I plan on rebuilding a 12 bolt rearend (adding new gears, posi, axles etc.) and I have a couple questions after doing some reasearch:

1) Why is it necessary to measure pinion depth (and spend money on a pinion depth tool) if the amount of shims you use on the pinion is ultimately dictated by the wear pattern of the gears (after applying that paint to the gears)?

2) In using a dial indicator to check the backlash, where on the tooth do you position the tip of the the dial indicator? It seems that if you were to position it on different spots on the tooth of the gear you could get a different backlash reading.

So, closer to the heal or toe (or right in the middle)?

I know not near the root, but exactly how close to the edge if the gear tooth? If someone had a picture that would be cool.


Thanks in advance guys

The pinion depth gauge gets you close otherwise you could spend a lot of time and crush collars before you find the right one. As far as backlash you are measuring the play between the gear set I put it on the outside top of one of the ring gear teeth. If you measure the pinion depth right you should not have to change the shim just adjust the side bearing shims untill you get the right backlash and then you can run a pattern and check the tooth contact. If you are unsure how to set up a rearend I would advise you to get a GM service manual and read though the procedure it can be very frustating if you don't know how and there is very little room for error.

Using a pinion depth tool gives you a very close idea of the shim to use. Not using one would require you to guess where to start. Pressing pinion bearings on and off is a job. Aftermarket gears have the pinion depth engrave for you to use. Mount the dial indicator at the top of the housing over the ring gear. I like to measure at the top of the tooth. Getting straight on (or parallel with the pinion) to the tooth will eliminate deflection of the dial indicator and give an accurate reading.

The best place to start is with the original pinion shim that was on the pinion that came out of the housing. A 12 bolt is usually around .037 on the pinion shim, so you can start there if you don't have the original shim. As for the pinion depth gauge, I got rid of mine a long time ago. The contact pattern is the only accurate way to set the pinion depth. Always set the pinion bearing preload to the correct amount each time you do a trial assembly. Don't install the crush spacer until the final assembly, and use the old pinion nut for the trial assemblies. I'll email you a picture of the correct way to set up the dial indicator. I'll need your email address.

Thanks for all the replies guys!

Big gear head, here's my email address: JacobM99@aol.com

Also, would you mind answering a few more questions since it seems that you're the authority on rearend rebuilds on the forum?

1) Pinion depth: In setting the pinion depth, when doing the trial assemblies, I should use the original pinion shim, the original pinion nut, and no crush sleeve, and set the preload to the specified amount. The preload is supposed to be set at 24-32 in/lbs of preload, correct? Also where can I find the other torque specs (caps, ring bolts, etc.)

a) I read somewhere that instead of pressing the pinion bearing on and off everytime you add/remove pinion shims when adjusting the pinion depth you can hone the inside of the old bearing and slip it on and off for the trial assemblies until you get the correct pinion depth. Is there anything wrong with this process?

2) Backlash: Should I reuse the same shims and return them to their original positions as a starting point to setting the backlash, and then adjust the shims as needed, or should I use new ones and/or distribute the shims in some other fashion such as an equal distribution to either sides?

3) Bearing removal/installation: Should I leave the bearing removal and installation to a rearend shop or is there a tool (perhaps an in expensive and compact press) that I can use/rent to do the work? I am not only referring to the pinion and carrier bearings but also the axle bearings.

That's all for now, I'm sure I'll have more questions later.
Hope you don't mind.

Thanks,

As for the pinion depth, start with the origianl shim, but it will probably need to be adjusted as much as .005 to .010. Be patient and get it right. Don't settle for close enough. The correct bearing preload is 19 to 22 inch pounds rotating torque. When the pinion depth is correct install the spacer, seal and the new nut. As for using the old pinion bearing for the trial assemblies, I don't recommend it. The old bearing is going to have wear on it and it will not be the same thickness as the new bearing. We are talking about getting the pinion depth correct to within +/- .001 inch, so you need to use the bearing that will be installed on the final assembly. Honing out a new bearing would be better, but I still recommend using the bearing that will be run in the rear end.

As for the backlash, if you will be using the original differential again then the original shims might be a good starting point, but be sure to use the thinnest shim on the ring gear side so that you don't jam the ring gear into the pinion and damage it. If you will be changing the differential then you are starting from scratch and you need to use the shims from the installation kit. Start thin on the ring gear side.

The differentail bearings will only have to be removed and installed one time, so having someone do that for you isn't a problem. Be sure that the new bearings are completely seated when they are pressed on. I have seen people install these bearings and leave them about .020 from being seated. This WILL cause problems later. The pinion bearing will need to be removed and installed several times, so that might be a problem. If you have no other choice then you might want to get another new inner pinion bearing and hone it for a slip fit. Be sure it is exactly like the one that you will use in the findal assembly (same brand and partnumber). You can "borrow" a tool from Auto Zone to remove the axle bearings.

Torque the bearing cap bolts to 60 foot pounds and the ring gear bolts to 45 foot pounds. Use Lock Tight on the ring gear bolts and pinion nut. I'll email the picture.

Absolutely awesome!

Thank you so much BGH.

BGH
If you were buying 3.73 gears for your 12 bolt Chevy, what brand would you buy? I've been looking at Strange (US Gear).
Thanks
Jim

I usually install US Gear or Motive Gear. Those are the brands that my supplier handles most. I have installed every brand available at some time and I haven't found much difference in them. The installation kits vary a lot in quality from one place to another.

Thanks for the reply. I plan on using Timken bearings and GM hardware.
Jim

There are installation kits that have very good parts in htem. They are cheaper than buying the parts seperate. Send me an email if you want one of these kits.

Thanks BGH
I'm not planning to do this for a little while. My wife has it very low on the priority list but I will certainly call you when I'm ready.
Thanks
Jim