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HELP! broke off a Grade-8 bolt inside my block, What drill bit do I buy to remove it?

Discussion in 'The Hokey Ass Message Board' started by Payne, Jan 29, 2012.

  1. Hello Everyone,

    I figured I would ask on here before I spent the rest of the day searching online for the right answer. I broke off about a half-inch of a 5/16" Grade-8 Allen bolt inside the front of my 241 Dodge Hemi block, and I was wondering what type of hardened drill bit to buy?

    I spent the better part of yesterday tearing down the front of the motor and with the help of some friends, attempted to drill a hole in the broken bolt. Our plan was to drill a small hole and step up in drill bit size until I had a whole big enough to use an easy-out to remove the broken bolt.

    All was going well until we broke off a drill bit in the broken bolt, so now I'm guessing I'll have to buy some type of harden drill bit that will be able to drill into both the Grade-8 bolt and the chunk of drill bit inside of the bolt? If anyone one know what type of harden drill bit I need to buy it would be greatly appreciated?

    The motor is fully assembled, painted, and ready to run. I''m hoping I don't have to completely tear down the motor just to remove a broken stud.
  2. I've done something very similar on a SBC. I had to use a EDM (electrical discharge machining) to get it out. I spent hours trying to drill. Like 8-12 hours. Then broke the bit and finally took it to work and had it done in 2 hours.. I did not have to disassemble the motor.
    Last edited: Jan 29, 2012
  3. Payne, tungsten carbide or just regular carbide. Remember carbide is brittle and will shatter, but will drill most any hard steel. TR
  4. Left hand carbide if you can find it. Regular carbide if the drill bit wasn't there. The carbide drill will be very heavy compaired to other.
    Center punch can break out (shatter into fragments) the busted bit and then have at it again.
    5/16 bolt is small to drill, grade 8 is hard to drill , that's going to take great care. Good luck
    j-jock and i.rant like this.
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  5. rogmoseley
    Joined: Jan 7, 2009
    Posts: 58


    There was arecent post just about this very thing. You can mig a nut to the broken bolt and it should come out. research the tech archives. haven't tried it yet tho myself.
    j-jock and Rich S. like this.
  6. Russco
    Joined: Nov 27, 2005
    Posts: 4,131

    from Central IL

    Is it close enough to the end to weld to?if so weld a nut or weld a washer then a nut to the broken off bolt.
  7. Good one on the left hand carbide 31, hopefully he can find one. A true solid carbide twist drill will be very expensive, so you will probably get a carbide tipped twist drill. The carbide segment will be brazed to the tip of a ordinary high speed steel twist drill.

    The key to your sucess will be centering the bit to start the cut. If you have access to a small die grinder and a small ball end burr, this is best. Make yourself a small concave entry point for the carbide bit to start in the dead center. This will give you a much better lead in than a center punch only in grade 8, TR
  8. NASTY 1
    Joined: Jul 24, 2011
    Posts: 76

    NASTY 1

    I would use a mig welder/arc welder if you find you can't drill it, weld on the broken end of the bolt carefully a little at a time until you have a stud that you can get visegrips on, do it while it is still warm if possible. I use this method on tiny 10/32" screws to large bolts, I have about a 99% success rate, good luck
  9. handyandy289
    Joined: Sep 19, 2010
    Posts: 354

    from Georgia

    It works.
  10. Here's a couple cell phone shots of the broken bolt and the motor. I don't want to put any heat to block as it's already been ground smooth and painted gloss black. And I really don't want to have to disassemble the entire motor to have machine work done.

    Attached Files:

  11. I used a masonry bit (carbide tipped) one time after breaking off a drill bit. I don't think the bolt was grade 8 though.

    Pretty engine. Sorry for your woes.
  12. I purchased carbide burrs for my Dremel to grind out a broken off screw extractor in a M6 (<1/4" dia) screw threaded into aluminum. Proceeded carefully and patiently. Worked perfectly.

    Why did the grade 8 screw break off in the first place? Answering that question will help you understand just how stuck the broken off piece is in its threaded hole, and how to best remove it.
  13. 201
    Joined: Dec 17, 2002
    Posts: 344


    Had the same scenerio with a busted rear axle stud on a class 8 truck rearend, busted drill bit and all. Heated the exposed stud red hot with torch, which softens the hard metals and was able to continue drilling. A left hand drill would of helped. Didn't have to worry about paint, though.

  14. I agree, knowing how and why it broke can lead to a more effective answer as to how to remove it.
  15. hvywrench
    Joined: Sep 29, 2011
    Posts: 158

    from N.W. Conn.

    I agree about finding out what broke the bolt first. If it was overtightened and snapped without cross threading, it should back out fairly easily with a left hand bit.
    I have used the MIG trick before with small spots until there is a stub sticking out, then weld on a nut. Spotting small deposits on the bolt doesn't have to make a lot of heat, just go slow.
    A Grade 8 bolt should have drilled without a lot of drama as long as the bit was sharp. I have drilled quite a few. The problem now is the drill bit in the hole, carbide burrs have worked for me in the past. The trick is to stay centered.
    Hope this helps.
  16. pug man
    Joined: Apr 9, 2007
    Posts: 1,010

    pug man
    from louisiana

    I would place a nut over the hole and try and weld it to the broken bolt and you should be able to back it out..... good luck......
    Rich S. likes this.
  17. thaugen
    Joined: Sep 18, 2007
    Posts: 174


    Billy, are you sure you want to mess with it at all, instead just leave it? Can't tell from the photos if it's absolutely critical to the motor. (Bell housing?) Since the motor is all assembled, can it wait until the next time the motor needs something? I've seen motors that ran and didn't leak even with a totally loose bolt or two.

    That other broken bolt thread isn't too old, but these guys already told you the most popular fixes.
  18. gallagher
    Joined: Jun 25, 2006
    Posts: 180

    from califorina

    since its on a stand just load it up in a truck and have it burned out by a shop about 20 bucks . i have had a few taps burned out dident even hurt threads
  19. Mr48chev
    Joined: Dec 28, 2007
    Posts: 26,971


    Yep, Inquiring minds want to know how the bolt broke off. Bottomed out? or over torqued?

    Along with the carbide tips that they have in the Dremel tool rack at Ace and other hardware stores they have small round grinding stones for sharpening chain saws. They work great to cut hard metal in tight places as long as you don't apply too much pressure and snap the stone. I've cut a number of bearing races out with them when they were stuck on the spindle or in the hub. You might be able to cut the center out of the bolt along with the bit with one of those. It will take some patience though.
  20. CH3NO2JAY
    Joined: Feb 28, 2008
    Posts: 244

    from Chicago

    Do you have access to a screw extractor set???
  21. Have you tried needle nose pliers or some thing to back the drill out? Like using a punch and tapping it in reverse direction. Most times you can loosen the drill bit that way. If you can drill threw the bolt and put some penetrating oil down the hole.
    Then i would try heating the broken bolt slowly and then try to back it out.
  22. Yep it's critical. It's one of four 5/16 bolt holes in the timing chain cover used to hold the front "Hurst" style mount to front of the motor, since I switch from a stock water pump to a sbc water pump.

    kidcampbell71 likes this.
  23. 204carcrazy
    Joined: Nov 12, 2010
    Posts: 8


    i would talk to engine or machine shop i'm sure they do this all the time ,show them a pic . to drill exactly on center is hard and it could cause more damage and if you break an easy out they are even harder. i try to do things myself but would be really mad at myself if i screwed this up. , at least get there opinion. good luck . jim
  24. I don't know to be honest, but I would have to guess I over torqued the bolt putting it in, and I'm hoping that's the reason why. The main concern I have is the bolt broke after setting the hemi motor and t-5 tranny in the chassis and driving around with the car on a trailer for a day in Spokane. What will happen when I get the car running and apply some torque to those four small 5/16 studs?
  25. It looks like it is broke down in the hole...usually if it botts out it will snap right at the surface when the stud stops and you twist the exposing off.

    I would say it was already fatigued, broke and wiggled loose trailering it around, I can't see we're weight and vibration woul cause one to do that and not harm the others. I will say 5/16 is pretty small for motor mount bolts.

    After you get it out (you will, just a pain in the ass) can you drill and tap all 4 holes to a larger size?
  26. Judging by the pics, the bolt looks to be broken deeper than the face. That leaves the weld tricks out, the pliers tricks out. Carbide drill in 1/4" is not cheap, about 30.00.

    Hurst mounts to timing cover, I believe that puts those bolts in a shear condition.(2 single mounts or is that the horseshoe set up?) 5/16 bolt isn't that difficult to break. Figure 1/2 of the engine weight + 1/3 trans weight on a side. Now multiply that by the length of the mount. That will give you some idea of what you are asking that bolt to do. It will be much more taxing on the bolt if its pulling or riding on the threaded parts or previously been stretched and re torqued. Botls are strongest in compression not shear.

    Maybe up the bolt size or find another way. Remember that some bolts are only case hardened and react differently to loads.

    My hemi has 6, 7/16 " bolts for mounts, they are in shear but direct, without any additional cantilever leverage.
  27. Hot Rods Ta Hell
    Joined: Apr 20, 2008
    Posts: 4,062

    Hot Rods Ta Hell

    I agree with increasing the bolt size from 5/16" if you can. Possibly the stress of the Hurst mount has yanked the hole threads to be a bit slopped and caused issues?

    I've never cared for the design of Hurst cradle motor mounts. Think about all the weight and torque of that engine (hanging) on those bolts. The lateral shear force is not good for the primary design of bolts (yeah, I know early Chevy V-8's were front mounted). Side mounts seem to be a better design.
  28. Engine man
    Joined: Jan 30, 2011
    Posts: 3,476

    Engine man
    from Wisconsin

    I'm the guy who gets to remove these things. It is rather strange for a bolt to break that deep in the hole. Was it too long and bottomed out, was it over tightened or was it just a bad bolt? If it bottomed out into taped threads, it will come out hard

    As has been said, you can usually break the drill bit with a punch. If not, carbide burrs in a Dremel tool or air grinder will likely be your best bet. Did the bit break as it went through the end of the bolt?

    I would stand the motor on end, set a 5/16 nut on the surface and use a 1/16 welding rod in the center of the hole to build weld up to the nut. If you stay fairly centered, the flux on the rod will go to the outside and protect the threads. It takes practice and a steady hand.
    j-jock and Rich S. like this.
  29. timmy25252
    Joined: Sep 15, 2007
    Posts: 315


  30. 5/16 is probably too small, considering how much a hemi actually weighs, and I've heard that from a couple of people but only after I had all the machine work done, had the engine built, ground it smooth, had it painted, fully assembled, and bench fired. I guess this is why you shouldn't go full show on the first motor you've ever had built. When you know absolutely nothing about them!;)

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