I had a similar problem with my Lambda, having to Helicoil a few stud holes. I had to hire a magnetic drill in order to be sure the threads were straight. I subsequently found adverts on the Internet for such drills, not a lot more expensive than the cost of hiring one. If I ever have to do a similar job, I'll buy a magnetic drill. By the way, I would not even consider running the engine before putting the stud in properly.