Looking for some affirmation (8 3/4 rebuild not successful)

Discussion in 'Transmission and Drivetrain Tech' started by zakimodo, Apr 16, 2018.

  1. zakimodo

    zakimodo Well-Known Member

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    Hi All

    Just rebuilt and installed my first 8 3/4. On the test/ break in drive, I noticed a few things.
    Here is what I am working with
    -489 case
    -3.55 gears from Yukon (rebuild kit from them as well)
    -Eaton Tru Trac
    -Ratech crush sleeve eliminator

    1. Under ~60 MPH the gears were quiet (car is noisy but I didn't notice any unusual gear noise. Over this speed and with the car in gear I noticed a humming noise (Felt more like harmonics than anything). If I put the car in neutral to coast it got quiet again

    2. On my return trip, I started to notice a lot of gear whine when coasting to a stop.

    When I set this up, I had a tough time getting the pinion preload set. At first, I could hit preload with only 90Ft lbs of torque on the pinion nut. A thinner shim would all but lock the pinion up at the same torque. I ended up filing an honing down the pinion spacer and was able to get 175 ft/lbs of torque on the pinion nut w/ the correct preload. I left it at this with some blue Loctite. Looking back, I was losing patience with constantly reinstalling the pinion to get this set... All that said, I feel like I screwed up on the preload (nut loosened up or the bearing cups settled). So, am I thinking correctly, is there anything else I should look at once I have a chance to pull the 3rd member out?

    Just for reference, here is my contact pattern after final assembly.

    20180325_133057.jpg

    20180325_133104.jpg
     
  2. TrailBeast

    TrailBeast Slightly Twisted Member

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    Pinion running too deep in the teeth, and needs to be moved away more.
     
    • Agree Agree x 1
    • AJ/FormS

      AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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      IMO, I think all you gotta do is give it a tad more backlash and watch the pattern climb up the tooth a bit. The pinion depth looks pretty dar n close, but it could use a tiny bit more
      shim, but I mean tiny.So I would increase the backlash first, to the max allowable, and see what happens.On the street you can get away with quite a bit of backlash.
      The spec is .006 to .008, but I've had to run as loose as .012/.013.. Some patterns want what the want. If that concerns you, call the manufacturer
       
      Last edited: Apr 16, 2018
    • pishta

      pishta I know I'm right....

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      what was your backlash with that pattern?
       
    • mderoy340

      mderoy340 Well-Known Member

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      Best to set the pinion depth to spec scribed on the pinion, then set backlash to gear manufacture spec.
       
    • zakimodo

      zakimodo Well-Known Member

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      That was at .007 backlash. I think the Yukon manual spec'd .006 to .010.

      It's interesting you guys bring this up. A local "mopar nut" (best way to describe him) told me that the tightest he will ever go with an 8 3/4 is .010 backlash.
       
    • AJ/FormS

      AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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      I've had no problems at up to .012/.013, in going on 40 years. And that includes the ones I have been abusing with the CFII clutch and splitting gears at WOT with the GVOD.
      But; your results may vary......so
      I'm not gonna tell you to run that loose, cuz I'm not gonna buy you new gears if the teeth start unzipping.
       
    • john-9

      john-9 Well-Known Member

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      your pattern looks really good ,,what did you set the preload at should be 20-25 in/lbs with new bearings ,,,, but your thinking is off on the pinion bearing preload set up --- you stated using the solid spacer and a thinner shim locks up the assembly ( which is correct ),, with that situation you would then ADD shim ( increase the thickness ) maybe .002 at a time ,, torque the nut to at least 150 ft/lbs and check the preload ,, by cutting down the spacer you are taking shim out and going the wrong way.
      one other thing to check is the I.D. of the solid spacer at the big end .. sometimes these spacers do not sit down flush on the bearing and have to be opened up at tad ( honed out ) to completely seat flush onto the large pinion bearing.
       
    • zakimodo

      zakimodo Well-Known Member

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      Yeah, I didn't specify that the next shim option up wouldn't get me anywhere near preload (too loose). This is why I shaved the spacer down a little.
       
    • zakimodo

      zakimodo Well-Known Member

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      I haven't had a chance to pull the 3rd member back out yet. Hoping I can get it pulled out this weekend.
      The day I installed this I was probably over sensitive to any noise/ vibration. I had to move my car this weekend and figured I would take it for a short cruise just to confirm my symptoms.

      First off, the harmonics I mentioned @ 60 MPH is all from the engine. Going to have to get used to the engine being wound up a bit more with the 3:23 to 3:55 ratio change.
      Outside of this, the rear end was quiet until the diff oil warmed up (again on my return route to home). The gear whine is back, although not near as bad as I had remembered it and is only present at ~40MPH (when coasting and under load). Above or below this speed it's silent.