How do you fix a stripped bolt thread?

What Causes Stripped Bolt Threads?

Stripped bolt threads can occur for a few different reasons:

• Over-tightening – Applying too much torque when tightening a bolt can cause the threads to strip.
• Cross-threading – Forcing a bolt into a hole at an angle can damage the threads.
• Corrosion – Rust buildup in threads can weaken them and cause stripping.
• Poor quality hardware – Inferior metals or manufacturing defects can lead to easier stripping.
• Incorrect tools – Using the wrong size wrench or poor fitting hardware leads to stripping.

The most common cause is over-tightening. It’s easy to apply too much force with tools that give mechanical advantage like wrenches and impact drivers. Cross-threading and corrosion are other leading culprits that ruin bolt threads.

Can Stripped Threads Be Repaired?

In many cases, stripped bolt threads can be repaired rather than replacing the entire bolt or threaded hole. The key is assessing the level of damage. Light stripping may only flatten or deform the threads while completely ruined threads may be unsalvageable.

Here are some guidelines for whether stripped threads can be fixed:

• If less than 25% of the threads are damaged, the bolt can likely be re-tightened after cleanup without losing structural integrity.
• 25-50% damaged threads can be repaired depending on the application, using thread repair inserts or helical coil inserts.
• More than 50% thread damage often requires replacing the bolt, nut, or threaded hole.

The strength requirements of the application also determine if repair is possible. Light duty bolts in non-critical applications have greater repair tolerance than high strength structural bolts.

Cleaning Stripped Threads

Before attempting repairs, you’ll need to thoroughly clean the threads. This removes any debris, grease, metal fragments, or rust from the damaged area.

Follow these steps to clean stripped bolt threads:

1. Remove the bolt if still threaded in hole, or the nut/other hardware from the bolt.
2. Use a wire brush, either handheld or in a drill, to scrub the threads. This helps clear particles and smooth any burrs/deformations.
3. On external threads, use a thread chaser tap to realign and redefine the thread form.
4. For internal threaded holes, run the correct size thread tap through to clear out threads.
5. Blow out the threads with compressed air to remove loose particles.
6. Use a thread lubricant/cleaner to prep threads after brushing. Let it soak for 5 minutes before wiping clean.

Once cleaned, examine the threads again. If less than 25% are damaged, skip to re-tightening the fastener. If repairs are needed continue to the next steps.

Bolt Thread Repair Options

Here are some proven ways to repair stripped threads on bolts:

Helical Inserts

Helical inserts, also called coil inserts, are tiny coiled springs with external and internal threads. The insert screws into a threaded hole, locking in place to provide new, full, internal threads. Helical coil inserts work in blind and through holes.

When to Use:

• 25-75% stripped thread damage in nuts or threaded holes.
• Damaged threads in aluminum or other soft materials.
• Restoring/upgrading worn out threads.

Installation:

1. Drill out hole to required diameter for insert using appropriate tap drill size.
2. Run helical insert into drilled hole using installation tool.
3. The insert displaces material and cuts new threads into hole walls as it screws in.
4. Insert tangs hold the coil firmly in place.

Helical inserts are permanent thread repairs best installed with the proper tooling.

Thread Inserts

Thread inserts provide pre-formed internal threads in an outer sleeve that screwed into a hole to add thread strength. Made of stainless steel or brass, inserts work in both blind and through holes.

When to Use:

• 25-75% stripped threads.
• Frequent use/assembly in soft metals.
• Reinforce extra-large or oddly sized threads.

Installation:

1. Drill and tap hole to appropriate size per insert specs.
2. Screw insert into tapped hole until it seats using installation tool.
3. The outer threads displace material to lock insert in place.

Thread inserts provide durable, reusable threads for frequent assembly/disassembly applications.

Thread Repair Files

A thread repair file has tapering, serrated teeth to quickly reshape and sharpen thread forms. The proper size file can restore mild to moderately damaged external bolt threads.

When to Use:

• Light thread damage less than 50%.
• External threads only.
• Quick field repair when inserts aren’t practical.

Using Thread Repair Files:

1. Select the correct thread repair file for the bolt’s thread pitch, diameter, etc.
2. Run the file along the bolt threads with moderate pressure using forward strokes.
3. Let the file teeth conform to the existing thread profile.
4. Repeat until the threads are restored.

Thread files provide a temporary fix for retrieving stuck or damaged bolts to remove fully later.

Thread Locking Compounds

Anaerobic thread locking adhesives fill stripped threads and increase friction. They prevent loosening and restore some stripping resistance.

When to Use:

• Minor thread wear when disassembly isn’t required.
• Difficult access fasteners where inserts aren’t possible.

Applying Thread Lockers:

1. Clean and dry threads thoroughly.
2. Apply sparingly along thread surface, avoid globs.
3. Assemble bolt and nut with normal torque.
4. The compound cures and adheres to threads.

Thread lockers provide a convenient, low-cost thread repair option in some applications. Disassembly will be extremely difficult afterwards.

Nut and Female Thread Repairs

Similar options exist for repairing stripped threads in nuts and threaded holes:

Nut Replacement

For stripped nut threads, easiest fix is to replace with a new nut of the same size/specs.

Helical Nut Inserts

Coil nut inserts repair stripped threads inside nuts. They install like helical inserts with similar benefits.

Nut Repair Kits

Kits use taps and dies to rethread damaged nut threads or increase major diameter for oversized bolts.

Weld Nut Replacement

Stripped female threads can be repaired by welding on a threaded nut collar or other threaded fitting. Requires machining/grinding to re-dimension for bolt clearance.

Helical Coil Inserts

These function the same in female threads as bolt holes. Great for repairing damaged threads in threaded holes.

Drill/Tap Thread Repair

For stripped holes in metals, you can drill out and retap with next size up threaded hole. Uses oversize bolts.

Preventing Thread Stripping

To avoid thread damage in the future:

• Carefully align threads before tightening.
• Lubricate threads to reduce friction and torque needed.
• Use a torque wrench for critical applications.
• Choose quality hardware with correct dimensions.
• Inspect for thread wear and replace before stripping occurs.
• Use anti-seize lubricant on threads prone to rust and corrosion.

Proper thread care and bolt tightening technique helps prevent annoying stripped threads. Repair any minor damage early before it escalates. Correct any installation issues like poor alignment or wrong sized hardware.

Conclusion

Stripped bolt and nut threads can often be restored using thread repair inserts, thread locking adhesives, thread files, or specialty products for nuts. The key steps are thread cleaning, damage assessment, and using the appropriate repair method for the situation. Prevent future problems through proper torque control, quality hardware, and maintenance. With some effort and the right repair techniques, stripped threads don’t mean the end of the world.