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Thread: IRS differential mounting problems

  1. #1
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    IRS differential mounting problems

    Went to mount my 2015 mustang IRS diff today and found the welded frame mounts off by at least a quarter of an inch. The diff mounts directly to the mounts at 4 points so there is no room for slop. The diff sure won't flex. Not sure what to do short of cutting off the mounts and rewelding in the right place. I guess I could grind some slop into the urethane bushing. Any thoughts?

  2. #2
    Senior Member edwardb's Avatar
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    I'm doing a Roadster with the 2015 IRS and have the center section mounted along with the rest of the IRS components. I too thought the mounts were off about that amount when I first tried to put in the center section. Additionally, it was so tight front to back it was very difficult to get it lined up. After wrestling with it for some hours, I made these tapered pins from 5/8 inch hardware store bolts and I was able to get it aligned and what seemed like misalignment actually wasn't. After getting the center section into the general right location, I used the pins to get the rear bolts aligned first since they're threaded into the rear aluminum cover. With the rear bolts started but not tightened, I used the pins to get the front lined up. Then one at a time, removed the pins and pushed in the mounting bolts. It's really tight, but it all worked out. Without something like these pins to help move it into perfect alignment, I'd probably still be wrestling with it. The tolerances and alignment are all really close. It's extremely important IMO to not tighten any of the bolts until all four are in place and threads started. Many more pictures in my build thread, link in my sig.

    The other thing I will mention is that I've been in contact with another Roadster builder doing the 2015 IRS. It turned out OK, but his actually was loose front to back by 1/4 inch or so. After discussing and doing some comparison measurements, Ford either changed the dimensions on the center section or there are differences between the different center section versions or even model years since the 2016's are out now. If the above doesn't work, maybe something like this is happening with yours.

    Last edited by edwardb; 11-07-2015 at 07:02 AM.
    Build 1: Mk3 Roadster #5125. Sold 11/08/2014. Build 2: Mk4 Roadster #7750. Sold 04/10/2017. Build Thread
    Build 3: Mk4 Roadster 20th Anniversary #8674. Sold 09/07/2020. Build Thread and Video. Build 4: Gen 3 Type 65 Coupe #59. Gen 3 Coyote. Legal 03/04/2020. Build Thread and Video
    Build 5: 35 Hot Rod Truck #138. LS3 and 4L65E auto. Rcvd 01/05/2021. Legal 04/20/2023. Build Thread. Sold 11/9/2023.

  3. #3
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    Quote Originally Posted by Carolina Hot Rod View Post
    Went to mount my 2015 mustang IRS diff today and found the welded frame mounts off by at least a quarter of an inch. The diff mounts directly to the mounts at 4 points so there is no room for slop. The diff sure won't flex. Not sure what to do short of cutting off the mounts and rewelding in the right place. I guess I could grind some slop into the urethane bushing. Any thoughts?
    I figured sooner or later someone else would be running into this problem. I put a call into FFR and got a weak response of just shim it and don't worry. Not what I expected, but I followed their direction. I then put in a call to Mike Forte, who is offering, new from Ford, IRS center sections. Told him to do some measuring on the new units. Like Paul said, I suspect that Ford changed the center section cover slightly. I will bug FFR to see if they could investigate this issue.

  4. #4
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    Thanks for the input. I went at it again today and was able to get it in. I left the bushings and inserts backed out away from the differential until the bolts were started. That gave me some room to align the bolts. The differential is very heavy and awkward to get in place! That install is will give you a good backache!

  5. #5
    Senior Member edwardb's Avatar
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    Good news! Glad it worked out.
    Build 1: Mk3 Roadster #5125. Sold 11/08/2014. Build 2: Mk4 Roadster #7750. Sold 04/10/2017. Build Thread
    Build 3: Mk4 Roadster 20th Anniversary #8674. Sold 09/07/2020. Build Thread and Video. Build 4: Gen 3 Type 65 Coupe #59. Gen 3 Coyote. Legal 03/04/2020. Build Thread and Video
    Build 5: 35 Hot Rod Truck #138. LS3 and 4L65E auto. Rcvd 01/05/2021. Legal 04/20/2023. Build Thread. Sold 11/9/2023.

  6. #6
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    My new 2017 cast iron diff has the same 1/4' issue. We also have an all alloy 2015 and it is the same. So maybe its the <2014 diffs that fit without the gap. Anyway Im going to laser cut some spacers. and put them in the rear.

  7. #7
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    I have cheated. I took the diff to a machine shop. They removed 2mm from the inside of all the bolt holes, they now have a very slight oval shape to them but it all goes together very easy now, no stress on any of the housings, front or rear. The threaded holes in the rear mount have been machined out and now have bolts that go all the way through, cone nuts and same size bolts just longer. Still needed a 6mm spacer in the rear. Sorry for the metrics but I dumped the imperial measurements when I joined the Air Force, they beat it out out of me with big sticks.

  8. #8
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    I did exactly the same.
    FF5 dissaponted me when they told just beat into place! WTF.

  9. #9
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    I 3D printed TPE offset bushings with the same durometer as the FFR supplied ones. I agree the FFR solution to just power it in there was not satisfactory to me. I also had to remove the diff cover to get my diff to fit into the frame location. I fought it for over an hour before I gave up and removed the cover. It went right in place with the cover off.

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