We’ve all been there at one time or another—tightening a fastener to specifications, only the specified torque is never reached. That unsettling feeling of a stripped thread. For most of us, it means taking a head or block to the machine shop for a fix.
But did you know thread repair is something you can do yourself?
Thread repair kits are a quick and easy fix for damaged bolt hole threads. You need patience and a steady hand long on accuracy and you can do this yourself. Damaged threads have to be carefully drilled out to where you have a clean straight hole—then—cutting in new threads to support the threaded insert.
This is easier than you think.
A visit to your favorite performance parts website reveals plenty of thread repair kits as well as stud/bolt/plug removal tools to remove anything broken off in the casting. We’re going to show you how to remove a seized or broken bolt/plug without inflicting damage to the casting. Then, we’re going to show you how to replace/repair damaged threads.
Dissimilar Metals and Destructive Corrosion
We often see the corrosive combination of steel and aluminum, which don’t get along because they are vastly different metals. Steel plugs and fasteners, when screwed into aluminum, suffer from dissimilar metal corrosion. They seize up and become impossible to remove.
The key to successful removal is proper installation to begin with.
Bolts, screws, and plugs should be coated with something like Permatex’s “Anti-Seize” lubricant, which is engineered for use during assembly to prevent galling, corrosion, and seizing between fasteners.
Anti-Seize works very well for spark plugs, cylinder heads, exhaust manifold bolts, and other extreme heat installations. It is also effective on dissimilar metal combinations because it keeps these metals separated. Generously coat threads and torque fasteners to the proper specifications for ease of disassembly later on. This is especially important in highly corrosive environments such as salt exposure and corrosive chemicals.
You may also coat plugs and fasteners with Teflon tape or paste as a protective barrier between dissimilar metals. Anytime you’re combining dissimilar metals, regardless of type, you are courting destructive corrosion.
Save yourself a lot of grief and protect fasteners and plugs during installation. Teflon provides sealing in places such as wet decks where coolant can wind up where you don’t want it, such as the garage floor or your engine’s oil pan. You may also use Permatex’s “The Right Stuff” on bolt and plug threads for sealing and corrosion resistance. This is especially important with screw-in rocker arm studs with holes that go directly into water jackets.
Removing Broken Bolts and Seized Plugs
Removing broken fasteners and plugs is a simple task that must be taken in baby steps beginning with a small pilot hole and gradually increasing size until what’s left is thin enough to remove without damage to the hole. It is best to begin removal with a soaking with a penetrating lubricant such as WD-40 or Liquid Wrench. If you have the luxury of time, you will want to do this over a period of days or even weeks. Keep the fastener or plug soaked with penetrating lubricant and keep trying to remove it.
When preparation does not work and you’ve drilled out the fastener or plug, apply heat with a torch to the casting around the hole and be conservative about heat. Aluminum melts around 1,300 degrees F. With heat applied, remove the fastener. If you’ve been unsuccessful at removal, drill it out to as thin as you can get it without damaging the casting. You may have to cut new threads and/or install a threaded insert to save it.
Jim Smart is a veteran automotive journalist, technical editor, and historian with hundreds of how-to and feature articles to his credit. Jim's also an enthusiast, and has owned and restored many classic vehicles, including an impressive mix of vintage Ford Mustangs.