My DIY differential gear change

Discussion in 'SRT10 and SRT10 Coupe Discussions' started by cowger, Nov 28, 2012.

  1. cowger

    cowger Viper Owner

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    I am going to attempt to document my DIY differential gear change project. Like many, I intend to swap out the stock 3.07:1 (too-tall) gearing in my 2005 Roadster for some 3.55 gears. From what I gather, not many choose to do all this work themselves, but my love for wrenching and learning new things has caused me to take this on myself, or at least attempt to. I’m hoping that with some guidance from those on this forum who truly know what they’re doing, we might be able to come up with a decent illustrated upgrades thread.

    For those who care, I’d consider myself of average mechanical ability. For example, I’ve dug into an engine and rebuilt it, and enjoy working on cars and other machines. However, I have no formal training in this, so please take everything I put in this thread with a grain of salt. I will defer to those who know and attempt to edit / correct anything I state that’s misleading or otherwise inaccurate.

    With that said, here we go…

    This first post will be to get the differential out of the car. As others have previously stated very helpfully on this forum, it’s not a difficult task and can be done with surprisingly little disassembly of the rear end.

    First, get the car safely up in the air, take off the belly pan, and remove the driver side wheel while the diff is draining (I open up both plugs as a fair amount comes from the 2nd even after the first has stopped flowing.) From there, only the upper A-arm bolts (18mm socket) and the brake line bracket (10mm socket) need to be removed:

    [​IMG]

    Next, under the car, use a pry bar to pop the driver’s side half shaft from the splined output of the differential. This didn’t require much effort on either side of the car for me, and while I used a rather crude tool (a ~16” nail pulling tool), it didn’t damage either contact point.

    Engaging the pry bar:
    [​IMG]

    And popped loose:
    [​IMG]

    Now you should just *barely* have enough room to pull the left halfshaft completely off the differential. The knuckle swings and rotates out to give you the clearance, and you need to ensure that the CV joints themselves are fully compressed at both ends. Some have stated that they also remove the tie rod in order to get some more working room, but I didn't need to. Use caution with the hard-plumbed portion of the brake line. And have some extra-large zip ties on-hand in order to secure it up and out of the way.

    With the car in gear, remove the bolts that secure the driveshaft to the diff flange. I struggled to find a socket that fit these tightly, but an 8mm, 6-sided socket on a 1/4” drive (w/o a u-joint on it needed in order to fit) seemed to be the best. A pneumatic impact wrench really made this task a lot easier.

    [​IMG]

    You’ll obviously need to shift into neutral for a bit in order to rotate the driveline around to get the other 2. I believe that it is recommended to replace both the straps and the bolts for installation.

    Now remove the screws that hold the 4 clamps for the e-brake cables.

    [​IMG]

    There are 4 bolts that hold the diff in the frame. The two lower are 18mm and the two upper are 21mm. Don’t do what I did and sit there wrenching the upper bolts for 2 minutes, wondering why they weren’t getting easier… You’ll need to get at *both* the bolt and the nuts toward the rear, as shown here. :)

    [​IMG]

    Support the diff with some type of jack while you shift it toward the driver side in order to pop off and then slide off the passenger side halfshaft. Tie both it and the driveshaft up and out of the way with zip ties. Here's the passenger side halfshaft strapped up and out of the way, with the upper arm of the differential free of its mount:

    [​IMG]

    Now the diff is floating around in the rear-end frame and it’s time to get it out and down. Perhaps someone else can come in with a nice, easy, elegant way to do this, but for me, lying on my back, it was essentially a bench press contest between it and me, and it took a good 10 minutes for me to maneuver it out of the car. I worked it to one side, slipped the opposite side e-brake cable up, around, and over the mounting arm, did the same for the other side, and then it finally flopped out, onto my chest, and then down and onto the floor. Whew!

    [​IMG]

    More to come as I dig into the differential...
    Bryan
     
  2. viper04

    viper04 Viper Owner

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    WOW! nice write up with pic's keep us posted. Did this upgrade this summer and have to say one that you will enjoy.
     
    Last edited: Nov 28, 2012
  3. Sonoman

    Sonoman Viper Owner

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    Nice job, Bryan. Brings back the memories of when I did my diff swap 18 months ago. While I did the "bench press" method as well, in the interest of safety, should probably take some extra precautions when handling that large hunk of metal over your chest. Just sayin'! I used some motorcycle tie downs hanging from the frame to limit the travel of the diff and put on a chest protector as well. Having someone around to assist (or call 911 for you!) is not a bad idea. If you're working on your back like this, a good creeper is worthwhile as you are much more mobile on it. The Viper aluminum diff carrier isn't that heavy, that is, until it falls on you. :omg:
     
  4. cowger

    cowger Viper Owner

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    Okay, I made a bit more progress tonight. And found some not good news...

    First thing was to build some type of fixture so I could work on this thing. I've seen the blue commercial ones but couldn't find where to buy one, so I made my own:

    [​IMG]

    It's a few pieces of flat bar stock welded to a 1.5" square tube that slides into some pieces welded to my metal workbench. I can flip it up or down (or sideways, I suppose). I used the bushings from the actual installation and the bolts from the upper A-arm mounts. Seems to work for now.


    With it mounted, I removed the cover bolts and used a soft mallet to pop the cover off:

    [​IMG]


    The first thing noted was the factory markings that keep the diff bearing caps straight:

    [​IMG]


    I then popped the stub shafts out, using this tool from Lowes to remove the retaining clips:
    [​IMG]

    I'd give this tool about a C- grade for this job. You really need some heavy duty circlip pliers with a 90 degree bend in them. But I eventually got them out.


    Then the next thing I noticed was that my spider gears are not happy. This is a compilation of the 4 parts of those gears that I could see from the open sides of the diff carrier:

    [​IMG]

    Ruh-roh!!! So much for a simple gear change...

    I'm guessing this might be one of the things (or the thing?) associated with the weakness of the Gen3 diff. And is this what happens when you wheel hop these cars? At any rate, these clearly need to be replaced.

    So what are my options at this point? I don't race, track, or drag my car, so I'm not crazy about dropping $1500 for a racing diff. Nor am I crazy about getting replacement OEM gears (if those are even available), since those clearly aren't very good...

    What are your thoughts?

    Bryan
     
  5. cowger

    cowger Viper Owner

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    Good tips, Sonoman! I'll work those into the final write-up...

    Thanks!
     
  6. mjorgensen Woodhouse

    mjorgensen Woodhouse VCA Member - West

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    I think Unitrax has a bunch of take out diffs if you want a replacement cheap, give Devin a call 800-622-4327
     
  7. cowger

    cowger Viper Owner

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    Thanks, Mark, I'll give them a call today. I think you're suggesting I put an OEM differential back in, so I'll ask this: Is every set of Gen3 spider gears destined to fail like this, or did I get a bad batch, or was this diff somehow abused at some point? My car has 30k miles, and while I get it that's somewhat high for these cars, that seems to me like not many miles on a differential...

    Bryan
     
  8. mjorgensen Woodhouse

    mjorgensen Woodhouse VCA Member - West

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    Yes that is to get a stock replacement take out unit, they do not sell new stock ones. If you want an aftermarket one then as you know it will be much stronger and more expensive, worth it if you will keep the car awhile though IMO.

    Yes this is relatively common with this diff, we do not see many Vipers with higher mileage, but I do know a shop in California that replace many of these for this very issue. That being said lots of still running ones without problems out there so might be a chance you will never have a problem with the next one.


     
  9. cowger

    cowger Viper Owner

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    Thanks, Mark. I think what I'm trying to figure out is what are my choices for aftermarket.

    At the low-end, an OEM take-out, as you suggested

    At the high-end, racing units like Quaife, Wavetrac, etc.

    Anything in-between? For example, are there replacement gears that go into the stock housing that are of higher quality?

    What about the Gen4 diff? Is that compatible and can it be back-fitted into a Gen3?

    Thanks,
    Bryan
     
  10. cowger

    cowger Viper Owner

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    WOW! Just spent an awesome 15 minutes on the phone with Devin. He's clearly the man when it comes to this stuff, so thank you Mark for the direction to him!!!

    New Wavetrak and bearing/seal kit on their way to me now... :D
     
  11. ACRucrazy

    ACRucrazy Enthusiast

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    What did he share? This is a great thread.
     
  12. cowger

    cowger Viper Owner

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    Thanks!

    He convinced me that the Wavetrak is the best bang for the buck at this point. It's not cheap, but it comes with the output shafts, a lifetime warranty, and he likes the way it is "always operating", versus some other units where you have to induce wheel spin before they start doing their job. There's probably more and better information available out there for the unit, so I won't attempt to describe it here (mostly since I don't know all the advantages).

    Before ordering, we covered what my other options would be:
    - You can't service the existing unit -- it's serviced at the carrier level, i.e. no replacement spider gears, for example
    - You can't find the OEM unit new -- they apparently don't sell them anymore
    - The Gen V unit would fit, but would be more similar to what I had in there (though sturdier), i.e. it doesn't have the torque transfer capabilities. More $$$, too.
    - I might be able to find a used unit, but they won't sell them. (They just threw 4 used units in the trash yesterday.)
    - Quaife and OS Giken were listed as alternatives, more expensive, more race-oriented

    In the end, the Wavetrak appeared to be my cheapest option (other than finding a used take-out unit somewhere), and with this I expect to have a better driving experience + a lifetime warranty.

    So I bit the bullet and chalked another one up to Viper Tax... :)
     
  13. mjorgensen Woodhouse

    mjorgensen Woodhouse VCA Member - West

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    Great choice and of the few that I have sold everyone really likes the Wavetrak, glad I could help and if I can in the future please just ask, I can offer you the same pricing on most things you would want for the Viper.


     
  14. cowger

    cowger Viper Owner

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    I was able to get some more disassembly work done this morning. But before I went too far, I wanted to take some measurements of the existing setup.

    First is backlash. I had this dial indicator set from Grizzly:
    [​IMG]

    And set it up as such:
    [​IMG]

    Obviously the magnetic base didn't do any good on an aluminum case, but thankfully the threaded rod from the kit matched the 10mm threads of the diff housing. Whether by universal design or pure dumb luck, I don't know, but I was happy regardless.

    You want the end of the dial indicator touching a face of the tooth, at its heel, and as close to perpendicular as you can get.

    I used a clamp on the pinion flange to ensure that it didn't turn and then measured the backlash as shown here:
    [​IMG]

    I read this as 0.012" of backlash; two other readings spread evenly around the ring gear yielded 0.014" and 0.013". Now this seems high to me, but maybe that's to be expected from a used setup with 30k miles on it.

    Question to the experts: What should I aim for with new gears? I wasn't ever able to find this spec anywhere.


    Next measurement was existing bearing preload. Here you need a small torque wrench, needle-style, ideally with a range of 0-50 in-lbs. Mine reads 0-250, but I still seemed to be able to see the total preload turning torque as 16 in-lbs (ignore the blue needle -- that's a reference you can set. The orange needle is the measurement):
    [​IMG]

    The idea is that you turn the pinion/ring gear combo around 4 turns with as even a pressure as you can, then note the reading as you're turning.

    With those done, I built a case spreader out of some .25" x 1.25" bar stock (I had a lot of this lying around, in case you haven't noticed yet), and cut up a 3/8" drill to make the two pins that fit into the holes in either side of the case. I also drilled and tapped a hole for my dial indicator setup, and used a smaller deflection-type dial to measure the case spread:
    [​IMG]

    I believe you're supposed to spread the case 0.010", but I'm not sure of this number.

    Remove the two bearing caps, and from there it was relatively simple, though not easy, to lever the differential out of the case:
    [​IMG]

    Take special care to retrieve the carrier bearing preload shims and note which came from which side:
    [​IMG]

    I redid the preload measurement now with only the pinion, and noted 9 in-lbs.


    Finally, here's a closeup of the oil drain holes from the inside. Note to self: Use the driver side hole to drain... it's lower!

    And yes, that's a piece of gear sitting there... :( :)
    [​IMG]
     
    Last edited: Nov 29, 2012
  15. Paul Hawker

    Paul Hawker Viper Owner

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    From what I understand, setting up the gears is a bit of an art/science.

    Set them up too loose, and they are weak from banging.

    Set them up too tight, and they are howlers.

    Also must use certain gear lubes with friction modifiers in the right ratio to make them work well and be long lasting.

    Break in is also essential for long quiet running. Run them for a bit, at varying speeds till they get hot, then park it for a while till it is cool. Do this at least twice. Then change out gear oil in about 1,000 miles for fresh, and expect a long life.

    Wheel hop is the worst things for these gears. If you have wheel hop, it is usually easier to fix that than the gear set.

    Devin at Unitrax is my expert on all things drive line related. Most of the Viper tuners have Devin on speed dial. They make a great team.
     
  16. JAY

    JAY Viper Owner

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    Great thread ! Thanx for sharing ! :2tu:
     
  17. cowger

    cowger Viper Owner

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    Thanks, Paul. I'll definitely try to cover the setup process during reassembly. The correct pinion depth + ring gear position = a good mesh pattern, plus how to set bearing preload.

    I already bought 2 quarts of the OEM fluid plus the friction modifier, but now Devin informs me that the Wavetec doesn't need the additive. Any want a bottle at a discount? :)

    Also, Devin is including a full break-in procedure with the new diff. I had thought something similar to what you write, but apparently it's different, and I'll share what I do.

    Bryan
     
    Last edited: Nov 29, 2012
  18. cowger

    cowger Viper Owner

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    Okay, I could use a little help figuring out what I'm looking at on the ends of the pinions.

    Here's the old one:
    [​IMG]

    And here's the new one:
    [​IMG]


    I expected to see a number showing how far off (plus or minus) the ideal running position of the pinion gear is. On the new one, I think I see "K + 1", which I think means that the pinion should run 0.001" farther away from the centerline of the axle, meaning subtract 0.001" from the thickness of the pinion depth shim. Does the "K" mean something, or, like on the bearing cap markings, is it simply used as a directional character (meaning it shows which way is up)?

    On the old one, it's less clear. I think I see "P1" with a couple of misc marks to the left and right of this. Does this mean "Plus 1", meaning that the two have identical running positions?

    What about the other characters? New one has maybe an "OL" on the bottom of it; old one has what looks like an "F". What should I be able to tell from all this?

    I understand that the mesh pattern will be the final determination of running position, but for now I'm just trying to figure out what thickness of pinion shim I should start with...

    Thanks,
    Bryan
     
  19. cowger

    cowger Viper Owner

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    One more tool needed that I made last night -- this one is a flange holder, needed to hold the pinion from turning while you're torquing on the nut.

    Here's what I came up with:
    [​IMG]

    The first version of this tool just had a 3/4" pipe screwed into a cast iron coupler, and in the first attempt, the pipe broke where the threads entered the coupler. As you can see, I had to add some more bracing after welding the pipe back into place...

    This then, combined with a 1-5/16" socket on a 20" 1/2" breaker bar PLUS a 2' pipe as a cheater, allowed me to undo the pinion nut. I thought the 1/2" breaker bar was going to go first, but in the end, the nut came loose.

    Here's everything apart:
    [​IMG]

    Note the fluffy white cotton-looking stuff inside the seal:
    [​IMG]

    Turns out it's a bunch of spider egg sacks, some of which had fried spider carcases inside. Felt like a miniature version of Aliens in there... :)

    This weekend I'll drive the two output shaft seals out, press out the two pinion bearing races, and get the case all cleaned up.

    BTW, I can't even describe how bad the oil out of a differential smells, especially when I pulled the carrier bearing caps off (for some reason). And I had just changed this oil out 3000 miles ago. I'm looking forward to getting everything cleaned up and my shop smelling better...

    TGIF!
    Bryan
     
  20. Allan

    Allan Viper Owner

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    The synthetic gear oil smells like that for some reason, like it's toasted. That's normal. Initial backlash setting is .008-.012 typically. I usually set up on the tighter side. Haven't ever had a gear set whine from being too tight. Setting up ring/pinion gears is more art than science to me. Something that you learn/develop a feel for with experience. I commend you for your well documented awesome attempt at a high skill repair. You are obviously not dumb. ........thanks for sharing the job with everyone. -Allan
     
  21. Camfab

    Camfab Viper Owner

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    Great write up, love the fact that your making your own tools! I couldn't help but notice the difference in the number of teeth on the new pinion vs the old. Who is the manufacturer of the new gear set. Looking forward to your progression.
     
  22. cowger

    cowger Viper Owner

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    Thanks, Allan! Great to know the backlash setting range, and that you tend toward the tighter side.

    BTW, here are the torque settings I have so far. I never was successful in finding a service manual specifically for this unit, which I believe is a Super 44 / or Dana 226. However, I did find a Dana / Spicer manual that yielded the following torque settings:

    - Drive pinion nut: 220-500 ft-lbs (collapsible spacer / preload determines final torque)
    - Drive gear bolts: 70-90 ft-lbs
    - Diff bearing cap bolts: 55-70 ft-lbs
    - Cover bolts: 28-33 ft-lbs

    - Pinion bearing preload: 25-35 in-lbs
    - Diff bearing preload: +8-10 in-lbs (added to prior setting)

    Pinion seal adds 3 in-lbs to total torque to rotate

    Look okay?
     
    Last edited: Dec 1, 2012
  23. cowger

    cowger Viper Owner

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    Thanks! Yes, I, too, was surprised by the delta in pinion teeth between the old and the new. I bought an OEM gearset from VPA (Jerry was a huge help!)

    Here are the ratios.

    Old: 14/43 = 3.0714
    New: 11/39 = 3.5454
     
  24. MtnBiker

    MtnBiker Viper Owner

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    Oh that explains that! I never knew what the 3.07 vs 3.55 numbers meant! Thanks!

    Awesome thread!
     
  25. Camfab

    Camfab Viper Owner

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    Maybe it's just the photograph, but I couldn't help but notice the generous amounts of chamfering on the original pinion vs. the new one.
     
  26. Allan

    Allan Viper Owner

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    Those specs look fine. Lean to the high side on the torque specs for the fasteners (bolts). You will have fun with the pinion nut against the crush sleeve. During that process, you will say a lot of in-appropriate words. It will seem like an impossible task, that's normal.
     
  27. cowger

    cowger Viper Owner

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    Thanks for the heads up on this apparently joyful process. :)

    BTW, here's what I'm planning to do:
    1) Install the pinion w/o the seal or crush sleeve. Torque enough to seat the bearings (50 ft-lbs?)
    2) Install the carrier and check backlash and the mesh pattern. Iterate on the 3 spacers until it looks good.
    3) Remove everything, install the seal and crush sleeve, torque to a minimum of 200 ft-lbs and then continue torquing the pinion nut (in very small increments) until I get ~30 in-lbs of rotating torque (this sets pinion preload)
    4) Install the carrier, set backlash to 0.008-0.010
    5) Set carrier preload to [pinion preload + (8-10) in-lbs + 3 in-lbs (to account for the seal)]
    6) Verify the mesh pattern
    7) Install output seals and output shafts

    You mention the "art" part of this and I know I'm trying to define it as a science, as I'm new to this. Anything you can point out here that you think will help me would be great!

    Bryan
     
  28. Sonoman

    Sonoman Viper Owner

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    The art part comes from years of experience with gears... I know a guy who's done bevel gear assemblies for 20 years and he can "feel" when the gear mesh is right (while newbies struggle with constant re-shimming and lots of Prussian blue dye-- and still don't get as good, consistent results). It sounds like you are analytical enough to get through it well enough with science, but it's hard to beat pure hands-on experience and a well-seasoned hand.
     
  29. cowger

    cowger Viper Owner

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    Yup, I know that I'm stretching it here in attempting this myself. I expect to spend a good deal of time on my step 2) above, I have a tube of Prussian blue and I know where to get more... :)
     
  30. JAY

    JAY Viper Owner

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    Sounds like you have the Right Approach ! Like Thomas Edison said " 1% inspiration / 99 % Perspiration " ......lol ! :2tu:
     

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