Eaton Posi unit question

[Mark P.]

I am having a posi rear end built to replace the non-posi unit installed in my 1960.

I bought a correctly numbered and dated posi 3rd member (originally it had 3.70 gears), an Eaton posi unit (from Sadler Powertrain for $449 if anyone is interested vs $600 at Summit), a US Gear 3.70 R & P and new pinion yoke (from Tom's Differential) and the Master rebuild kit (Timken bearings, seals, shims, bolts, crush sleeve, etc).

We set the backlash at .005 and used a .028 R & P shim to get the optimal pattern on the drive side of gears. Run-out is 2.5 thousands measured in 5 different places. The guy helping me has done done over 100 rear ends from various muscle cars and is a diesel truck mechanic by trade.

Here is my question:

Has anyone heard of having to grind some of the lower inside of the 3rd member case to increase the clearance with the attachment flange on the Eaton posi unit ? There is no bevel on the edge which would have prevented this problem. The clearance is just a few thousands from the lower (bottom) side of the case.

Should we grind a bit out to give 20 thousands clearance or so ?

[Gary C.]

Re: Eaton Posi unit question

Mark, Tom Parson's is probably the best source to answer your question. Tom's a NCRS and TDB member so you can send him an e-mail from the Member's List. Gary....

[Mark P.]

Re: Eaton Posi unit question

Gary - I have sent a PM to Tom Parsons. Hope to hear from him soon.

Tom from Tom's Differentials and a product support technician at Eaton both recommended using a die grinder to give 20 thousands or so of clearance. They said on occasion this needs to be done.

We will cut a 1/16" groove .020" deep in the bottom of the case to give ample clearance.

Mark

[Tom P.]

Re: Eaton Posi unit question

Large, cast iron castings, such as blocks, heads, rearend cases, intake manifolds, etc, etc, are frequently not exactly the same. If you will take notice, all of these kinds of cast iron castings, SOMEWHERE, will have a GM and 1 or 2 numbers. For example GM13 or GM 7, etc, etc. This GM followed by 1 or 2 numbers (really, I'm going somewhere with this) is what identifies the actual mold that the molten iron was poured into at the foundry. As a result of the SAME part (ie a head, rearend case, etc) being cast from DIFFERENT molds, minute variations may occur and these variations are corrected during the machining processes. For those areas of a cast part which are of no significance (casting flash, mold parting lines, etc), the cast part is not machined and thus from one like part to another like part, there may be a slight variation.
For the past 40+yrs, when building a rearend, and using factory Chevy posi units, I've never seen the need to grind any clearance in the rearend case for the ring gear flange. BUT, when installing an Eaton posi unit in a posi (P) case or a NON-posi case, I have noticed that the ring gear flange of the Eaton units are much closer to the edge of the case. Although, so far, I've not needed to do any clearancing for the ring gear flange of the Eaton posi unit. There may be a few cases out there which DO require some minor clearancing.

And next, while we're on the issue of installing posi units in 56-62 Vettes, ESPECIALLY those with NON-posi rearend center cases, there is a particular area that MUST be removed with a grinder. I've not posted that information here in the past (but I have on Corvette Forum and Chevytalk http://www.chevytalk.org/fusionbb/sh.../1/tp/0/all/1/ ), because this is a hardcore NCRS site, and modifications (such as installing a posi unit into a NON-posi case) are heavily frowned upon! BUT, if I'm not mistaken, that will be a major article in the upcoming RESTORER (only because I was asked to contribute that information).

[Mark P.]

Re: Eaton Posi unit question

Thanks Tom. This is just what I was hoping to hear.

[Tom P.]

Re: Eaton Posi unit question

Mark,
This is a recent NON-posi rear that I converted to posi, using an Eaton posi unit.
You can just see how close the Eaton ring gear flange is to the case. It's close, but not touching. This is the area where some clearancing COULD be done, but so far, I've never needed to do it. These pictures are in the link above, but I though I'd break these out since you specifically asked about this clearance issue.

[Roberto L.]

Re: Eaton Posi unit question

Mark,
You can just see how close the Eaton ring gear flange is to the case. It's close, but not touching. This is the area where some clearancing COULD be done, but so far, I've never needed to do it. These pictures are in the link above, but I though I'd break these out since you specifically asked about this clearance issue.

Thanks for sharing this!!

Best regards

[Mark P.]

Re: Eaton Posi unit question

Tom - do you like to install lock washers when bolting the ring gear to the posi carrier ? No washers came with my master rebuild kit but we decided to install grade 8 lock washers.

Lock washers were originally installed I believe and are mentioned here:

http://chevy.oldcarmanualproject.com/trans/56spcra/56spcra19.html

I've heard some rebuilders don't like to use lock washers for fear of an uneven torque load or future lock washer breakage.

I'll post some pictures of the grinding that was done. A very slight radius was made on each edge of the case for the 3/4" or so edge where the posi unit and ring gear rotate near the case.

Thanks,

Mark

[Tom P.]

Re: Eaton Posi unit question

All I can say is that for 40+yrs of rebuilding the 55-64 style rear (56-62 Vette), I've always used lockwashers on the ring gear bolts. I have never seen a failure nor problem due to bolt torque.
They were, and still are, a good rearend design. Many people turn their noses up at the 55-64 style rears, which has an 8.2in ring gear. These same people won't hesitate to built up and run an 8.2in 10bolt rear such as used in the 60s-early 70s cars such as Chevelles, Novas, Camaros.
And almost no one remembers that the 55-64 style rear was installed behind the 63-64 425hp/409 as well as the handful of Z-11s!

[Mark P.]

Re: Eaton Posi unit question

Here is a picture of the grinding done on each side. The edge was just rolled a bit to give adequate clearance.