I believe you are speaking of the Bolts that go through the bearing housing to the exhaust housing.

Did you snap off the Ezee out type tool or a Tap during re-threading?

It is quite common for turbos that use bolts in a similar manner for them to rust/corrode in and snap off when you try to remove them. Note that that would not have happened at all if when it was assembled a Never-seeze type compound had been applied to the bolt threads.

For others one of he ways to decrease the chance of shearing the bolts is to do the best you can to put a punch over the top of the bolt head and give hit the punch several good wacks with a hammer.

Next is penetrating oil and heating the are the bolt threads into with a Propane Torch.

If they won't turn off normally after that. If possible use the Box end of the wrench and take a Hammer and tap with the hammer on the wrench in one direction of rotation and then the other direction. If it loosens a bit get some penetrating oil in the gap formed and heat again.

Having already been broken applications of penetrating oil and heat several times trying to get the oil to seep down into the threads followed by the proper sized drill for the Ezee out type tool would have been next.

As you indicated the Hole needs to be drilled down the center and the right size for the ezee out your chances of success decrease. I have always had trouble with the drilling part of the job.

Ezee outs are usually so hard they cannot be drilled through with normal drill bits.

Not sure if it would be cheaper for you to get another Exhaust housing from one of our Members or taking it to a shop that specializes in burning broken bolts and so on out.

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84 300D, 82 Volvo 244Gl Diesel