8.75, 742 Pinion Preload Question

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ValiantOne

ValiantOne

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This is the first time I have set the Pinion Preload on a 8.75 rear (742 case). First I tried .040 worth of shims as that was the thickness of the original shim pack. (Talking about Preload shims, not Pinion depth shims)
.040 was too tight even when only torqued to 100 lbs. So I added another .004 thou. The resulting .044 shim pack was perfect at 20 inch lbs of pre load (on new bearings) torqued to 240 foot lbs, with no Pinion seal as stated in the FSM.

I also did all this with the original Pinion nut as the new nut was a squish nut and I didn't want to wear it out.
After disassembling, installing Pinion seal and reinstalling Pinion, torquing to 240 lbs with new squish nut, spinning bearings and then rechecking torque, my preload had climbed to 40 inch pounds.

Jumping from 20 to 40 inch pounds because of the seal friction seems excessive to me. Can any of you rear axle gurus confirm this for me?

I'm not too worried as the FSM allows 20-30 inch lbs for new bearings, but some confirmation from a seasoned veteran would be nice.

Thank you,

CE
 
I'm not seasoned, well maybe, if it were mine I don't think I would worry about it. That's just me.
 
That's fine. Run hell out of it.
 
too much preload. a lubed seal adds about 5-see specs with seal like from strange and ford-they say 25-35. add .002 more and reuse that nut no problem. u do not want too much preload
 
Marcohotrod said:
too much preload. a lubed seal adds about 5-see specs with seal like from strange and ford-they say 25-35. add .002 more and reuse that nut no problem. u do not want too much preload
Click to expand...

I wasn't too worried about it at first but the more I play with it, the more it just "intuitively" seems too tight. I am a machinist by trade and used to be an auto mechanic and the "feel" of it is bothering me.

I am inclined to add another .002 as I am thinking I may have put a crush on the shim pack the first time and after reassembly it actually is too snug.

Problem is that the pinion seal is now installed and I really don't want to destroy it to test again without it. What to do, what to do..............

I'll probably put the .002 in and see how it "feels" as well as inch lbs even with the seal installed.

CE
 
are you setting up a new gear set or just rebuilding your stock diff --- the reason I ask is aftermarket gears seem to need more (thicker ) shim to achieve the proper preload -- e.g. .040 for your stock gear,, aftermarket gear may need .055 + ,, of course new bearings may also change the shim thickness required even for stock gears.
back to your question -- pull it apart to check the shims they may have been cocked on the pinion gear shaft - you will see they have caught the shoulder and are bent giving you a false reading -- what I like to do is to have the pinion vertical when installing the shims so they sit flush with the shoulder they sit on and then install the new bearing and yoke and then tighten the pinion nut at least to 100lbs/ft., again in the vertical position-- final tightening can be done in any position -- check your preload and repeat if necessary
do not worry about the new seal as you can remove the yoke and press out the pinion shaft and reinstall without damaging the seal .
a new seal will add 3-5 in/lbs. to your preload
 
John thanks for the tips. In this case I am just replacing bearings and keeping the stock 2.93 gears.

I tried adding .002 and that gave me a Preload of 13-15 inch lbs w the seal installed. I don't have the correct assortment of shims to only .001 to the original pack thickness of .044. So I am stuck with either .044 @ 40 in lbs or .046 @ 13-15 in lbs (a little hard to get exact reading on my torque wrench)

I opted for the .046 set up. We'll see!

Thanks,

CE
 
2 times I tested the preload before and after a seal install and found it climbed 12-13 inch lbs. IMO I think your fine with 40 with the seal installed
 
Ok, just got off the phone with Cass over at Dr. Diff. Love that guy! As busy as he is, he really helps you figure out what you need to know.

He says MAX preload on the 742 w/ new bearings should be 25 in lbs. Not the 30 mentioned in the FSM.

Ideal range 15-25. So I am going to try and bring mine up a bit to 20 lbs.

Also, I should have been using 170 lbs of torque on the yoke nut (for the 10 spline) not 240 lbs mentioned in the FSM.

He also gave me a ton of good info on setting up my used gears, which varies in several ways from the set up of new gears.

So, it all comes back apart! Whoo! :banghead: But I sure am glad I called him to be sure.

CE
 
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