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What size of drill bit do i need

P

pinball

Well-known Member
I have an exhaust manifold bolt broke. I have drilled out a small hole in the bolt. It is suppose to have a 3/8 stud in it. What size of drill bit do I need to drill out the hole and then use a 3/8 tap to thread it. THANKS nOPRM
 
you need 5/16 drill for 3/8-16 tap. so drilling the hole at 3/8 will not allow you to us 3/8 tap. if you drill it to 3/8 you will need to us 7/16-14. check tap drill chart. this is just off top of my head. Would try to use a 5/16 tapped hole. instead of 3/8 if possible.
 
Yes, it is a 3/8". Be sure to go exactly down the middle. Then try to peel out as much thread as you can, be very careful taping, take a little at a time.
 
Steve nailed it. I doubt seriously you hit the exact middle of the broken stud with the small hole. That being the case, it would more than likely get into part of the threads, on one side of the hole, if you drilled to the full, tap size.

The best thing to do is as Steve says, and progressively step up in size until you can see the tops of the threads in part of the hole. From there, and sharp cape chisel will do wonders to get the remainder of the bolt out of the hole.

I've done hundreds this way, some in extremely tight spots. It may take a little bit, but I have yet to have one I absolutely couldn't get out.
 
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