I am replacing a GM 4:56 with a GM 3:73. All the parts are used. I realize that used Gears are frowned apron and If I took it to a shop they probably not want to work with the parts I have. This car is a restoration and won't see much aggressive driving. The majority of its future millage will involve a trailer.
I don't have the Pinion depth setting Tools and if I did the face of the Pinion Gear is so poorly machined I'm not sure it would do much good. There are GM numbers stamped but none of the look to be pinion height. I started out with the 0.32 Shim that was on the 4:56 and I had 0.15 Backlash. Slowly adding shims I'm now at .050 and I'm down to 0.06 Backlash. But now the Pattern on the Gears is way out at the ends and looks terrible. I have read a lot about setting the pinion height but not much about moving the Shims left and right on the carrier. For what its worth I checked the 4:56 Gears in what looked to be a fresh rebuilt never driven rear end and they looked terrible too.
So I guess my question is what what effect does Pinion height have Vs Carrier Shims. I realize that this is not an ideal situation I'm just trying to make the best of it. If you haven't figured it out this is my first time setting one these up. Like most people is the only place I have not ventured before.

 

Can you put a 3 series gear on a 4 series carrier?

 

You are opening a huge can of worms here. Keep the 4.56 gears for what you intend to do with the car.

 

Some thoughts.
Good that you're going to rebuild it but you're lost here. Sorry I am not trying to insult you in any way but everything you listed is concerning and is not going to work as you are approaching it.

GM 4 series gears require a 4 series posi case. GM 3 series gears require a 3 series posi case. Now if you had a 3 series case and wanted a 4 series gear without changing to the correct case the old 60's trick of using a spacer could be used but they often failed when the ring gear bolts loosened up.

Tom Watt had US Gear make a 411, 433, 456 gear set that had thicker ring gears and would bolt onto a 3 series case without issue, I used a lot of them.

The difference between a 3 & 4 series case is the thickness of the flange.

GM gears used pinion shims in the range of 028-032 most of the time. Setting up 456 or 373 is the same, the principle of gear setup don't change. I have never used a pinion setting tool in any of the 100's of vette diff's I built. I go by pattern and experience. The housing machine work varies from one to another. The old trick of laying a parallel across the pads and measuring to the pinion for depth it misleading since it depends on the pads being at the diameter of the bore of the housing and that isn't the case a lot of times. It's not off much but 010 is a mile in setting up a diff.

It sounds like you are trying to dial in the lash by increasing the pinion shim size and that is not correct. The only vette diffs that I found requiring thicker shims, say in the 035-040 range were some 12 bolt conversions I setup and those are all custom hand fit. Yours should not be anywhere close to that. You need to go back to the 032, set the drag between 17-20, add side shims to snug up but not totally preload the case bearings, and adjust the side shims until you get to 006-008 lash and then run a pattern. Anything else is going to fail.

It doesn't matter if the car is only going to be rolled on and off a trailer, someone at some point may actually want to drive it someday and it won't be very much fun to see a nice, "correct" car lock up the diff 15 feet down the street.

The best manual is the GM overhaul manual. If you don't have one get one, even the old chiltons had a chapter on diff setup. These are the books from the mid 70's. I have a couple of them and they are very good, not the junk that was later published.

Good luck.

 

If you need a chassis overhaul manual here is a link to mine:
https://drive.google.com/file/d/1W3r...ew?usp=sharing
It is a big 200MB download, so rather than use up forum bandwidth, I put it in my google drive.

The only thing I would add is do you have the original pinion shim? The one under the pinion bearing? Was that the .032" shim? I would start with that one. Especially if both of the used gears are GM. Then leave that alone and change carrier shims until the backlash looks good.

 

I guess I lied or in todays political world I mis spoke when I said I had all GM parts. When I removed the 4:56 I also removed the Spacer Ring that was with it. I guess this means the 4:56 is an aftermarket after all and that I have a 3 Series Case.

 

So I think I need to go back to where I started out at about 0.30 shim on the pinion gear. This will improve the pattern but also increase the backlash. Can I work on the backlash with Carrier Shims ?

 

this should help with your pattern.

 

Thats a very simple and direct way to see what to do.. As I added Shims to the pinion I went to a Heel Contact just like the diagram shows. So I'm going to go back to where I started and work from there. Thank You.

 

You need to follow the procedure I laid out above but this video series is about the best I have seen on YT. Make sure to view the whole series including the one on case runout. I have had to machine posi cases more than a few times.

 

First of all, you dont set backlash with the pinion depth. You set the pinon depth, then adjust the backlash with carrier shims (they're on the outter edge of the carrier bearings. You add/subtract to them to move the carrier left/right, which adjusts your backlash. Now, if you do change your pinion depth, it will affect your backlash, to which then you'd adjust via the carrier shims.

The pinion depth is set either via the dimensions etched into it, along with measurements of the carrier depth. It takes special tools to measure/adjust. Or you can do trial and error. 1) Set pinion depth, set backlash, mark the gear with marking compound, and then check the pattern (both heel and flank). Then start at 1) again if it's wrong.

 

Don't you think that over the course of 1 year the OP may have finished his task?

 

Sure hope so! But I think other people looking for advice would want the info.

 

I did get a pattern I was happy with. But I am also interested in more information for the next time I do one.

 

Gee, maybe a search on 'GTR1999' would give all the info you ever wanted. He has huge write-ups out on the web.