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Gear Pattern Setting

Now we are ready to test fit the gear in the housing and check the pattern.  Shove the pinion into the housing with the new or old bearing and shim installed.  For a test fit, I did not put the pinion seal in place on the housing, instead I just put on the yoke and my ground down pinion nut on and tighten it up until the bearing slack was all taken up.  Make up your shims ready for a test.  Then pop the carrier with bearings into the housing.  The rebuild kit came with adjustable shims for setting the back lash.  

 

I bought this Ratech tool to check pinion depth.  It is not very accurate at all.  Do not simply slap it in and measure.  Because the pinion head on a GM pinion is not a machined surface it is not perpendicular to the pinion axis.  I took multiple measurements with the pinion at 90 degree increments and averaged them.  Even still, 2 sets of readings produced a scatter up to 0.001".  Since we are trying to set the pinion with +/- 0.002" this is pretty bad.  Using the tool, set to the numbers stamped on the pinion head, the pattern was quite far off.  You really cannot use this tool as an absolute measure of pinion position.  The tool was only useful for comparing changes in pinion height.  I found more than once that when measuring a thicker shim with a caliper that it resulted in the pinion head moving the wrong way.  So the tool is useful that way.  

 

Adjustable carrier shim - thick holders with thin shims to make up a total shim

 

 

Adjustable carrier shim.  I ground a chamfer on the right edge to make it easier to start into the housing.

 

 

Lift the carrier into place and push it forward.  I insert the left shim and then push everything over to the left.  While holding it all with my left hand, I start the right shim pack into place.  I tap it in with a plastic headed mallet.  At this point I make the shim packs so that there is no preload, the shim packs are just snug, not tight.  We are only checking pattern at the point, no need to make the job harder by having to hammer in the shims.

 

 

Carrier in, right shim almost in

 

 

Once the shims are in the housing, keep holding it in place until you can get the bearing caps on or you risk having it drop out suddenly.  Torque these roughly to about 20 ft.lbs for pattern checking, just enough to seat them fully and pull the shims down if they are not fully in.

 

Trial Gear Patterns - 3.73 AAM Gear

As I said at the beginning, I started this with no experience so I made some mistakes.  The biggest mistake was in believing I had a good pattern and then putting it all together and going driving.  I found reading the gear patterns much more subtle than I thought it would be.  The second big mistake was driving too far with a bad set up and then changing it.  Hopefully you can learn from my errors.  You have to remember that my factory gear whined and was not set up right.  Where most people just re-use the factory pinion shim and are happy with the result, this did not work for me at all.  I suspect my housing is out of spec, the hypoid distance is probably off.

The factory manual suggest setting back lash at .005"-.009".  The AAM instructions said, "remember, with back lash, aim for .004".  which was a strange way to list a tolerance.

 

Trial 1

Trial 1 - Coast Side .035" pinion shim, back lash at .005"

 

Trial 1 - Drive Side .035" pinion shim, back lash at .005"

Looking at Trial 1, I now know that the pattern is too deep in the tooth.  On both the drive and coast side, the pattern is well centered, but too deep.   You can tell this by looking at how the pattern cuts off abruptly at the bottom of the teeth.  The was silent up to 90 kph and whined from there until about 110 kph and then went quiet again.  The noise was quite loud but not terrible in any way and only occurred on the drive side, never at all on coast.  The gear break-in causes the teeth to rub on each other and smooth each other to some degree.  Also the surfaces of the teeth work harden and once set will no allow you to fix the gear no matter what pattern you achieve because the tooth hardness varies over the surface.  I gingerly drove it like this for about 40 miles.  This shim was thicker than the stock shim of .029"

 

 

Trial 2

Trial 2 - Drive Side .029" pinion shim, back lash at .0055"

 

Trial 2 - Coast Side .029" pinion shim, back lash at .0055"

Mad at myself for thinking I need a thicker shim than stock, I pulled it all apart again and changed back to the stock shim of .029".  Most people do this and it works fine.  The pattern is higher up the tooth as expected.  So the the noise was better but not perfect at all.  Noises occurred in the same speed range.

 

 

Trial 3

Trial 3 - Drive Side .029" pinion shim, back lash at .004

 

Trial 3 - Coast Side .029" pinion shim, back lash at .004"

 

 

This time I tightened up the back lash and the it improved the situation even more but it was not what you would call silent.  So now I called AAM tech support and after a long delay (I had to bully them) they responded that based on the above photos, my pinion was too far out, I needed a shim between the two I had tried.  So I tried again with Trial 4.  By this time however I realized I had put on about 500 miles total on the new gears, oops.

 

 

Trial 4

Trial 4 - Coast Side .031" pinion shim, back lash at .005"

 

Trial 4 - Drive Side .031" pinion shim, back lash at .005

It takes a bit of staring but the pattern is the best one so far.  It is deeper than Trial 2 but not too deep.  It still makes noise but is the quietest yet.  It does not meet my personal standard.  I am pretty sure it cannot be fixed since I drove too far with the gear in the wrong set up too many times.  I drove on it for the season and after about 6000 km it was quieting down ever so little.  I decided to try again over the next winter and go to Motive 4.10:1 gears.

Goto Motive 4.10 Gear Pattern Setting