Fixing bolts - upper & lower arms to subframe

[marinabrian]

Originally Posted by Typhoon190

Oh dear.

I thought that DMGR's would have sorted this by now.

I purchased the complete kit to fix my Dad's ZT-T. It failed it's MOT on a fractured coli spring, and having removed the coil spring small sections of the upper arm had rusted through. I got the bolt kit out of the envelope and should have trusted my better judgement but assumed (my mistake) that these would have been supplied as a suitable alternative.

Snapping the upper arm to sub frame bolt whilst tightening to the required torque I realised my mistake.

They have an email from me.

I will now have to drop the sub frame again to try and get the sheared thread section out of the captive nut and arm. Not happy is an understatement.

I'm sorry to hear your tale of woe Ben, this is why I warned originally about using the correct fasteners, and this was apparently dismissed as unimportant, or otherwise by DMGRS, quoting yield strength is one thing, understanding it quite another.

Preload of any threaded fastener is important, as when radial torque is applied what you are actually doing is applying torsional load to the fastener, and placing it in tension.

As a rule of thumb, the preload should not exceed 75% of the yield value of the fastener.

So when a 10.9 tensile strength cadmium plated bolt, is substituted by a 8.8 bright zinc plated one, the yield value is reduced by 68% of the original fastener, meaning the elastic limits of the fastener are easily exceeded as you have found.

In short without delving too deep into semantics, the original fasteners are carefully calculated in design to use the minimum diameter of bolt to effect the correct clamping load to the joint.

Substitution without understanding the concept that tension is the important factor, and the applied tension of two otherwise similar components, but of different tensile strength, fastened to an identical torque figure is completely different.

Brian

[Typhoon190]

Hi Brian.

I'm annoyed at myself. I work in engineering and saw the bolts were marked up 8.8. I should have stopped the job.

[Arctic]

HI Ben.
You are allowed to be with yourself we all make little mistakes now and then, what comes after is how you learn from them lets hope DMGRS will now remove these from sale with the arms and either add the correct ones or point purchaser in the right direction from where they can be sourced, hopefully you will be able to remove the snapped bolt without to much trouble.

[trikey]

Originally Posted by Arctic

hopefully you will be able to remove the snapped bolt without to much trouble.

With a new bolt this should hopefully be an easy task, once the pressure is off the bolt (As it will be when it has snapped) The threaded part shouldn't have too much resistance.

[Typhoon190]

Originally Posted by trikey

With a new bolt this should hopefully be an easy task, once the pressure is off the bolt (As it will be when it has snapped) The threaded part shouldn't have too much resistance.

Hi Andy. The issue is access now. The head and shank will come out no problem, but it depends where is has sheared. If there is any shank remaining I won't be able to rotate it clockwise and out through the captive nut. If there is still part of it holding the arm in, it won't be easy to get hold of to 'undo'.

I'll sort it, not impossible just much more work than it was supposed to be, and it's my Dad who is without a car for a few more days.

DMGR's have acknowledged so they're are on it. This need sorting out for their sake as well, as a failure at another time could have much more severe consequences.

[macafee2]

may be quicker to remove sub frame and do it on the floor

macafee2

[trikey]

Originally Posted by Typhoon190

Hi Andy. The issue is access now. The head and shank will come out no problem, but it depends where is has sheared. If there is any shank remaining I won't be able to rotate it clockwise and out through the captive nut. If there is still part of it holding the arm in, it won't be easy to get hold of to 'undo'.

I'll sort it, not impossible just much more work than it was supposed to be, and it's my Dad who is without a car for a few more days.

DMGR's have acknowledged so they're are on it. This need sorting out for their sake as well, as a failure at another time could have much more severe consequences.

Yes, access to these bolts is a pain, I have just removed one that had been rounded by someone trying to remove the arm (Luckily an imperial socket did the trick)

I would try and see if you can get a punch or a small chisel on the broken bolt to see if you can turn it enough to allow a small pair of long nosed pliers in..

Good luck.

[clf]

would a screw extractor work? I imagine it would on the floor, I cannot remember how much space there is fitted

[breakfastinsmethwick]

I had the same thing a few years ago with DMGRS and told him about it. Not only are they the wrong property grade, they have insufficient corrosion protection too. I would not sleep at night supplying inferior replacements for safety critical parts. It would seem a little knowledge could be dangerous.

[Blink]

Ben - here's a pic in case it helps.

1. Captive nuts as viewed from above the subframe.




2. Blow up of above (split)




3. Clearer view of the captive nut cage (lower arm shown)




4. Above taken from this wide shot (lower arm again)