Stud Removal Tool
Double nutting sometimes does not provide enough clamping force to remove the stud. You need to borrow a collet type stud removal tool. I think its a 5/16" coarse thread. And, one lesson that I learned was, do not return the tool until you have used it to insert the new stud to the correct height. Be aware that studs have a slightly larger diameter thread on the engine side than on the nut side, and use a thread sealer before inserting the stud.