The Official 1LE 3.91 gear swap thread

[Pro Stock John]

Thanks!

[spd2612]

I have done allot of gears in my previous life and needless to say I have had a couple come backs Either from a defect, just getting in a hurry or someone deciding to tow a trailer before putting any miles at all on the new gear set.

I just did my rear in my SS with the 3.91 1 LE gears and noticing the gear cut was different I did some research and posted the results below.

And I might add that doing your own ring and pinion with the 1 LE gears can be done by a novice, it is time consuming as you always want to take the time to check and re check. If it can go wrong it will..

The 2 cut gear is the reason allot of these installs are being successful

Pinion Bearing Pre-Load

Rotational force with no ring gear installed

15-25 in lb spec for new bearings

9-15 in lbs on used bearings

Go to far on the crush sleeve then replace sleeve and start over it cannot be backed off.

Backlash on AAM OEM Gears

.003 to .005 ( It is tight because these are 2 cut gears ) NOT 5 cut like most aftermarket gears which will have there run settings marked on the pinion.

Pattern on 2 cut gears

Will give a more diamond shape pattern which should be center on the ring between toe and heel and face and flank that is how you can determine if pinion depth and backlash are correct.

Difference between a 2 cut gear and a 5 cut.

2 cut gears are the same height at the toe ( Inside Of Gear ) as it is at the heal ( Outside of Gear )

5 cut tooth height is shorter at the toe (inside of gear ) and is taller at the heal ( outside of gear ) ....

With this said you can see how a 2 cut gear would not be as problematic with pinion depth because of the lack of the additionl taper in the ring gear as it meshes.

[2SS2010]

Originally Posted by spd2612

I have done allot of gears in my previous life and needless to say I have had a couple come backs Either from a defect, just getting in a hurry or someone deciding to tow a trailer before putting any miles at all on the new gear set.

I just did my rear in my SS with the 3.91 1 LE gears and noticing the gear cut was different I did some research and posted the results below.

And I might add that doing your own ring and pinion with the 1 LE gears can be done by a novice, it is time consuming as you always want to take the time to check and re check. If it can go wrong it will..

The 2 cut gear is the reason allot of these installs are being successful

Pinion Bearing Pre-Load

Rotational force with no ring gear installed

15-25 in lb spec for new bearings

9-15 in lbs on used bearings

Go to far on the crush sleeve then replace sleeve and start over it cannot be backed off.

Backlash on AAM OEM Gears

.003 to .005 ( It is tight because these are 2 cut gears ) NOT 5 cut like most aftermarket gears which will have there run settings marked on the pinion.

Pattern on 2 cut gears

Will give a more diamond shape pattern which should be center on the ring between toe and heel and face and flank that is how you can determine if pinion depth and backlash are correct.

Difference between a 2 cut gear and a 5 cut.

2 cut gears are the same height at the toe ( Inside Of Gear ) as it is at the heal ( Outside of Gear )

5 cut tooth height is shorter at the toe (inside of gear ) and is taller at the heal ( outside of gear ) ....

With this said you can see how a 2 cut gear would not be as problematic with pinion depth because of the lack of the additionl taper in the ring gear as it meshes.

Any sepcial tools needed other than dial indicator and torque wrench?

[Pro Stock John]

Had mine installed by Speed Inc. I was there the whole time and am writing it up for an article. Daniel what backlash did you shoot for?

[spd2612]

Originally Posted by 2SS2010

Any sepcial tools needed other than dial indicator and torque wrench?

A press or they do have hand held gear pullers and I would definitely have shims and a caliper on hand also shim kits are cheap the carrier shims are the same as a 12 bolt or 8.8

[danielblack60]

Sorry for the late reply. Gm specs on backlash are .003 to .010 but .005 to .007 is preferred. I usually shoot for 4-5 on high performance applications especially with aluminum housings because on a hypoid cut gearset the ring and pinion try to separate from each other under a heavy load.

[golfking]

Hey Daniel I just received a set of 3.91 gm gears and I noticed the pinion doesn't even have a pinion depth setting stamped on it . Also no markings on the ring gear but I am assuming the backlash spec in the gm manual shows .005 backlash as you said earlier. I have assembled 8-10 rearends before but never dealt with aluminum center section. The carrier bearing preload I found in the Manual shows adding around .006 to the housing opening to gain preload with an added torque spec of 10 inch pounds of pinion drag. That all sound about right ? I'm a little gun shy. Just went threw my 4.33 junk gears from Richmond on my first past attempt rounded all the gears off both ring and pinion so now I will be installing these 3.91 gm gears but other than the pinion shim I didn't record the shims originally in the housing when I removed the 3.45 gears any input or tips would be greatly appreciated

[Tenring]

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[h018871]

What is the torque for the rig gear bolts?
is it a stepped torque or go to spec once the gear is seated?

thx in advance

[robbiegordon]

It Depends on which axle you have but you should be able to look at yours and see pretty easily ... Hope this helps

[h018871]

Originally Posted by robbiegordon

It Depends on which axle you have but you should be able to look at yours and see pretty easily ... Hope this helps

Perfect, Thx Robert. I have the 218

[robbiegordon]

also I put the ring on the BBQ and the posi in the freezer and they fell together ... No need to step torque or press it on then I torque to per the spec I posted above (based on my axel)

[bleavens1962]

Question for guys who have done this swap and have v6 manuals . Any issues starting out in 1st gear ? Thanks

[h018871]

Originally Posted by robbiegordon

also I put the ring on the BBQ and the posi in the freezer and they fell together ... No need to step torque or press it on then I torque to per the spec I posted above (based on my axel)

Robert
How did you spread the case?
I heated the case to be nearly hot to the touch (with a heat gun) and the dif is cold. I'm afraid that I will break the shim trying to push it in