Oscar--If all you need to know is how large the coil should be if everything is in an unbroken state, the coil needs to be about an 1/8" over the diameter of the bolt in question. However, It sounds like you're addressing the situation where a bolt or stud has broken off INSIDE the engine block (i.e., below the surface of the block). If that's the case, this tool won't work for you. The Bolt Buster is not designed to heat a portion of an engine block to loosen a broken bolt. You will have to remove it by carefully drilling a starter hole about a half-inch deep into the CENTER of the broken bolt/stud, then inserting an extractor and "setting" it with several strong raps with the hammer. Once set, you then need to use a tap wrench or an adjustable wrench to turn the extractor out in reverse (i.e., by turning it counter-clockwise) to loosen and remove it. You start this process by spraying some penetrating oil into the hole. Then, using a center punch and a ball-peen hammer, you hit the center punch into the bolt two or three times. That action, by itself, may loosen the bolt. You may be lucky and be able to loosen or even back the bolt out a few turns while drilling the starter hole, but don't count on that happening. You can watch videos here on KZfaq which will show you how to do this. In the "Search" box, type in "Removing broken bolts in an engine block" for the step-by-step procedure. If you've never done this before, you might be better off by taking the car to a mechanic and let him tackle this. It can be difficult to do, and the mechanic will have the necessary tools and experience to remove the broken stud or bolt for you. Good luck.

Did you watch the video or just post looking for someone to save you when you did this?

Capitalism

Still has a head so sure. Its ass end is also sitting in the sump surrounded by, something. So as long as that somethings not flammable. or something with a low flash point, Yeah go nuts. And if you haven't figured this out in two years. Sell your tools and pick up knitting.