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  1. #1

    Default 7.5 roaring (84 GT)

    Howdy everyone —

    Recently got my '84 GT vert running and am having a loud roaring noise coming from the rear. It starts as soon as the wheels turn. I have no history on this rear, so before driving I removed the differential cover to check it out and replace the leaky gasket. Oil that came out was really clean. Nothing looked concerning — no metallic sheen in the oil or debris in the bottom. Put a new gasket on and filled up with 80/90 along with a bottle of ford friction modifier.

    Attached a couple of videos of me spinning the axles by hand. The noise is immediate. Never heard anything like this. If it's an axle bearing I'd be willing to change them out and keep running this unit for now. If it ends up being more complex than that, I'll probably just swap it for an 8.8 I have sitting in the corner. It would likely need rebuilding as well, but would rather put the $ in that one instead of the weaker 7.5.

    Any suggestions on the noise? Thanks in advance!

    https://youtu.be/ujq8ZhnxWic
    '84.5 GT vert / roller 5 liter 4v / t5
    '86 LX coupe / 331 / t5
    '86 F-150 - 5 liter / efi / aod
    '56 F-100 - 272 y-block / 3 on the tree

  2. #2
    FEP Senior Member
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    yup sounds like axle bearings. once you pull the axles out you can look at the bearings for pitting, also look at the axle surface . mine was shot also. they make a axle bearing that rides on a different part of the axle so you do not need to buy new axles. on my 8.8 i needed an axle bearing puller to get the old bearing out. good luck.

  3. #3

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    Quote Originally Posted by bob85gt View Post
    yup sounds like axle bearings. once you pull the axles out you can look at the bearings for pitting, also look at the axle surface . mine was shot also. they make a axle bearing that rides on a different part of the axle so you do not need to buy new axles. on my 8.8 i needed an axle bearing puller to get the old bearing out. good luck.
    Yeah, seems to be the consensus after sharing the video with a few other folks. I'll pull them and report findings.

    Thanks

  4. #4

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    In all likely hood it's the pinion bearings if you hear it immediately when the vehicle starts to move. Get the wheels off the ground and get underneath it and turn the pinion flange, my guess is you'll feel the roughness. I run into this a bunch of times when I was a tech.

  5. #5

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    Quote Originally Posted by gt pony View Post
    In all likely hood it's the pinion bearings if you hear it immediately when the vehicle starts to move. Get the wheels off the ground and get underneath it and turn the pinion flange, my guess is you'll feel the roughness. I run into this a bunch of times when I was a tech.
    Thanks. Easy enough to check. Hopefully not the issue, but wouldn’t be surprised.
    '84.5 GT vert / roller 5 liter 4v / t5
    '86 LX coupe / 331 / t5
    '86 F-150 - 5 liter / efi / aod
    '56 F-100 - 272 y-block / 3 on the tree

  6. #6

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    Pulled axles and verified noise was coming from the center section… likely pinion bearings. Went ahead and pulled the thing out and am prepping the 8.8 to go in.

    Besides changing the oil and axle/pinion seals, would y’all recommend anything else while it’s out? Axle bearings? Everything sounds quiet when rotating by hand… especially compared to what it’s replacing. Brakes look brand new too




    Sent from my iPhone using Tapatalk
    '84.5 GT vert / roller 5 liter 4v / t5
    '86 LX coupe / 331 / t5
    '86 F-150 - 5 liter / efi / aod
    '56 F-100 - 272 y-block / 3 on the tree

  7. #7
    FEP Power Member qikgts's Avatar
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    A a minimum I'd say do the axle side upper control arm bushings. Also, don't forget that going to the 8.8 will require you swap out the hard line on the body side from the side location (single side exhaust) to the center location the 8.8 is set up for. LMR has that piece on their site and you'll also need a new soft line for it.

    I'll also say this is perfect time to swap it over to discs. It's not that expensive or a big deal. You might even be able to find a buyer for those new brakes already assembled and ready to go on the backing plates to get a few bucks back.

    For the discs, North Race Cars has everything you need in one box for $650.00 shipped with the exception of the adjustable prop valve and the plug for the factory valve. Or, you can pick up the brackets and cables from NRC and piece together the rest.

    https://northracecars.com/c-clip_brakes.html
    '85 GT

  8. #8

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    Quote Originally Posted by qikgts View Post
    A a minimum I'd say do the axle side upper control arm bushings. Also, don't forget that going to the 8.8 will require you swap out the hard line on the body side from the side location (single side exhaust) to the center location the 8.8 is set up for. LMR has that piece on their site and you'll also need a new soft line for it.

    I'll also say this is perfect time to swap it over to discs. It's not that expensive or a big deal. You might even be able to find a buyer for those new brakes already assembled and ready to go on the backing plates to get a few bucks back.

    For the discs, North Race Cars has everything you need in one box for $650.00 shipped with the exception of the adjustable prop valve and the plug for the factory valve. Or, you can pick up the brackets and cables from NRC and piece together the rest.

    https://northracecars.com/c-clip_brakes.html
    I actually just ordered the bushings you mentioned along with the removal/install tool & coil spring isolators from Maximum Motorsports.

    I did snag the later hard brake line that runs the length of the car down the tunnel. I did overlook that bracket though. Is this the bracket you're talking about?

    Due to lack of funds I'm staying drum for the foreseeable future. Thanks for the heads-up on the disk conversion though.
    '84.5 GT vert / roller 5 liter 4v / t5
    '86 LX coupe / 331 / t5
    '86 F-150 - 5 liter / efi / aod
    '56 F-100 - 272 y-block / 3 on the tree

  9. #9
    FEP Power Member qikgts's Avatar
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    Jan 2010
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    Rockledge, FL
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    That's the bracket!

    BTW, I've been following your build thread and the car looks great! Congrats on the fantastic progress and outcomes!
    '85 GT

  10. #10

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    Quote Originally Posted by qikgts View Post
    That's the bracket!

    BTW, I've been following your build thread and the car looks great! Congrats on the fantastic progress and outcomes!
    Thanks for confirming that part.

    Appreciate the kind words. Progress is super slow, but inchin’ my way forward
    '84.5 GT vert / roller 5 liter 4v / t5
    '86 LX coupe / 331 / t5
    '86 F-150 - 5 liter / efi / aod
    '56 F-100 - 272 y-block / 3 on the tree

  11. #11
    FEP Power Member Saturn V's Avatar
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    Quote Originally Posted by qikgts View Post
    A a minimum I'd say do the axle side upper control arm bushings. Also, don't forget that going to the 8.8 will require you swap out the hard line on the body side from the side location (single side exhaust) to the center location the 8.8 is set up for. LMR has that piece on their site and you'll also need a new soft line for it.

    I'll also say this is perfect time to swap it over to discs. It's not that expensive or a big deal. You might even be able to find a buyer for those new brakes already assembled and ready to go on the backing plates to get a few bucks back.

    For the discs, North Race Cars has everything you need in one box for $650.00 shipped with the exception of the adjustable prop valve and the plug for the factory valve. Or, you can pick up the brackets and cables from NRC and piece together the rest.

    https://northracecars.com/c-clip_brakes.html
    Note that when upgrading the rear end from 7.5 to 8.8, swapping out the body side brake line is not strictly required. When I did the 8.8 upgrade on my 84 GT, I simply transferred the hard lines from the 7.5 to the 8.8 housing. As I recall, all mounting points for the housing brake lines were the same. I did replace the body side flex hose in the passenger side, since it’s age was unknown. All of that said, I chose to keep my original style single exhaust on the driver side and do not plan to change that. If you are running dual exhaust, then I agree that swapping the body brake line to the 85-93 style (or extend the original body side brake line with a custom piece to reach the special bracket/fitting linked above) for center hose mount is needed in order to eliminate the original flex hose that would be too close to the passenger side dual exhaust pipe.


    Sent from my iPhone using Tapatalk
    Present: '84.5 Mustang GT T-top, '06 Mazdaspeed6
    Past: '79 5.0 Capri, '86 Buick GN, '90 Mustang GT, '92 SHO, '95 SHO
    Browse cover pages of my Fox Chassis related library

  12. #12

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    Quote Originally Posted by Saturn V View Post
    Note that when upgrading the rear end from 7.5 to 8.8, swapping out the body side brake line is not strictly required. When I did the 8.8 upgrade on my 84 GT, I simply transferred the hard lines from the 7.5 to the 8.8 housing. As I recall, all mounting points for the housing brake lines were the same. I did replace the body side flex hose in the passenger side, since it’s age was unknown. All of that said, I chose to keep my original style single exhaust on the driver side and do not plan to change that. If you are running dual exhaust, then I agree that swapping the body brake line to the 85-93 style (or extend the original body side brake line with a custom piece to reach the special bracket/fitting linked above) for center hose mount is needed in order to eliminate the original flex hose that would be too close to the passenger side dual exhaust pipe.


    Sent from my iPhone using Tapatalk
    Hey Erik - Thanks for chiming in.

    Yes, I'm running dual exhaust, so I'll go ahead and swap lines on the car and add that bracket. Hopefully this will get me where I need to be. Lots of cleaning/painting for the next few weeks

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