A broken bolt or screw is the bane of auto restorers, not to mention anyone else trying to fix any piece of machinery. The job can't proceed until the fastener is removed and, of course, you didn't allow for all that extra time, did you? What to do now? What alternatives are there to get the bolt out?
There are, in fact, many different ways to get broken bolts and screws out. Which method you use will depend upon the circumstances and also your available tools. At a minimum, however, we recommend every hobbyist own a small Easy-Out kit, set of left-hand drill bits, propane torch, electric drill (3/8ths chuck, variable speed and please, don't buy one of those expensive battery-powered drills!) and Vise-Grip pliers.
We'll talk about all these momentarily, but first a word about penetrating oils. These are light oils that (supposedly) work their way down between rusted threads and help to release the bolt. While it's true that — given sufficient time — these oils do help, you will find that using them seldom gives instant gratification. They generally take a lot of time to soak in and are useful in situations where you can leave the seized part for days or even weeks. You should have a good selection of lubricants and cutting oils and penetrating oils, but don't assume you're going to get anywhere with them when you're pressed for time.
Okay, let's look at some of the more routine steps in removing a broken bolt. In all cases the best rule of thumb is to start with the simplest procedures and work your way up from there, as each approach fails to get results.
First, let's assume part of its shaft is protruding from the metal surface. In such cases it's always best to try to remove it by clamping the Vise Grips (very tight!) onto the shaft and attempting to turn it out. In our years of experience we've found that in about half the attempts the bolt or screw comes out, so it's well worth trying.
If it doesn't come out it's time to get the propane (or MAPP) torch and heat the shaft and surrounding metal to a dull red color. This will, in most cases, break the molecular bond between the bolt threads and those in the material and allow you to turn the bolt out using the Vise Grips. Heat also expands the metal a little, and all this helps. Heating too much, however, causes the fastener to soften and it might break off, so don't overdo it. Do we need to mention that this method shouldn't be used in situations where heat will harm or destroy the finished surface?
Didn't work or the bolt broke off again? Now it's time to get out the drill, but first you need to grind or file the bolt's shaft perfectly flat. Once you do you can use a drill bit or center punch to locate the center of the shaft, then to create a starter-hole for the drill bit. Choose a bit that's smaller than the diameter of the bolt shaft and start drilling.
In this case it's going to work better if you have a left-hand drill set, because once you've drilled far enough into the bolt the drill will bind up and probably unscrew the remaining shaft. If the drill goes all the way through the bolt shaft you can drill again, using a slightly larger bit. Eventually you will be left with thin remnants of the bolt's threads which you can remove with a dental pick or run a threading tap into the hole to clean it out.
If you don't have left-hand drill bits you should have an Easy-Out set. These are little hardened bits with twisted flutes arranged in a left-hand pattern. The top of each bit is squared to allow turning with a wrench. The bits are tapered, so the several sizes in the kit will fit a large range of bolt sizes. The procedure is to drill into the broken shaft a short way, hammer the appropriate Easy-Out bit into the hole until it wedges and then turn it out with an adjustable wrench.
If all these methods fail to remove the broken bolt (yes, we've had this happen plenty of times over the years!) then the only choice is to drill it out completely, which means drilling away the mating threads where the bolt was fastened. In this case you will have to tap new, larger threads into the hole and find a larger bolt to use in that location.
Can't use a larger bolt because of specific fit or appearance reasons? Well, in such cases you can obtain a Helicoil kit. A Helicoil is a ready-made threaded adapter that will screw into the larger hole you tapped. The adapter has its own internal threads of the original size, so the proper fastener can be used. Helicoil kits are available wherever you find Easy-Out kits (good hardware stores and tool stores, online, etc.)