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Thinner control arm bolt make any difference? #5424886 05/09/20 04:32 PM
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tookien Offline OP
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I had to cut one of the bolts while changing a front control arm.

I installed a high grade bolt in replacement but it's a tiny bit thinner. The diameter is 0.97 mm smaller (0.038 inches) than the original bolt.

I torqued it down and drove it around and it's holding up with no issues, no torque steer or alignment problem.

Is that any cause for concern and should I just leave it?

Last edited by tookien; 05/09/20 04:34 PM.
Re: Thinner control arm bolt make any difference? [Re: tookien] #5424890 05/09/20 04:36 PM
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The wrong size bolt is never the right answer.
Vibration will eventually lead to fatigue.
Fatigue will lead to failure.

Can you afford that??

Re: Thinner control arm bolt make any difference? [Re: tookien] #5424894 05/09/20 04:38 PM
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spasm3 Offline
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I would get a replacement. There could be slight movement that wears and either fails or wallows out the new control arm.


13 elantra 84k 5w30 synpwr
03 chevy avalanche 83k synpwr 5w30
17 mazda cx-5 19000 miles m1 0w30
Re: Thinner control arm bolt make any difference? [Re: Imp4] #5424897 05/09/20 04:43 PM
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tookien Offline OP
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Originally Posted by Imp4
The wrong size bolt is never the right answer.
Vibration will eventually lead to fatigue.
Fatigue will lead to failure.

Can you afford that??


The original bolt isn't exactly vibration free either as it still has play within the bushing of the control arm.

I'm thinking that the small difference shouldn't be a problem because the outer ends of the bushing compresses the outside mounts when it's torqued down.

So yes I agree with you in most cases but in this case I'm just not sure it applies.

Re: Thinner control arm bolt make any difference? [Re: tookien] #5424899 05/09/20 04:43 PM
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Nothing to add to the reasons already stated- they are correct

Replace it with the proper dimension and grade fastener

Re: Thinner control arm bolt make any difference? [Re: spasm3] #5424904 05/09/20 04:49 PM
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tookien Offline OP
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Originally Posted by spasm3
I would get a replacement. There could be slight movement that wears and either fails or wallows out the new control arm.


To break a grade 10.9 bolt would take quite a bit of force. The difference here is so tiny in terms of the thickness.

Re: Thinner control arm bolt make any difference? [Re: tookien] #5424905 05/09/20 04:50 PM
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Originally Posted by tookien
Originally Posted by Imp4
The wrong size bolt is never the right answer.
Vibration will eventually lead to fatigue.
Fatigue will lead to failure.

Can you afford that??


The original bolt isn't exactly vibration free either as it still has play within the bushing of the control arm.

I'm thinking that the small difference shouldn't be a problem because the outer ends of the bushing compresses the outside mounts when it's torqued down.

So yes I agree with you in most cases but in this case I'm just not sure it applies.

Yes, it still applies.
Especially here with a control arm and the movement and vibration it sees.
Do it right.
Change it out.

Re: Thinner control arm bolt make any difference? [Re: tookien] #5424907 05/09/20 04:51 PM
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spasm3 Offline
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Should not be much, check Rock Auto or a u pull it yard.

Last edited by spasm3; 05/09/20 04:51 PM.

13 elantra 84k 5w30 synpwr
03 chevy avalanche 83k synpwr 5w30
17 mazda cx-5 19000 miles m1 0w30
Re: Thinner control arm bolt make any difference? [Re: tookien] #5424908 05/09/20 04:55 PM
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Originally Posted by tookien
Originally Posted by spasm3
I would get a replacement. There could be slight movement that wears and either fails or wallows out the new control arm.


To break a grade 10.9 bolt would take quite a bit of force. The difference here is so tiny in terms of the thickness.



Incorrect but it depends on the definition and direction of that "force". Fasteners are designed for tension loading for engineered joints. When shock and bending loads ( shearing) are introduced- they all break with very little force.

That thickness could ( not saying absolutely will) be enough clearance from design to introduce shearing or shock loading over time and cause failure.

Re: Thinner control arm bolt make any difference? [Re: ABN_CBT_ENGR] #5424915 05/09/20 05:08 PM
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tookien Offline OP
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Originally Posted by ABN_CBT_ENGR
Originally Posted by tookien
Originally Posted by spasm3
I would get a replacement. There could be slight movement that wears and either fails or wallows out the new control arm.


To break a grade 10.9 bolt would take quite a bit of force. The difference here is so tiny in terms of the thickness.



Incorrect but it depends on the definition and direction of that "force". Fasteners are designed for tension loading for engineered joints. When shock and bending loads ( shearing) are introduced- they all break with very little force.

That thickness could ( not saying absolutely will) be enough clearance from design to introduce shearing or shock loading over time and cause failure.



Thanks. I'll keep an eye out on it.

Re: Thinner control arm bolt make any difference? [Re: tookien] #5424917 05/09/20 05:15 PM
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Originally Posted by tookien



Thanks. I'll keep an eye out on it.


Other than safety, the reason I point that out is that I frequently have to do failure analysis on sheared bolts.( especially on high impact equipment at mines and lumber mills)

By the time I get there they have gone from 5 to 8 to B-7 ( even welding heads sometimes) and they are all still snapping.

Its almost always a case of the joint and/or the fastener losing dimensional tolerance and creating those loads which result in failure.

May not be the case in your application but its a very common failure mode. Not worth the risk.

Re: Thinner control arm bolt make any difference? [Re: ABN_CBT_ENGR] #5424924 05/09/20 05:24 PM
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tookien Offline OP
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Originally Posted by ABN_CBT_ENGR
Originally Posted by tookien



Thanks. I'll keep an eye out on it.


Other than safety, the reason I point that out is that I frequently have to do failure analysis on sheared bolts.( especially on high impact equipment at mines and lumber mills)

By the time I get there they have gone from 5 to 8 to B-7 ( even welding heads sometimes) and they are all still snapping.

Its almost always a case of the joint and/or the fastener losing dimensional tolerance and creating those loads which result in failure.

May not be the case in your application but its a very common failure mode. Not worth the risk.



Personally I've used slightly thinner bolts (the same ones in fact) on control arms in the past on my other vehicle and drove it for years without failure.

IMHO, as I don't really want to start an argument, it really depends on the context. As I mentioned the original bolt has play within the control arm bushing as well, so the logic clearly doesn't really follow that in this case play will cause failure.

I understand the argument from safety so I will look out for any pplay, I would really be concerned if this was a low grade stainless steel bolt.

Re: Thinner control arm bolt make any difference? [Re: tookien] #5424925 05/09/20 05:25 PM
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skyactiv Offline
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This is another thread where the OP is looking for an answer he agrees with. In this case, he wishes people would say, "you'll be fine". I concur with the majority here.


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Re: Thinner control arm bolt make any difference? [Re: tookien] #5424937 05/09/20 05:42 PM
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The biggest problem with using a smaller diameter bolt is that it does not fit precisely into the bore of the bushing and holes in the frame mounting points. Using a smaller diameter bolt may work for awhile, but eventually it was loosen and start elongating the frame mounting point holes. Then you're screwed.

Scott


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Re: Thinner control arm bolt make any difference? [Re: skyactiv] #5424938 05/09/20 05:44 PM
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Originally Posted by skyactiv
This is another thread where the OP is looking for an answer he agrees with. In this case, he wishes people would say, "you'll be fine". I concur with the majority here.

Precisely.

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