Oops! - broken subframe stud

[Can'tWeld]

While removing the front subframe from my Mazda MX-5, I managed to snap one of the four mounting studs inside the shock towers. No idea why - this particular one was a real git to turn and eventually snapped right off about halfway down - might have been a damaged thread!

I didn't pay it much heed at the time because I assumed I'd just be able to back out the stud and replace it with another, but it turns out it passes through a crush tube welded to the inside of the frame rail... and is then spot-welded to the crush tube. So basically, it can't be replaced.

The way I see it, I have three options:

1) see if there's enough thread left, and if the thread is in good enough condition, to just put the nut back on anyway - I must admit I haven't checked this yet

2) cut and drill out the stud, tap the hole and thread in a new stud

3) Cut the entire thing out of the frame rail, top and bottom, and make a new assembly - crush tube with stud welded to it, plates top and bottom, then weld it where the old one went.

Not my image, and this is a rear subframe mount rather than front, but it's this kind of arrangement: http://www.miataforumz.com/attachme...lp-my-differential-ppf-bolts-stuck-004888.jpg


Any advice for dealing with this? I'm sure there's some way to fix it but since it literally holds the front suspension and engine in the car, I don't want to go the wrong way here...!

 

[rtbcomp]

1) IMO not safe unless you have more than the nut thickness of good thread left.

2) You probably won't have enough meat to tap a new hole, it would be a sod to drill accurately in situ

So that leaves 3

Would it be possible to use a nut & bolt rather than a stud? Allow plenty of clearance through the crush tube and fill it with grease to stop the bolt seizing in. If it shears again you could just knock the bolt out.

Can you grip the stud and turn it to break the stud clear then drill down the tube and through the chassis rail to fit a nut & bolt?

 

[God mode restorations]

Any chance of some pictures of the offending item. Sometimes helps people get a visual on the problem and be able to advise better. A shame you never backed it off as it was tight. Could of possibly worked it out.

 

[MrFluffy]

you forgot 4) drill it out with a left handed drill bit in the hope that the heat and vibration causes it to snag and unwind leaving the thread unmolested...

 

[Rocketman]

1) IMO not safe unless you have more than the nut thickness of good thread left.

2) You probably won't have enough meat to tap a new hole, it would be a sod to drill accurately in situ

So that leaves 3

Would it be possible to use a nut & bolt rather than a stud? Allow plenty of clearance through the crush tube and fill it with grease to stop the bolt seizing in. If it shears again you could just knock the bolt out.

Can you grip the stud and turn it to break the stud clear then drill down the tube and through the chassis rail to fit a nut & bolt?

I wonder if a shoulder bolt would be better than a standard. With a whole load of luck a standard length shoulder bolt might be the right length.

 

[Can'tWeld]

Thanks for the suggestions all.

I took some pictures showing the problem a bit better.



View from inside the chassis leg (which is the bit I was supposed to be repairing in the first place)...



You can just about see the crush tube peeking behind the cover plate. Apparently the stud is spot welded to this in more than one place to stop it rotating.

Looking at how far back the rust goes... I'm wondering if I'll end up rebuilding this entire section anyway :|

 

[Screwdriver]

Well if it's broken off at one point it's probably weakened along the entire length...

 

[doubleboost]

Heat will be your friend with this
Plenty of heat around the stud in to the (captive nut)
Then mole grips it should come out easy with some propper heat
If you have no Oxy / Fuel gas heat Weld a nut on to the stud giving it plenty of amps so the heat soaks up the stud in to the nut

 

[arther dailey]

I would agree DB,but think you have misread this, stud is spot welded into tube rather than screwed into a nut unless I have read it wrong!

 

[colnerov]

Hi, Can you get a drill in there to drill that plug weld out? Then extract the broken stud, insert new and re-plug weld.

Colin

 

[Screwdriver]

Yeah I'd go with that. I don't think you want to reuse that stud under any circumstance so destroy it out.

 

[rtbcomp]

Post #7 - he says there's more than one spot weld.

I think it's going to be a case of drill, chisel, file, carbide burrs, a lot of swearing and use a nut & bolt if possible to make it easier next time.

 

[Can'tWeld]

Thanks all.

So basically the consensus is "get the stud out any which way possible, then drill through and use a bolt from the top".

I'm wondering if, as an alternative route, I could cut out the crush tube itself and rebuild it with a new one, attached to a plate welded into the frame rail from the top. Basically reconstructing what's already there but hollow so I can just use a bolt.But less work is always better, especially with something like this as I'm not a hugely confident or experienced welder

Won't get a chance to work on it now for a week but I'll let you know how it goes, thanks again.

 

[Can'tWeld]

New development.

Seems I was wrong about them being studs inside a crush tube at all. They are actually one piece bolts, like shoulder bolts with a large flat head and a countersink at the bottom where they fit through into the bottom plate of the rail.

They are spot welded to a large bracket inside the rail which holds both of them in position and stops them rotating.

I have cut away the top of the rail and I am trying to remove the bracket by drilling out the spotwelds which should free both of them. The main thing is a) keeping the strength in the section and b) keeping the mounting points in the right place (should be fine as long as I keep the bottom plate in place).

I'll then have to try and source new bolts, or get someone with a lathe to turn some matching ones down for me. Replate the top of the rail, drill through and weld the bolts back in as they were.

Talk about a faff... all for a stupid snapped stud! Should have been more careful

 

[Dragos Marcai]

Hi there and greetings from Romania. I’m in the process of restoring my MX5 and I am facing the same bloddy issue. It’s been driving myself and my mechanic / welder nuts. I haven’t found much information on the matter besides your thread here. I would like to ask you if the solution you proposed here worked? If not, was there a solution at all? I’m afraid that if there is no solution that will maintain the rigidity and safety of the car I will have to bin this project and a lot of money that went into it. Any ideas / tips are highly appreciated. Thank you! Dragos