posi install difficulties

[Alex Hayley]

I am replacing the open differential with a posi unit while reusing the original ring gear and pinion.

I put the new pinion bearing on the original pinion shaft with the original shim. Installed the inner pinion race in the housing. Set the pinion in
the housing with the correct preload(15 pounds/inch). I Installed the original ring gear on a new posi unit. Then I installed the new posi unit with new
bearings in the housing with the original shims. The carrier would not turn once the bearing caps were snugged down loosely. The posi unit did not
seem to seat properly in the housing with the shims installed. With all shims removed on the side opposite the ring gear, the posi unit will seat but barely turn. Thinking that the position of the pinion was causing the binding, I removed the pinion, put the posi unit back in with all of the shims and it turned just fine. Could the posi unit be placing the ring gear too close to the pinion? I was under the assumption that the posi unit would sit in the same position allowing the use of the same ring gear and pinion. Is there something obvious I missed? Could there be a problem with the pinion?

[71chevylowrider]

what posi unit did you install? Did you mark the teeth on the gears with gear paint to see where they are meshing?

[cooters]

when changing center sections, you will not likely use the same shims. Use a die grinder on the inside of your old bearings so they slide on and off easy and find the shim stack you need to get the correct pattern. Do you have the correct series of posi ( 2,3, or 4 series) for the ratio of the gears?

[EFI70Blazer]

Your backlash is probably off now. You will need to buy a shim kit to adjust the carrier to the right or left to get the proper .008 - .010 backlash setting. To check this you will need a dial indicator with a magnetic base. When you get your shims measure each carrier shim and try to add the same amount back in (total of both shims). You will have to put in less on the drivers side if your carrier is too tight and more on the passenger side. If your carrier is too loose add shims to the drivers side and and take some away from the passenger side. Basically you want to keep the same amount of total shim thickness, but just swap shims side to side to get the correct backlash. When you get the proper backlash you will need to check your tooth contact patern to make sure your pinion is not too deep or too shallow. The ideal pattern to have is the contact pattern being the same length in the root of the tooth as the pattern in the top of the tooth. For new bearings, the drag on the pinion bearings should be 25 inch/lbs and 15 inch/lbs for used bearings. Just a little bit of advice, do not install the crush sleeve, locktite, or pinion seal until you get done getting your contact pattern correct and your backlash correct.


Good luck. If you have more questions feel free to ask.

[EFI70Blazer]

Originally Posted by cooters

when changing center sections, you will not likely use the same shims. Use a die grinder on the inside of your old bearings so they slide on and off easy and find the shim stack you need to get the correct pattern. Do you have the correct series of posi ( 2,3, or 4 series) for the ratio of the gears?

If you are doing a Dana setup then grinding the inside of your old bearings is a good idea to keep from having to pressing and pulling off your bearings to change backlash. This is only a good idea if you are using new bearings for your final setup.

If you are doing a 10 or 12 bolt chevy setup the shims are on the outside of bearings and this step is not needed.

This process could also be used for pinion depth when you will be using new bearings for final setup.

[cooters]

I should have specified grinding bearings for the pinion. Alex did say he has new bearings.

[Alex Hayley]

Thanks for the great feedback! There was a lot of information I inadvertently left out. I am replacing the open differential with an Eaton posi in a 12 bolt truck axle. I have not even been able to mark the teeth with compound because if I have any shims beside the posi unit(on the side opposite the ring gear) it will not seat properly or turn. When I purchased the Eaton posi I told the person on the other end of the phone what I had and he matched me up with the correct unit. I hope a mistake was not made there. I had a 3.73 rearend and am sticking with the same ratio. Is this a series 3 or 4? I have noticed in the Summit Racing catalogue that there is a 3.73 unit for both a series 3 and 4. Perhaps I have been sent the incorrect unit. If there is any other information I have missed please let me know.

[71chevylowrider]

I think maybe Cooters hit it on the head, a series 4 carrier starts at 3.73 & goes up, a series 3 starts at 3.07 & can go up to a 3.73, but.....I think you have to do/buy something to make it accept a 3.73, I know EFIblazer knows his stuff on these (I learned alot of what I know from talking with him) so hopefully he can shed slome light on what's nesseasary to make a series 3 carrier accept a 3.73 ring/pinion

[Alex Hayley]

How do I determine if I need a series 3 or 4?

[71chevylowrider]

a serie 4 carrier will bolt right in with a 3.73, but I know I've heard with a series 3 you had to do/have something different, sorry i don't want to guess, here's a very good right up, look down at references #15 & down.

http://www.hotrod.com/techarticles/c...ntial_install/

[71chevylowrider]

Dude, I'd call this place, Tom's knows everything about these, & here's a section of their website that shows a series 3 goes from 2.76 to 3.42 but a sereis 4 starts at 3.73

http://www.tomsdifferentials.com/catalog.asp?pg=12

[cooters]

double check your part number to see what you got for a carrier. If you got the wrong one sent then return it.

[cdowns]

are you sure the ring gear is seated all the way down onto the carrier?? i always heated the gear in an oven at 400degrees or so for an hour to allow it to expand for proper seating on the carrier

[Alex Hayley]

The part numbers I have are:

original unit 3866118
new unit 19556-010

The new unit is a series 4. I believe that the old unit is a series 3. Can anyone verify that for me? I have attempted to search for something that says so but no luck so far. If the original unit is a series 3 then I have an obvious problem.

[Alex Hayley]

I am sure the ring gear is seated correctly.

[EFI70Blazer]

Originally Posted by Alex Hayley

The part numbers I have are:

original unit 3866118
new unit 19556-010

The new unit is a series 4. I believe that the old unit is a series 3. Can anyone verify that for me? I have attempted to search for something that says so but no luck so far. If the original unit is a series 3 then I have an obvious problem.

If your old ring and pinion was 3.73's then your open carrier was a 4 series truck carrier. Unless somone put aftermarket thick gears on a 3 series carrrier.

Could you take some pics of your ring gear part numbers and your old open carrier. To veryify that you do have 3.73 gears count the number of ring gear teeth and divide by the number of pinion teeth. Also maybe take a pic intalled so we can see what is going on.


Looks like your new carrier is correct for 3.73 gears and up. It is a 4 series for a truck 12 bolt rear. Her is a link from Summit.

http://store.summitracing.com/partde...5&autoview=sku

[EFI70Blazer]

One other quick thing to note. Car 12 bolts and truck 12 bolts are totally different. 3.73's on a car 12 bolt call for a 3 series carrier and 3.73's for a truck call for a 4 series carrier. The two may look similar, but are not interchangeable. Gears for both are also different. Car 12 bolts have a larger pinion shaft, making them beefier than a 12 bolt truck rear.



I actually just got done doing the rear on my 70 Blazer. I changed the rear axel out with a later model one. I welded in new perches offset back 1 inch so my tire's were more centered, relocated my shock mounts, sanblasted the whole assembly, rebuilt an older eaton posi to use instead of the open one i had, changed my gears from 3.73's to 4.10's, put in new axels and new brakes. I have to the front end still, but have not had time lately to do so.

Here is a pic of the final product. I should have taken my pics during my teardown and install but i did not.




Here is a pic of my old 3.73 pinion. It has 11 teeth and 41 teeth on the ring gear. The ring gear should have a stamping on it that says 41:11 telling you what gears you have for sure.

[71chevylowrider]

Sorry to highjack this thread, but while mike is in here...hey Mike? His old one has a different number then my 3.73, mine is 3817778 does this mean anything? mine says 11:41 so that's a 3.73 right?

Oh, & I love the way you did your rearend, looks awsome! Maybe when I get ready to do my series 4, I could buy you some beer & pick your brain some more?

[EFI70Blazer]

Originally Posted by 71chevylowrider

Sorry to highjack this thread, but while mike is in here...hey Mike? His old one has a different number then my 3.73, mine is 3817778 does this mean anything? mine says 11:41 so that's a 3.73 right?

Oh, & I love the way you did your rearend, looks awsome! Maybe when I get ready to do my series 4, I could buy you some beer & pick your brain some more?

Yes, 11:41 is for a 3.73 rear my new ones had 10:41 stamped on them which is for a 4.10 ratio. Usually both the ring and pinion will have a matching serial number so you know that they are a matched set unless they are factory gears. Most aftermarket should have matching serial numbers though. As far as open carriers go am not real sure, because my part number for my old carrrier is different than both of yours, except mine is out of a 77 Chevy. There may have been some design changes in there. The best way to tell is by measuring. Here are some pics to illustrate.


In picture 1, left hand side is a 4 series open carrier. The measurement to the ring gear mounting flange is about 2 inches.



In picture 2, right hand side is a 3 series posi carrier. The measurement is about 1 3/4 inches. So there is about 1/4 difference between the two.




Picture 3 is just showing the pinion gear stampings for a 3.73 ratio.



Picture 4 is showing the ring gear stampings for a 3.73 ratio.




Picture 5 is showing a 10 bolt carier with a 4.10 ratio. Notice how thick the ring gear is. This is how they make thick ring and pinion set to fit on lower ratio carriers. So, if you want to keep your 3 series carrier and install lower gears on a 12 bolt rear end, you would get a thicker ring and pinion set or a ring gear spacer to make up the difference.




I hope some of this helps.

[Alex Hayley]

EFI70Blazer, this has really helped. In fact, it has made apparent a very dumb mistake I have made. So dumb in fact that I am almost too embarrassed to continue this post but I might has well so others do not make the same mistake. I checked my ratio a couple years ago by simply jacking up the back of the truck and spinning the wheels while counting the rotations. It appeared to be 3.73. I guess I was so convinced that I had performed the check correctly that I did not not even bother to count the teeth to verify that I did have a 3.73 before I ordered the new posi unit. I counted the teeth yesterday and have 43 on the ring and 14 on the pinion which is a 3.07. So what I have is a new series 4 unit which I am attempting to use with an old but clean series 3 ring and pinion. Obviously this will not work. Can I simply order the 3.73 ring and pinion and expect it to correctly fit in my current housing? I am hoping for a "yes" but prepared for "no". If "no", what are my possible options? TO EVERYONE, MEASURE TWICE(or even once correctly), CUT ONCE or you might end up in my position!!!!

[71chevylowrider]

EFI70blazer know his stuff, I have learned alot from talking to him........Mike YOU ROCK!
There's always someone looking for a series 4 Eaton Posi "Limited Slip" maybe you could sell it if you can't return it & then buy a series 3.

[EFI70Blazer]

You can either go two routs. Return the posi unit and get a 3 series unit or get 3.73 gears for the 4 series that you have now.

[EFI70Blazer]

Almost forgot to mention, when you set up your gears it is usually never the same as the original ones. Sometimes everything works out, but out of 15 that i have set up only 1 worked out without having to change at least one thing. Installing your original pinion shim gets you close, but most the time you do have to change the carrier shims to get the correct backlash. Basically it is just trial and error until you get the right pattern.


Here is a nice little cheat card to verify correct pattern when you get to that point.