Some of you may already have seen this infamous forum thread on Honda-Tech. For those who haven’t, a hapless (and clueless) would-be mechanic damages one of the lug bolts on his VW, and after a couple of misguided attempts to extract it, uses a sawzall to cut away the wheel to free the broken fastener. A truly astounding and hilarious example of poor judgment.
Broken fasteners are a fact of life when wrenching on cars, and the older the car, the more likely you’re likely to experience one. Fasteners–bolts and screws–break in a variety of ways: bolt heads round off, screw heads strip, bolt and screw heads break off, threads strip, and fasteners as a whole can seize. Causes vary as well: overtorquing, rust and corrosion, heat and vibration, etc.
Tap and die set: use these to cut new threads into a blank hole, or clean up existing damaged threads.
It’s important to understand how fasteners work: bolts, when properly tightened, are under tension, which means that the metal is slightly stretched (but generally not beyond its ability to spring back to its original shape). Overtorquing a fastener stretches it beyond its ability to hold its shape and permanently damages it, often causing it to break (most often, the head will break off). Overtorquing can also damage the metal a fastener is screwed into; most commonly I see bolts in aluminum overtighened which damages or destroys the threads, whereupon a larger hole must be drilled to tap threads for a larger bolt, or a thread-repair tool such as Helicoil or Timesert must be used.
Most screws in automotive applications tend to be Philips-head (“cross” shaped hole for the screwdriver) or sometimes Allen-head (hex key) or Torx (“star”), or some other variant. It’s important to use the right tool and right size of tool to tighten and loosen these screws; “improvising”–in my experience–with the wrong tool is what most often damages the head. The tool can also slip, or sometimes the screw has seized up and too much torque is applied in an attempt to break loose the screw; the result is that the hole for the tool will end up rounded and the tool will no longer turn the screw. For smaller screws that are easy to reach, I’ll sometimes use a Dremel tool to cut a slot into the screw head, allowing me to use a flat-head screwdriver to undo them.
Otherwise, or for larger bolt heads that have been rounded off (rounded-off bolts most often occur as a result of a 12-point socket slipping on a 6-point bolt head–ask me how I know), an extractor tool (e.g. EZ-Out) can sometimes help. You drill a hole into the fastener and screw the extractor bit into it; the bit is threaded counterclockwise so that as you tighten it, you loosen the damaged fastener.
Screw extractors; use these to remove a damaged or broken screw or bolt.
In my personal experience, extractor tools don’t often work, for the simple reason that the fastener head was damaged in the first place because the fastener has seized or “galled” against the surrounding material and the bolt/screw head was damaged because too much torque was applied to try to loosen the fastener. Therefore, it’s unlikely the extractor tool will allow you to apply even more torque to loosen the fastener.
Instead, I prefer simply to drill out the head of the damaged fastener. In the case of screws or other “female” headed fastener, the rounded-off hole provides a natural centering guide for the drill bit. The goal is not to drill out the entire fastener, but rather to break off the head by using a drill bit slightly wider than the threaded or shoulder portion of the fastener. Because the screw is under tension, often it’s not even necessary to drill entirely through the material–drilling deep enough weakens the material enough for the fastener (which is still under tension) to break. Once the head has broken off, the part can be removed and the remaining threaded part of the fastener is usually easy to unscrew because it is no longer under tension.
The best way to deal with broken fasteners is of course to prevent them from happening in the first place; I use the following rules of thumb:
- Don’t overtorque (overtighten) fasteners. Use a torque wrench when and where possible, and otherwise use common sense taking into account the size of the fastener and the material it’s in (aluminum is very soft and easy to damage by overtorquing).
- Start screwing in fasteners with your fingers (and not a socket wrench, impact gun, etc.) to make sure threads are engaging easily. Except for special fasteners, screws and bolts should initially turn easily by hand until they are nearly tightened. If not, the threads may be damaged or dirty, fastener holes in multiple parts may not be properly lined up, or the fastener may be angled incorrectly.
- Keep fasteners clean; don’t let them sit on the ground where they can pick up dirt and metal shavings and other debris.
- Use anti-seize compound on fastener threads, where appropriate.
- Use the right tools (sockets, wrenches, drivers) to loosen and tighten fasteners. “Universal” tools such as adjustable wrenches, vise grips, etc. should generally be avoided when initially breaking loose or finally tightening the fastener.
- Use a penetrating lubricant such as WD40, PB Blaster, or Liquid Wrench  before undoing fasteners that may be seized and/or corroded. Give the stuff time to work (minutes to hours depending on the exact situation).
Broken fasteners will still happen. When they do, it’s most important to apply the right tools and techniques to correct the problem. Remember, fasteners are generally much cheaper and easier to replace than the parts that they hold, so it’s always preferable to damage the fastener rather than the part, if you have to choose between the two. Something the Honda-Tech poster should have kept in mind before he picked up the sawzall.
