Thinner control arm bolt make any difference?

Joined
Jun 7, 2016
Messages
275
Location
Toronto
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:
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.
 
Nothing to add to the reasons already stated- they are correct

Replace it with the proper dimension and grade fastener
 
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.
 
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.
 
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.
 
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.
 
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.
 
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.
 
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.
 
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
 
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.
 
Originally Posted by tookien
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?



Install the correct size bolt , even if you must purchase it at the dealer .

Not saying it will or will not fail . But it can not be very $$$ . Why take the chance . You would not have asked , if you were not concerned .
 
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.


Not at all.

I'm just looking for feedback and then weigh it all in to make a decision.

It's better to replace it for sure, but I am skeptical that it's absolutely critical given that the original bolt design has play as well and not exact fit.
 
Last edited:
Originally Posted by SLO_Town
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


Yeah that's a good argument that the frame mounting holes would be larger in diameter and could introduce some play overtime.
The only thing in this case is the nut is welded center on one end so the bolt wouldn't have any play at least on that end.

But yeah, I'll eventually replace it, it's just one bolt...

Thanks
 
Originally Posted by WyrTwister
Originally Posted by tookien
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?



Install the correct size bolt , even if you must purchase it at the dealer .

Not saying it will or will not fail . But it can not be very $$$ . Why take the chance . You would not have asked , if you were not concerned .



For sure.
 
Back
Top