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Thread: Front differential mount crossbar - one stripped bolt hole

  1. #1
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    Default Front differential mount crossbar - one stripped bolt hole

    Alright ever since the diff swap last week, subconsciously this thing has been nagging at me to a point where I'm reaching out for feedback.

    - I'm not 100% sure whether the bolt hole was stripped prior to removal, during removal, or during re-install. It may have well been in that condition for the 2 yrs I've been beating and autoX'ing the car, or it could have happened some point during the reassembly last week.

    - It's not completely jacked, and I can put maybe 15lb's of torque on it before it loosens again, for now the bolt is back in there at its tightest 10 - 15 lbs torque with a bit of blue loctite - but this is nowhere close to what it should be, or at least I feel that way.

    The problem in question is highlighted with the RED arrow, passenger side, outer. *this isn't my car lol, using a reference pic.





    * The other three bolts on the crossmember are in good condition, and are holding their torque specs nicely. The yellow dots, which includes the 2 on the lower control arm with total of 5, is my observation on supporting points for the pivot arm, when the control arm moves up and down.

    What would be the most reasonable fix for this? Leave it with lot of loctite and assume the 5/6 bolts will do their job? Heli Coil? Maybe drill the hole all the way through and put a big nut on the other side? Bigger bolt that'll bite harder?

    I don't foresee myself dropping that crossmember again anytime soon, but I'm sure at some point I need to access the part of the car again, where the crossmember does need to come back down.

  2. #2
    Barn Find Daily Driver Captain_Zeros's Avatar
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    What I would be more concerned about than 5 of 6 bolts holding in the differential would be there being 3 of 4 bolts holding in your right-hand control arm. That front crossmember holds the front side of both control arms, I wouldn't personally want to trust locktite holding the suspension together on my car.

    However, I don't remember if you can get to the top side of that at all, I'm gonna leave it to somebody else to give suggestions on how to fix it

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    If somehow that control arm swung back into the wheel well from doing anything spirited to your car u would be in a world of hurt and probably have an accident. If it was me i would re tap the hole and use an appropriate size bolt and torque to spec. Theres too much suspension load going on and only having 15lbs of torque applied to that bolt.

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    I dont know if I would trust a helicoil. Helicoils work best in blind holes. If the bolt binds a little on the helicoil, it might screw the helicoil all the way through.
    I would drill it out for the next size metric bolt M10, if I recall correctly. If a hex head wont give you enough room for a socket, then you can use a set screw (allen key head). The Larger bolt will also be stronger because it has more thread surface area. Just make sure you use a grade 8 quality bolt.
    Thats my 2 cents...
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    Original colour: 305 Light Blue. The PO changed it to Red

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    Crumudgeon
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    Timesert. And yes, you need that bolt torqued to spec.

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    I encountered the same problem and ended up drilling, tapping the next size up.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


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    Thanks for the suggestions gents, sounds like tapping for a next size up would be the safest. I definitely don't want to mess with critical stress points, it's not like the rest of the car where it's held on by zip ties

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    John C's suggestion was a good one. Timesert addresses the concern of inserting too deeply. Link below.

    Going one size up means one odd bolt to worry about forever in the future.

    ++ TIME-SERT Threaded inserts for stripped threads, threaded inserts, thread repair stripped sparkplug's, Ford sparkplug blowouts, threaded inserts threaded, repair stripped threads, stripped threads, inserts threaded inserts, Ford spark plug repair,
    1976 280Z, with some minor modifications

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    I checked that out, my only concern is holding a hand held drill perpendicularly since the problem lies underneath the chassis... I'm not sure what the tolerance would be for something like this, I could easily see myself drilling the hole slightly slanted or otherwise, and botching the whole operation.

    Being part of the chassis I can't otherwise 'remove and take' the problem to a machine shop and have them deal with it, and at $80 / kit, that 1 bolt seems like an expensive problem haha

    The good news is *possibly*, I just re-inspected the bolt which seems to be pretty stripped in itself of threads, I'd have to double check when I get home but there's a chance that the receiving end could be ok. In the meanwhile I bought couple of m10 x 1.25 that looks like the part, hopefully these will hold the torque.

    If not, the timesert seems to be my only realistic option, that or finding a shop to re-drill/tap properly underneath the car.

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    When I drill do it slow. When u tap, use oil on the tap. Slowly twist tap into hole about one turn. Reverse half a turn. Then twist into the hole a turn. Then reverse half a turn. This technique allows the metal shavings to move to the bottom of the tap. After ur done clean the hole with brake cleaner and a chase. Its really not that hard. You should probably have a tap and die set anyways as this probably won't be the last time u need it. Instead. Of paying someone money u can do it urself and use the money for tools u can keep.

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    Also make sure u use appropriate strength bolt and lock washer.

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    ^Thanks for the tip, it doesn't seem like rocket science, and if I could do a timing chain/cam job going in blind, I think I should be able to do this.

    The new bolt had no luck, will be shopping for a redrill/tap kit tomorrow. Thanks again guys.

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    John always knows:

    Cool Product
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


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    http://ZSportCanada.com


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    A magnetic level makes it much easier to drill things straight.

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    Quote Originally Posted by John Coffey View Post
    A magnetic level makes it much easier to drill things straight.
    Thanks John, ordered a kit from Torrance last night, should get here tomorrow. I watched their youtube vids and it does look pretty straight forward.

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    I did the exact same thing on one of those bolts. I walked over to the tool box got a 1/2 fine thread tap and an appropriate drill bit and retapped the hole. I have found that the metal in my 74 varies from soft to extremely hard. bolts all seem to be very soft, but I cut a hole in the hood for some aero catch latches, DANG! That metal was tough!

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    Quote Originally Posted by johnbell47 View Post
    I did the exact same thing on one of those bolts. I walked over to the tool box got a 1/2 fine thread tap and an appropriate drill bit and retapped the hole. I have found that the metal in my 74 varies from soft to extremely hard. bolts all seem to be very soft, but I cut a hole in the hood for some aero catch latches, DANG! That metal was tough!
    Yeah I had to use a drive extension on the tap wrench for leverage, but got it done.

    Update: The timesert kit worked perfectly! thanks again to John C. for the suggestion.

    I put some red loctite on the outer part of the insert (probably not necessary) to make it ultra permanent. Only problem I encountered was the threaded hole itself being somewhat short, the insert did not seat fully to the counter sunken hole. So I just drilled out a ~1/8" on the diff mount bar, using the same timesert countersink tool, allowing the mount to seat flush to the body. Torqued to spec nicely.

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