It’s only a matter of time before you run across a stripped bolt-hole or stripped threads in your DIY repair project. And if the threads are stripped, you can’t assemble the parts and achieve the correct torque. Here’s why this issue occurs and how to fix stripped threads so you can reassemble the components correctly.
Why Stripped Threads Happen
Stripped threads can occur for several reasons:
- Corrosion: Rust is thicker than steel, so running a rusty bolt into a nut or threaded hole can enlarge the hole and deform the threads, possibly cross-threading or binding the fastener.
- Misalignment: If parts are misaligned during assembly, the bolts may not go straight into the holes, resulting in cross-threading or deformed threads.
- Under-Torque or Over-Torque: Under-torqued fasteners can loosen and wobble, deforming threads and breaking bolts or studs. Over-torqued fasteners can stretch threads or even rip them out of a softer metal, such as an aluminum engine block.
- Mismatched Bolts and Holes: Using a fine-thread bolt in a coarse-thread hole or a metric bolt in an SAE hole can result in interference and stripped threads as the pieces try to conform to each other.
Depending on the damage done and the materials you’re working with, you can use a few different methods to fix damaged threads. Here they are from the least intensive to the most intensive:
- Thread Chaser: A thread chaser rolls and reforms the deformed threads. If possible, chase the threads starting from the undamaged section. This can be done for spark plugs or caliper bolts, for example.
- Drill and Tap: You can drill and tap the hole larger to accommodate a larger fastener. Going up one or two sizes is allowable in many circumstances, but be sure to use the correct drill bit to retain the strength of the attachment. Keep in mind that tapped threads don’t retain their strength as well as chased threads because a tap may remove too much metal. Nevertheless, a tap and die set is a necessary addition to anyone’s toolbox.
- Epoxy: For low-torque applications, high-strength epoxy can allow you to use the OEM bolt size. After cleaning and applying a release agent to the bolt, put epoxy into the damaged hole and on the bolt. Threading in the bolt will force the epoxy into the existing threads. Once the epoxy cures, install new bolts as needed.
- Threaded Insert: For high-torque applications, threaded inserts are best. A thread repair kit includes a specially sized drill bit, an insert tool and a few inserts. Bear in mind that solid inserts are stronger than coiled inserts. Enlarge the hole and rethread using the included drill bit and thread tap. Finally, thread in the new insert. Thread-locking adhesive is recommended for solid inserts, but not coiled inserts. The best part about this method is that you can use the OEM bolt and torque specifications.
Whether you work on cars in your spare time or your mechanic is telling you that you have some stripped threads that need repairing, knowing how to fix stripped threads for your own projects is a good idea. Of course, you can usually prevent threads from stripping by being careful with how you assemble things, but sometimes it’s totally unavoidable.
Check out all the thread repair products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on how to fix stripped threads, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
Photo courtesy of Flickr.
Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.