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Thread: have a question on rear axle issue

  1. #1
    Senior Member
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    have a question on rear axle issue

    Hey guys - working on a gen 1 hot rod equipped with new Moser 8.8, w. zero miles. I pulled the axles out to swap some brake components and while looking at things inside the diff I noticed that the spider gears seemed to be really loose on the cross pin. When you rock the pinion back and forth the spider gears wobble quite a bit. I called the Moser tech line and the guy just seemed to want to tell me how to measure the back lash (ring and pinion). He finally said he didn't have a spec for the spider gears and didn't seem to be concerned with it. I did make some rough measurements and came up with about .010 clearance between the id of the gears and the diam of the shaft (cross pin). .010 may not seem like a lot but on a 1 inch diam shaft that seems huge to me.
    I don't have that much experience messing around inside rear ends (no pun intended) but was wondering if anybody has ever noticed that before.

  2. #2
    Seasoned Citizen NAZ's Avatar
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    I see that no one has come up with a clearance spec yet. Well I don't have one either, but I can give you some general info that will help you make a sanity check of what you have.

    Generally speaking, a plane radial bearing (like a pin and bushing such as you would find on a spider) will typically be designed to have .0015" of clearance per inch of diameter (pin diameter in your case). If your pin is 1.000" O.D. then the installed bushing I.D. should be about 1.0015" +/-.0005".

    However, you don't mention how you arrived at that clearance dimension. Unless you actually mic the ID of the bushing and the OD of the pin you can't be sure of the clearance.
    Dart Little M 406" SBC 800 HP N/A & 1,100 HP on nitrous, 2-spd Powerglide with trans brake, 6,000 RPM stall converter, narrowed Moser 88 3.90:1 spool with 35-spline gun-drilled axles & Torino bearings, custom parallel four-link, custom tube chassis & roll cage NHRA certified for 8.5-sec (only two FFR Hot Rods have this cert).

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  3. #3
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    Hey thanks for the quick reply Naz. I did measure the pin with a cheap micrometer, but for the id of the spider gears all I had was a set of inside dividers . Certainly not gonna be accurate to .0001 but I'm pretty sure it is within .001 to .002. I have been a mechanic long enough to have a pretty good feel for fits and clearances of many different mechanisms but I have only really been deep inside one rear axle. I completely rebuilt the 8.8 in my fox body mustang and it worked great afterwards but that is about the extent of it. I did notice that the spider gears on that one seemed to be kind of sloppy but that rear end had about 350 k miles on it.
    The way I understand it, the spider gears don't really do anything until one wheel starts spinning faster than the other but I really don't know what a typical fit would be in this case. It sure doesn't seem to be a very precise fit.
    I'm hoping somebody who has done a bunch of them will chime in and say "don't worry about it - they are all like that"
    Last edited by sread; 07-18-2019 at 04:04 PM.

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