BB exhaust studs

33IMP said

Local quality hardware store. 3/8-16 setscrews In whatever length you need. (You may have to go to a bolt specialty shop, or order from a place like McMaster Carr.)
Also, I would check the threads in your heads. A 906 head has all the exhaust manifold bolts into water. I don't know for sure with 915s, but I'll bet they are wet too.
There should be a lot more the 3/8 thread depth. I'm guessing you have some serious rust or thread sealer built up.

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They all seemed to be wet, but after figuring that out I took out the rear bolt first since the front of the car is up in the air and that way I only had one that made a mess.

I was thinking the same thing about the bolt holes, something doesn't seem right but I'm being careful with taps etc because if I screw one up it changes my problem from can't get a stud to work into have to pull the the head(s). The weird part to me if there's something blocking the hole, is that they are all the same. I took them down as tight as I dared and they all stopped at pretty much the same place as witnessed by the thread sealer left on the studs when I took them out. I also ran a thread chaser in, and got the same result.