yes, yes, if ANY posi unit is installed into a NON-posi case, a VERY SMALL modification is required of the NON-posi case (partial removal of the gusset as shown above), and that is TOOOOOOOOOO easy!

Oh, and I forgot, yes, the cars that came with a factory posi rear did have an elbow with a length of 3/16 steel tubing installed in the axle housing, in the hole where there was normally a vent installed. Unless it is imperative for a person's peace of mind, there really is no need to replace that vent with the elbow and length of tubing. But if you install a posi, and you JUST GOTTA HAVE that elbow and tubing, then go for it!

Tom, thanks for the quick reply and your patience.

Great explanations. Great Pictures..

[Tom P.]

Re: C1 - Rear Axle Open to Posi Conversion

Tom, thanks for the quick reply and your patience.

Great explanations. Great Pictures..

By the way, when you are referring to the rearend of a 53-62 Corvette in the future, it will make it more clear to others if you are more specific about the parts that you are asking about.
For example, there is the AXLE HOUSING ASSEMBLY, which includes the AXLE HOUSING, AXLES (shafts) and the removeable CENTER SECTION, as well as other misc parts such as the brake components. Over the years, the removeable center section has acquired various names such as hogs head, pumpkin, third member, differential, etc.

This is the removable center section.


This is the axle housing (partial view).




And these are axles (shafts).

[Mike M.]

Re: C1 - Rear Axle Open to Posi Conversion

those were great explainations tom. regards,mike

[Joe P.]

Re: C1 - Rear Axle Open to Posi Conversion

Tom, Great of you to share you knowledge with a 'newbie' so openly.. Much appreciated

Joe

[Mark P.]

Re: C1 - Rear Axle Open to Posi Conversion

I am working on the posi conversion for my 1960. I bought the correct dated carrier 3743833P and had it rebuilt using advice from this TDB.

I will check that my axles are the correct length - Left 28-13/16" (3775689) and Right 30-5/16" (3775690).

My questions are:

1. I know that the axle bearings are the same for posi and non-posi, but are the axle seals the same for posi and non-posi cars ? If not, do I buy the non-posi version ? Is the correct part# for the axle seal "GM part no. 3772999" ?
2. Do I need 4 o-rings, 2 metal and rubber axle seals and 2 paper gaskets, 2 bearings and 2 retainers (aka lock ring), the posi differential carrier and gasket and tapping for the axle housing for the posi breather with 1/8" NPT threads to do this conversion ?

Thanks,

Mark

[Tom P.]

Re: C1 - Rear Axle Open to Posi Conversion

Mark,
If your car is a 60, and PRESUMING it also has a 60 axle housing, then FOR A FACT, the axles are correct for EITHER a posi or NON-posi rear----------------it does not matter which third member is installed in the axle housing.
1957 (Pass car or Corvette) is the ONLY, ONLY, ONLY year which had separate axles for a posi and NON-posi rear. Beginning with the 58 models, ALLLLLLLLLLLLLLLLLLLLLLLL rears used the same axle for both posi and non-posi.
Additionally, the seals are PART OF THE BEARING. When a new axle bearing is purchased for a 56-62 Corvette (or 55-64 passs car), it comes with the seal installed in the bearing and O-rings on the bearing, and it comes with a new lock ring. SOME vendors sell the seal----------------BUUUUUUUUUUUUUUUUT, to replace the seal, removal of the bearing from the axle is required. In order to remove the bearing, the lock ring must be destroyed----------------------------NEW LOCK RINGS ARE NOT AVAILABLE SEPARATELY!!!!!!! Thus, the ONLY way to get a new lock ring, it is necessary to buy the bearing. Also, MOST LIKELY, if the axle bearing seal is leaking, the bearing is very probably beginning to go bad (or is already bad).

NON-posi vents for the axle housing are NOT, repeat, NOT screwed into the hole in the housing. The original metal vents have serations and were pressed into the housing (probably lightly tapped in with a hammer). So, as you mentioned, to installed a posi style vent, the hole needs to be threaded with a 1/8in pipe tap.

[Mark P.]

Re: C1 - Rear Axle Open to Posi Conversion

Thank you Tom. That clarifies my questions. I can now see the seal and O rings on the bearing in the photo of the kit.

[Tom P.]

Re: C1 - Rear Axle Open to Posi Conversion

Originally, cars equipped with Positraction, from the factory, had ROLLER bearing rear axle bearings. All other cars got ball bearings. Replacement roller bearings were only available for a very short time and then discontinued. Locating a roller bearing today is nearly impossible and IS NOT worth the price!!!!!!!!!!!!!! The regular ball bearing bearings are perfectly fine and are readily available. For the 58-62 Corvettes (as well as 58-64 pass cars), the industry part number is RW607(with a suffix of BR, NR, etc) series. At one time these bearings were made by various manufacturers (SKF, Timken, Delco, Hyatt, Green, etc, etc). Some have thick O-rings, some have thin O-rings, some have one O-ring, some have 2 O-rings. ALLLLLLLLLLLLLLLLL are totally interchangeable. As long as it is an RW607 series bearing, it fits. I've only seen a FEW Green bearings, but for some reason, many people say they are an excellent bearing.

[Mark P.]

Re: C1 - Rear Axle Open to Posi Conversion

Thank you very much Tom.

[Tom P.]

Re: C1 - Rear Axle Open to Posi Conversion

I have posted this SOMEWHERE in cyber space (more than once) in the past. But, for those who do not have a posi in their rear, and want to retain their original, correctly dated rear--------------but have a posi-----------------------, here is an article I put together several years ago explaining how to convert a 56-62 NON-posi (or 55-64 pass car) rear to posi.

Converting a 55-64 non-pos to posi -Tom Parsons sm.pdf