Thread Repair Information

When to Repair vs. Replace

Repair when:

• Only threads are damaged (not the surrounding material)
• Component is expensive or difficult to replace
• Original thread size can be maintained
• Damage is limited to first few threads
• Part has adequate wall thickness for repair

Replace when:

• Surrounding material is cracked or damaged
• Wall thickness is insufficient for repair
• Thread damage extends deep into the hole
• Safety-critical application with unknown history
• Part has been repaired multiple times already

Insert Length Selection

The proper insert length is critical for achieving full thread strength:

• Minimum length: 1.0x the bolt diameter (acceptable for low-load applications)
• Standard length: 1.5x the bolt diameter (recommended for most applications)
• Heavy-duty length: 2.0x the bolt diameter (for high-stress or critical joints)
• Aluminum/magnesium: Always use 1.5x minimum, 2.0x preferred
• Blind holes: Leave 1/4 turn clearance at bottom for debris

Step-by-Step Process

1. Assess the damage: Verify that only threads are damaged and surrounding material is sound
2. Drill out threads: Use the specified drill size (found in sizing chart). Drill straight and perpendicular to the surface
3. Clean the hole: Remove all chips and debris. Use compressed air for blind holes
4. Tap the hole: MUST use STI (Screw Thread Insert) tap—standard taps will not work! Use cutting fluid and back out every 2 turns to break chips
5. Verify tap depth: Tap should go 1/4 turn past the desired insert depth
6. Thread insert on mandrel: Screw the HeliCoil onto the installation mandrel 3-4 threads
7. Install insert: Thread the insert into the tapped hole using the mandrel, keeping steady pressure
8. Seat the insert: Insert should end up 1/4 to 1/2 turn below the surface
9. Break off tang: Use tang break-off tool to snap tang at the notch. Remove tang from hole
10. Test installation: Thread in mating bolt by hand to verify smooth operation

Simplified Process

1. Drill and tap: Same as standard HeliCoil—use correct drill size and STI tap
2. Use special tool: Tangless inserts require a different installation tool (not the standard mandrel)
3. Install insert: Thread insert into hole using the tangless installation tool
4. Seat properly: Insert should be flush or slightly recessed
5. No tang removal: Tangless design eliminates this step—cleaner and faster
6. Ideal for blind holes: No tang debris to worry about

Key-Locking Insert Process

1. Drill to size: Use precision drill bit for the specified hole size
2. Broach keyways: Use proper broach tool to cut locking keys in the hole
3. Verify keyway depth: Keys must be deep enough to fully engage the insert
4. Apply thread locking compound: On external threads of insert (optional but recommended)
5. Align keys: Position insert so keys align with keyway slots
6. Drive insert: Use installation driver tool with steady force
7. Seat flush: Insert should be flush with or slightly below the surface
8. Verify installation: Insert should not rotate—keys prevent this

Simple Repair Process

1. Select next size up: Choose the smallest oversize that will clean up all damage
2. Drill to tap drill size: Use standard tap drill chart for the new size
3. Tap to new size: Use standard tap (not STI) for the oversized threads
4. Clean threads: Remove chips and verify thread quality
5. Test with oversized bolt: Thread in the new size bolt by hand first
6. Apply appropriate torque: Use torque specs for the NEW thread size, not original

Possible Causes & Solutions:

• Wrong tap used: Verify you used STI tap, not standard tap. If standard tap used, hole must be re-drilled oversize
• Tap not deep enough: Tap should go 1/4 turn past insert length. Re-tap to proper depth
• Insert damaged: Inspect insert for bent or damaged threads. Use new insert
• Chips in hole: Clean hole thoroughly with compressed air or thread chaser
• Cross-threaded start: Back out and restart carefully, ensuring straight entry

Possible Causes & Solutions:

• Insert too short: Use longer insert—minimum 1.5x bolt diameter, preferably 2.0x
• Parent material too weak: Consider solid bushing insert instead of wire insert
• Insufficient engagement: Verify tap depth and insert installation depth
• Material cracking: Inspect surrounding material. May require component replacement
• Oversized hole: Hole may have been drilled too large. Try larger insert size or fill and re-drill

Possible Causes & Solutions:

• Insert not straight: Hole was drilled at an angle. Must re-drill straight
• Insert not fully seated: Drive insert deeper until properly recessed
• Wrong insert thread pitch: Verify insert matches original thread specification
• Damaged insert threads: Remove and install new insert
• Bolt entering at angle: Ensure bolt starts straight—use alignment tool if needed

Possible Causes & Solutions:

• Key-locking keys not engaged: Remove insert, check keyway depth, re-broach if needed
• Hole oversized: Drill bit was too large. May need to go to next size up insert
• Insufficient thread engagement: Use longer insert with more external thread grip
• Thread locker needed: Apply thread locking compound on external threads of insert
• Wrong insert type: Consider switching to key-locking or self-tapping design

Possible Causes & Solutions:

• Using wrong tool: Must use proper tang break-off tool, not pliers
• Insert not fully seated: Drive insert deeper before attempting tang removal
• Tang damaged during installation: May need to drill out insert and start over
• Blind hole issue: Consider using tangless inserts for blind holes
• Insert bottomed out: Tap wasn't deep enough. Must re-tap deeper

Assembly Best Practices

• Start threads by hand: Always start bolts by hand to prevent cross-threading
• Use proper torque: Follow torque specifications—over-torquing is a primary cause of damage
• Keep threads clean: Remove dirt, chips, and old thread locker before assembly
• Inspect before use: Check both male and female threads for damage before assembly
• Use anti-seize: Apply anti-seize compound in corrosive environments
• Thread engagement: Ensure proper thread engagement—minimum 1.0x diameter for steel, 1.5x for aluminum

Maintenance Considerations

• Limit disassembly cycles: Repeated removal/installation wears threads
• Replace worn fasteners: Don't reuse badly worn or damaged bolts
• Chase threads periodically: Clean threads with chasers during maintenance
• Use inserts preemptively: Install HeliCoil in aluminum during initial assembly for frequently-serviced joints
• Protect threads: Use thread protectors during storage and transport
• Address corrosion early: Don't let threads rust—prevention is easier than repair

Design Considerations

• Specify inserts initially: Design with HeliCoil inserts in soft materials from the start
• Adequate material thickness: Ensure sufficient wall thickness for thread depth
• Avoid blind holes when possible: Through-holes are easier to repair
• Access for tools: Design in clearance for torque wrenches and installation tools
• Material selection: Choose materials appropriate for the load and environment