Checking the posts and read of if the bolt, stud, is broken off in the hole, I have seen many studs with about 1/2 inch of weld on one end, I was told they were from engine blocks and the studs had broken off about 1/2 down the hole, thus the 1/2 inch of weld and had been removed using a regular welding rod. The shop was not open at the time, the removed studs were scattered around on the ground, I thought perhaps after depositing the weld on the studs the holes would have to be cleaned out as the splatter would not stick to the cast block but could jam in the block threads. That was in the late 60s and shops kept it secret as it was a good money maker.