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This is the flange plate on one of my catalytic converters AKA exhaust manifolds where it joins the exhaust pipe.

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Before they broke off, the studs looked something like this, the driver head was reverse torx:

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The location of the flange is on the very underside of the vehicle with high exposure to water so no wonder it and the studs were badly rusted. I wanted to remove the two studs but the rusting/adhesion was so bad that they broke. First tried the proper method using a reverse torx socket, that broke off, then used a plumbers wrench around the stud but that broke off too. The plumbers wrench method worked on the other catalytic converter (V6 engine). Of course, I WD-40ed the hell out of it but it didn't help.

How can I remove these broken off rusted studs without going to a machinist to be charged an arm and a leg? I am trying to not have to buy a whole piece just because the broken studs ($240).

4 Answers 4

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Your best bet is to use heat.

Take a torch (propane will work just fine) and heat the stud portion until it is cherry red. Then douse it in PB Blaster or WD40 or just about any other lubricant. Then grab the stud with a pair of locking pliers. Even the short one should be long enough for you to do this. You should be able to walk it out, working it back and forth until it is removed. You want to unscrew it from the broken side, as if you try to screw it through to remove, you'll damage the threads in converter. To make the job a little easier, if you can cut the threads off flush with the back of the converter flange, it will mean less for you to remove and less rust to plow through during removal.

Once removed, you'll want to clean up the threads with a tap. Be careful, though ... you don't want to cut new threads, you just want to clean up the old ones. When putting new fasteners in, it is a good idea to hit them with some anti-seize. I use copper based anti-seize for exhaust parts as it stands up to the heat better.

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  • Unfortunately, that did not work. What's more I used MAPP torch, which burns even hotter than propane
    – amphibient
    Mar 24, 2022 at 20:29
  • @amphibient - Before you destroy anything, do the heat/cool cycle again and maybe even a 3rd time. You're trying to break the bond between the cat and the stud. Once that happens, it will unscrew. Mar 24, 2022 at 20:47
  • @Paulster2 -- how do we know it doesn't do the exact opposite, weld it together ?
    – amphibient
    Mar 24, 2022 at 21:27
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    Because you don't get it hot enough to weld it. Welding it requires melted metal. You can't get there on steel or cast iron even with map gas. Mar 24, 2022 at 22:25
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    @jnovacho - Do not let it cool down. There's two points to doing it when hot. The sudden shock of the cooldown helps with breaking the bond. Secondarily, when the hot metal is hit by the liquid, there's a capillary action which draws the liquid into the space between the two metals. The heat of the stud is relatively small and cools down quickly, as well as both of the liquids I mentioned, while flammable, aren't like gasoline. They don't explode or ignite with a large flame. There might be a small amount of flame, but really it's very insignificant. Mar 25, 2022 at 9:35
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Not exactly what you're asking for, but maybe a different solution. I had similar problems with a couple of Chevys. More-or-less, I cut the studs flush, and used a pair of 'split flanges' to join the ends. (picture below.) It took a lot of effort (at least for me) to find ones with the right geometry, but there seem to be a lot of choices. Walker makes some. I got mine from Exhaust Direct Ltd in Canada, who had a large catalog of them.

My most recent are still holding after almost 5 years.

split-flange coupling

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  • what is this product exactly called?
    – amphibient
    Mar 25, 2022 at 14:49
  • @amphibient - Split flange. They're listed on the EXD link under 'Flanges'. The pic has two of them, coupled with nutted bolts. (The horizontal threads at top & bottom.) The other ('vertical') bolts clamp the halves of each flange together around the pipes.
    – George
    Mar 25, 2022 at 19:56
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These bolts can be very difficult to remove due to the rust and heat they are exposed to. The threads are rusted both externally and internally.

I've had some success with the following:

  1. Soak with a penetrating oil like PB Blaster for a day or two. Then try to remove. You have enough bolt extending that a pair of locking pliers should be able to grip. If not, get an EXTERNAL screw extractor.

  2. If that fails drilling out the majority of the bolt while being careful not to damage the flange threads is usually successful. A drill press is your friend here as doing this freehand with a portable drill is tricky.

Good luck but you might reconsider your stance on taking this to a machinist.

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  • the machinist will charge me almost as much as it costs to replace it with a new part
    – amphibient
    Mar 24, 2022 at 16:27
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    Maybe shop around? I once had a similar situation with a rusted bolt on a suspension component. It broke off flush. I took it to a local machine shop and they drilled it out and even provided me with a new nut and bolt for $50. Saved me a bunch of money.
    – jwh20
    Mar 24, 2022 at 16:29
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Yes - use bolt loosener, let it soak for an hour. Make flat spots on the sides of the bolt with a file or, prefereably, an angle grinder (because it will also get it hot - crack rust) and then... locking pliers really tight - bang on end of bolt with hammer while turning. If it turns a little bit and jams, add more loosener juice - go backwards - more loosener juice - then go for it.

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