1967 Eaton Posi Check Information

[Timothy B.]

My 67 convertible has a 327/300 with wide box trans and 3.36 posi. I noticed when one rear tire was off the ground there is not much resistance in the posi stack. The car has 53000 miles so I don't think the posi is wore out but I have thought of tightening it up with a larger shim on each side.

I did some reading about how the posi is set up and viewed the Tom's differential you tube showing how to tune a posi without the four springs. I really just want to inspect the clutch pack and tighten it up so the break away is in spec.

Has anyone ever done this and can make any suggestions as I really don't want to change bearings etc. unless I find problems with closer examination.

Thanks as always, Tim

[Clem Z.]

Re: 1967 Eaton Posi Check Information

My 67 convertible has a 327/300 with wide box trans and 3.36 posi. I noticed when one rear tire was off the ground there is not much resistance in the posi stack. The car has 53000 miles so I don't think the posi is wore out but I have thought of tightening it up with a larger shim on each side.

I did some reading about how the posi is set up and viewed the Tom's differential you tube showing how to tune a posi without the four springs. I really just want to inspect the clutch pack and tighten it up so the break away is in spec.

Has anyone ever done this and can make any suggestions as I really don't want to change bearings etc. unless I find problems with closer examination.

Thanks as always, Tim

do a burn out and see if the pattern of both wheels show the same i would say the posi is OK

[Richard F.]

Re: 1967 Eaton Posi Check Information

Back in the day we used to remove the springs, and shim the plates until you had to drive the pin back in with a hammer. Dont ask me why. Somebody said it was the thing to do.

[Timothy B.]

Re: 1967 Eaton Posi Check Information

I think the you tube video shows if you remove the four springs and shim for the proper drag you eliminate the differential gear backlash so .005-.008 clearance each side in the pin/yoke end is very important. When cornering there needs to be some clearance at this junction so the clutch pack does not chatter.

[Gary R.]

Re: 1967 Eaton Posi Check Information

Tim
As you probably know that is how I build all my posi's. I will be holding a seminar on differentials this year, Saturday at noon, at Corvettes at Carlisle. I will be going over posi polishing and tuning. If you have an original 67 posi then it is one of the early designs and very prone to cracking. If you get into it be sure to check it very close for cracks, if any are found the posi case should be replaced. I pretty much build them like Tom shows as well, I finally built a posi tuning machine to save wear and tear on my hands. Tuning a posi can be done at home but I like to grind my shims to within 001 to dial them in.

[Timothy B.]

Re: 1967 Eaton Posi Check Information

Gary,

I was hoping you would respond to my post. Is it possible to just install a larger shim to the factory set up to bring the break away back into spec. I realize the differential gear blacklash also needs to be taken into consideration when keeping with the stock springs.

What's a good target blacklash spec that will keep the stock set up from hammering in corners?

[Gary R.]

Re: 1967 Eaton Posi Check Information

Hi Tim,
I have found the spring tension varies with the same rated springs used. Some I have been able to pinch by hand and remove, that is not common though. Assuming you have an original 67 differential, with posi, there are some things to check if you remove it.
1- The case again for cracks. The 65-68 castings were the postage stamp type and were not the best. Check the case side holes for steps or loose fitting shaft.
2- Spiders, these used the weaker 10-18 spiders. If you want the best you should switch to 10-17 heat treated.
3- Clutches, these used the better solid steels. Mic them up, they should be in the 068-069 range. Look at the pattern on the faces, you may see wear rings in them.

If you check the parts and reuse them you will be able to shim them tighter. New clutches have to be seated to properly setup the lash or tune. The spider lash spec is 001-006" however with a properly tuned posi the lash will be no more then 0015-002". The machine work varies from one casting to another. Equal shims per side are not always what is required that is why I have to grind them most of the time.

If you have the time and want to give it a shot go ahead. This will not affect R&P lash or pattern - unless you remove the RG from the case. Even then as long as you witness mark it and relocate the RG it will be very close. Once you get into replacing the bearings, crush sleeve, gears you will get into the pattern dial in which proves to be a frustrating thing at times.

I also have some of my own mods that I do but the Tom's video is what you need to watch.

A lot of rebuilders like to sell a new loaded Eaton posi to people when they need a new case. I prefer not to go that route. Talk about spider lash, the last new loaded case I had here I removed the springs to check the lash and found 030". Yeah those huge springs will tighten that up some but it's not how I wanted it.

The spring tension along with the spider lash will determine if they will hammer at corners. Just like 4x4 rock climbers, in a striaght line they are great, take a corner and chatter your teeth out of your head. A tuned posi will not do that, the wear will be much less without constant pressure from the springs, it will slip enough in corners, and should last longer. You can find very few who build them like that due to the lack of experience or added labor cost.

I do not use fiber coated or slotted (snowflake) clutches. ALWAYS seat new clutches, use GM posi additive,and I use Lucas 85-140 gear oil.

[Timothy B.]

Re: 1967 Eaton Posi Check Information

Gary,

Thank you for the very useful information. I have not decided if I want to tune the posi without the springs but it makes sense to me, I will make that decision when the parts are checked for condition. When you say a properly tuned posi should have spider blacklash in the .0015-.002 are you refering to the unit tuned without the springs?

I also plan on being at your seminar in August.

Thanks again..

[Gary R.]

Re: 1967 Eaton Posi Check Information

Tim,
You will see in Toms video he states "no backlash" in the spiders. Doing these for the first time can be a little challenging because you have to develope the Feel for the job. There is very little lash with these setups. The stock procedure is to shim the spiders to get in the 001-006" range then install the springs and plates. If you get into the 001 range and use springs chances are you will have chatter in the corners. Tension will be too tight and the clutches will not slip enough causing the chatter.

[Timothy B.]

Re: 1967 Eaton Posi Check Information

Thanks Gary, I just wanted to be clear on what's acceptable with the stock four spring set up.

[Gary R.]

Re: 1967 Eaton Posi Check Information

Tim
I would still keep the lash with springs in the 002-003 range rather then looser. I don't like a lot of lash in spiders.