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Broken Stud on Exhaust Manifold

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Post by Mcarlo77 Tue Feb 07, 2023 7:06 pm

Never had the need to do this before, but I need some expertise on how to replace a broken stud on a stock exhaust manifold that snapped off.  This is one of the three that are used to attach the front pipe.  The stud was absolutely rust welded to the manifold and nothing I tried would break it free; although I didn't have a map gas torch to work with.  Got impatient and snapped the bolt with a socket wrench.  Anyway, my specific question is how do I proceed now that I've got it drilled out to 1/4" dia. for a 3/8" threaded stud?  Manifold is sitting level in my bench vise (which is a good thing) and feel I did a pretty good job centering the drilled hole up to this point.  What is standard procedure is such circumstances...just drill it out to 5/16" and tap to next size up (in this case 3/8").  Any other options?  This is a thru hole, btw.


Last edited by Mcarlo77 on Tue Feb 07, 2023 11:43 pm; edited 1 time in total
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Broken Stud on Exhaust Manifold Empty sorry about your bolt troubles

Post by 76Chevelle2Tone Tue Feb 07, 2023 11:12 pm

I hope I didn't jinx you but i was looking at a vid several days ago about broken bolts in the exhaust manifold that did mention drilling out a bolt among other methods. The drilling out attempt was about 15 minutes into the vid. link below.

https://youtu.be/x_ky5JVxtqU?t=957
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Post by Mcarlo77 Tue Feb 07, 2023 11:33 pm

According to a chart I found online, I'd use a 5/16" drill bit for the tap for this size stud.  But, since that won't remove the existing threads that will remain (or, might remove them on one side of the hole if I'm not perfectly centered with the drill bit), do I just run the tap into the hole and hope that the remaining threads don't cause a problem when trying to cut new ones?  Or, will I need to find a next larger size stud...if they even exist?  Also, I seriously doubt an easy-out is going to remove what's left of the stud due to excessive rust around both ends and I don't want the hassle of trying to drill that thing out if it breaks off inside the hole.

I watched a You Tube video of a guy doing this job in his machine shop.  He had all the equipment and fixtures to dial in exactly the center of the broken off stud and was so precise with his drilling that he could remove the slivers of remaining threads with needle-nosed pliers.
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Post by Mcarlo77 Wed Feb 08, 2023 4:09 pm

Well, I forged ahead and got it done.  Fought with the tap bit to get it started with a good bite and was about to give up on this plan.  Don' t know if it was fighting the remaining threads, parts of the old stud left behind after drilling, or both.  Since cast iron is a softer metal, it might have been easier to just drill the hole clean and use an oversize 7/16" stud (which I found was available).  Now to decide if I want to tackle the other two...or, just let them be.

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Post by Limey SE Thu Feb 09, 2023 12:11 am

I would fix them other two as one looks bent already and not very good threads other farthest away in pics is worse SO its already out fix them I say

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Post by Mcarlo77 Thu Feb 09, 2023 1:29 am

Yeah, Limey...I know that is the correct answer. I'm sure I wouldn't be satisfied with just leaving it this way. Thanks for the nudge!
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Post by Limey SE Thu Feb 09, 2023 1:46 am

Mcarlo77 wrote:Yeah, Limey...I know that is the correct answer.  I'm sure I wouldn't be satisfied with just leaving it this way.  Thanks for the nudge!

I was gonna offer ya one of mine I took off the 305 BUT making sure the the later model 305 Manifolds would fit yours I have a spare set from an 82 305 from a B Body wagon

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Post by zucchi Thu Feb 09, 2023 9:55 am

76Chevelle2Tone wrote:I hope I didn't jinx you but i was looking at a vid several days ago about broken bolts in the exhaust manifold that did mention drilling out a bolt among other methods…
Good info indeed. I'm especially intrigued by the option of welding a nut onto the remnant bolt. I suspect the quick heating generated during the welding process aided loosing the bolt.

Limey SE wrote:I would fix them other two … its already out fix them I say
Completely agree.
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Post by Joe73 Thu Feb 09, 2023 2:05 pm

Id first try threading two nuts on and jamming them tight against each other. Then have the appropriate size box end wrench or socket available. Then Cherry the whoa nelly out of the manifold until she threads out. Then thread the new one in.
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Post by Mcarlo77 Thu Feb 09, 2023 3:55 pm

If I had access to an acetylene torch, I'd certainly give it a try.  My propane torch just wasn't hot enough.  Wonder if the Mapp-Pro would get it red hot?  I'm guessing not. My son-in-law has a mig welder...but, from what I've read regarding welding a nut to the stud, is that you need a tig welder to get a strong enough weld. I know nothing of either, so don't know why it would matter. Point of note...the older gentleman in his machine shop who made the video on how to remove a stud that I mentioned in initial post made mention that he has had very little luck using heat to dislodge these manifold bolts.


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Post by Joe73 Thu Feb 09, 2023 3:56 pm

Best would be acetylene and the quickest.
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Post by relic7680 Wed Feb 15, 2023 2:53 am

In the past, I have drilled to just shy of the underlying thread peaks in the hole, then chased the remaining shell out with a tap. I also stepped up to a "medium duty" oxy acetylene torch setup that turns things orange quickly. Good luck pulling the two studs that didn't snap.
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Post by g3chevy / Mr Pontiac Thu Feb 16, 2023 12:38 pm

I've had success with cutting off the rusted studs about a 1/4" above the flange. Then find some fuel hard line or similar tubing the just fits over the remaining stud. I cut my tube about an inch and a half long and used a locking pliers to hold it in place. This will keep the drill bit centered for the next step. Then find the drill bit that just barely fits inside the tubing and start a pilot hole. Then take everything off and inspect. The pilot hole you started should be pretty much centered on the remaining stud. Then I used an 1/8" bit to drill through the remaining stud. Then I worked with progressively larger bits until I could just see the threads in the flange. Then I removed the remaining metal with a pick. Some of it came out looking like a heli-coil. Then I ran my tap through it to clean up the threads. A lot of work but it turned out well. If you need a replacement manifold from southern car, I've got a couple of those too. And I have to agree with Joe. An acetylene torch is the best way to tackle this project if you have access to one. Hope this helps.
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Post by Mcarlo77 Thu Feb 16, 2023 3:34 pm

This was my first attempt at drilling/tapping a hole. My method was to level the manifold flange in my bench vise and use a center punch to get as close as possible to center for drilling. Thought I had pretty much nailed it. Then, using a 1/8" bit and my drill that has a leveling bubble on the end, proceeded to drill. Looked centered with that first and second drilling (3/16" bit). Stepped up to 7/32" and then 1/4". Still looked pretty good. 9/32" bit started to bind as I slowly drilled (brand new cobalt bit). As the remaining wall of the bolt got thinner, I could it was more apparent that the hole was slightly off-center (little or no threads on one side and part of the bolt on the other side of the hole). Finished with a 5/16" bit (also new) and it was obvious now that I had drifted off to one side of the hole a little. Think this is why I had trouble starting the tap bit as it was catching on the remaining part of the bolt. Wrestled with getting it to bite while trying to hold it straight up and down and just about gave up on the whole thing until I finally got it to catch. Haven't attempted the other two studs.
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Post by g3chevy / Mr Pontiac Thu Feb 16, 2023 5:02 pm

Mcarlo77 wrote:This was my first attempt at drilling/tapping a hole.  My method was to level the manifold flange in my bench vise and use a center punch to get as close as possible to center for drilling.  Thought I had pretty much nailed it.  Then, using a 1/8" bit and my drill that has a leveling bubble on the end, proceeded to drill.  Looked centered with that first and second drilling (3/16" bit). Stepped up to 7/32" and then 1/4".  Still looked pretty good.  9/32" bit started to bind as I slowly drilled (brand new cobalt bit).  As the remaining wall of the bolt got thinner, I could it was more apparent that the hole was slightly off-center (little or no threads on one side and part of the bolt on the other side of the hole).  Finished with a 5/16" bit (also new) and it was obvious now that I had drifted off to one side of the hole a little.  Think this is why I had trouble starting the tap bit as it was catching on the remaining part of the bolt.  Wrestled with getting it to bite while trying to hold it straight up and down and just about gave up on the whole thing until I finally got it to catch.  Haven't attempted the other two studs.

Sounds like you did everything right. These are a real pain in the arse to deal with. Let us know how you do on the other 2. Always looking for tips and tricks to make this a little less painful in the future.
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Post by jaybee Fri Feb 24, 2023 8:16 pm

Joe73 wrote:Id first try threading two nuts on and jamming them tight against each other.  Then have the appropriate size box end wrench or socket available.  Then Cherry the whoa nelly out of the manifold until she threads out.  Then thread the new one in.  

I agree. I'm a career mechanic of 40 years and the most used method is to double nut and plenty of heat ! Unless broken off flush, then - drill and tap and / or a thread insert. Or alternatively, with proper tools - center a nut on broken stud and weld in center of nut to stud. Then heat manifold and remove.

Also I've had good success with using copper anti seize on the new studs for removal later down the road. Good luck folks.
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Post by Mcarlo77 Sat Feb 25, 2023 12:38 pm

Appreciate the tips...but without access to an acetylene torch, I'm pretty much restricted to the drill and tap method.
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Post by jaybee Sat Feb 25, 2023 9:00 pm

Mcarlo77
I completely understand working with limited tool selection. As a low buck alternative to an oxy / act torch, you could purchase from your local hardware store a portable MAP gas torch. Very similar to the everyday propane torch used for plumbing - just a hotter burning gas.  Just a thought

Yes heating the manifold cherry red is kinda the preferred method, but bear in mind really all you need to do is break the rust / corrosion bond. Sometimes a couple of hi temp. heat cycles will do the trick. I've actually had to do as many as 4 heat cycles to accomplish said task. Good luck 🤞
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Post by Mcarlo77 Sat Feb 25, 2023 9:27 pm

I was just reading up about Map-Pro gas vs my propane soldering kit.  As you know, the hotter burning Mapp gas was discontinued 15 yrs ago.  Map-Pro is next best thing, but it only burns about 130 degrees hotter than propane.  Not sure that's enough to make a difference.  Still might make use of a gift card I got and replace my propane kit with one just to see.  On a side note, I was measuring the diameter of the worst of the remaining two studs at the exposed base where it threads into the manifold and it is a full .060" smaller due to corrosion.  Imagine they are also brittle from so many heat cycles and will snap off no matter what I try.  Anyway, I'm going to go for broke and what happens, happens.  No big loss if things don't go well.  It will be a learning experience if nothing else.
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Post by jaybee Sun Feb 26, 2023 4:09 pm

Hay Mcarlo77

Your reply got me thinking I saw a small homeowners series oxy / act torch kit somewhere in recent years. I did a quick internet search and found a small Lincoln Port A Torch kit advertised at Fleet / Farm for about $350. If ya can afford the extra cash, it definitely would be a better set up then a MAP gas kit. Also Harbor Freight has  similar items - probably questionable quality. I personally wouldn't chance there product with hi pressure gases. Also I think Menard's has a small kit also . Again just a thought.
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Post by Mcarlo77 Sun Feb 26, 2023 7:21 pm

I was reading about those oxy/acetylene kits, too. Just can't justify the cost for this one project...and, doubt I'd ever have a need for it in the future. I'll forge ahead with the drill/tap routine and see how this second attempt works out.
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Post by Mcarlo77 Sat Mar 04, 2023 3:09 pm

Well, got the second stud replaced. Went about the same as the first one. Tried hitting it with propane once I had drilled about half the center out and using an easy-out...but, had no effect. This drilling was dead nuts in the center and still no slivers from the old stud threads as I reached the 5/16" drilling required for the 3/8" tap. And, still tedious trying to get the tap to bite while trying to keep it straight up and down. Patience is the key using this method. But, so far all three studs still line up with the exhaust pipe flange holes. Little room for error on this last stud, though.

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Post by jaybee Sat Mar 04, 2023 9:17 pm

Slow and steady wins the race every time with these types of projects on a tight budget.
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Post by Mcarlo77 Sun Mar 05, 2023 5:16 pm

Success at last! The tap on the last stud bit immediately and easily worked its way down the drilled hole. Think the difference was that a short sliver of the old stud's thread came out with my final drilling and the tap was able to follow that groove to get started. Glad I was able to salvage this manifold.

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Post by Limey SE Sun Mar 05, 2023 5:30 pm

Great Job Buddy Now I will send the 3 sets I have so you can replace them YOU DID SUCH A GOOD JOB LMAO

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