To repair a stripped metal screw hole, re-tap new threads, fit a thread insert, or rebuild with metal epoxy when cutting isn’t possible.
Stripped threads stop a fastener from clamping, rattle loose under vibration, and waste time on rework. This guide shows proven ways to bring a damaged threaded hole back to full strength, when each method makes sense, and the tools that give clean, repeatable results. You’ll find quick wins for small repairs and the right steps for long-term fixes in structural parts.
Pick The Right Repair Path
Start by matching the fix to the part and the load. A light cover plate in mild steel might accept a larger screw and a dab of threadlocker. A bracket on a motor housing needs a real thread restoration, usually by re-tapping or using a threaded insert. Thin sheet behaves differently from a thick boss, and stainless needs different cutting fluid than aluminum. The table below helps you choose your lane fast.
| Method | Best For | Core Tools |
|---|---|---|
| Re-tap To Next Size | Thick bosses, mild to medium loads | Tap set, tap wrench, cutting fluid |
| Threaded Coil Insert | Aluminum or soft metals needing original screw size | Insert kit (drill, tap, mandrel), installation tool |
| Key-locking Insert | High vibration or high load in castings | Insert kit, countersink, driver for locking keys |
| Metal Epoxy + Tap | When you can’t cut a larger thread or heat is an issue | Steel-filled epoxy, drill, tap |
| Rivet Nut (Thin Sheet) | Sheet metal with no depth for a tap | Rivet-nut tool, correct grip-range insert |
| Through-Bolt + Nut | Accessible backside and high clamp needs | Drill, bolt, washer, nut |
Fixing A Worn Metal Threaded Hole: Quick Checklist
Before you cut or bond anything, confirm the original screw size and pitch. Measure with a thread gauge, or test with a known screw. Clean the hole with solvent and a nylon brush. If the part has oil in the pores, bake it low and dry, then cool to room temp. Shield nearby surfaces with masking tape to catch chips and epoxy squeeze-out.
Option 1: Re-tap To The Next Size
This is the fastest permanent fix when you can accept a larger fastener. Choose the next imperial or metric size with the same pitch style, then drill to the correct tap drill size. Keep the drill square to the surface. Use cutting fluid suited to the base metal. Start the tap dead straight, advance a half turn, back a quarter turn to break chips, then repeat. Blow out the hole, test-fit the screw, and set the final torque by spec or a known feel on similar joints.
Tips That Prevent A Second Strip
- Use a guide block with a square hole to keep the tap aligned.
- Stop early if the tap binds; back it out, clear chips, add fresh fluid, and continue.
- Deburr the entry with a shallow countersink for smoother starts and fewer raised edges.
Option 2: Restore The Original Screw Size With A Threaded Coil Insert
Coil inserts place hardened steel threads into a softer parent metal. They shine when you must keep the original screw size in aluminum housings, machine covers, and electronics enclosures. The workflow is simple: drill the damaged hole to the kit’s size, tap with the special insert tap, install the coil on the mandrel until it sits a turn below flush, then break the tang. Many kits include the drill, tap, installer, and a few coils. For step-by-step technique, see Henkel’s guide to Helicoil installation steps. This shows the drill-tap-install sequence and a threadlocker tip for added security in high-heat areas.
Coil Insert Do’s And Don’ts
- Pick insert length to match screw engagement: 1.0–1.5× diameter suits most joints.
- Stop the insert a touch below the surface so the screw head seats flat.
- Use the kit’s tap; a standard tap won’t match the insert’s profile.
Option 3: Lock It Down With A Key-locking Insert
Key-locking inserts (often called Keenserts) add a solid sleeve plus small keys that bite into the parent metal. They resist vibration, heat cycles, and repeated service. Drill, countersink, and tap to the insert’s external size, screw in the insert until flush, then drive the keys to lock it. This style suits engine covers, gearboxes, and cast iron frames where thread pull-out would be costly. The official catalog outlines the hammer-down key step and sizing notes for installation depth and countersink angle, which helps set repeatable results in production fixtures.
When To Pick Key-locking Over A Coil
- Loads cycle, and the joint sees frequent teardown.
- Base metal is weak or porous and needs more bearing area.
- Space allows the larger outside diameter and shallow countersink.
Option 4: Rebuild Threads With A Steel-Filled Epoxy
When you can’t cut a larger size and inserts won’t fit, a steel-filled putty can rebuild the hole. Degrease the cavity, scuff with a small burr or coarse file to add tooth, and dam any through-holes with tape. Mix the putty, pack it into the cavity, and form a pilot with a release-coated screw or a plastic plug. After cure, drill and tap to the original size. J-B Weld’s SteelStik is designed for this approach and can be drilled and tapped after cure; see the product page for the set and cure windows that guide your timing.
Bonded Thread Best Practices
- Clean until the wipe stays clean; any oil under the patch weakens the bond.
- Use a fresh mixing stick so you don’t seed oil into the putty.
- Let the full cure finish before tapping; rushing leads to crumbly threads.
Threadlocker, Torque, And Reuse
A light to medium threadlocker helps hold new threads under vibration. Blue grades suit serviceable joints; red grades target permanent assemblies that only release with heat. Henkel’s guidance notes that high-strength red needs heat for removal, which helps in joints that must not back out under shock. Use the smallest drop that wets the first few threads; excess squeezes into blind holes and can hydraulic-lock the screw.
When To Add Threadlocker
- Coil inserts in warm, vibrating assemblies.
- Key-locking inserts in cast housings that see impact loads.
- Re-tapped holes where engagement length is short.
Cut Clean Threads: Drill And Tap Basics
Fresh, straight threads start with the right pilot size and a square approach. Use a rigid drill, mark depth with tape, and keep the tool perpendicular with a drill guide or a block with a straight bore. Switch to the tap at low speed by hand. Advance slowly with short chip breaks. Always flush chips and test fit the screw by hand before setting torque.
Pick The Right Pilot Size
Use a reliable chart for tap drill sizes. For common UNC and UNF threads, manufacturer charts list the proper drill for a given tap. A trusted reference is here: tap drill sizes for UNC/UNF. It keeps you from over-weakening the root or leaving too much stock for the tap.
Set Up For Alignment
- Use a center punch to keep the pilot from walking.
- Clamp the part; movement yields tapered threads.
- Start the tap with a guide block; a crooked start strips again.
Lubrication And Metals
Use tapping fluid for steel and cast iron, kerosene or light oil for aluminum, and a sulfurized oil for stainless. This reduces galling and makes chip breaking smoother. In blind holes, don’t flood; a small brush dip reduces hydraulic lock and leaves space for chips to pack and exit.
Tap Drill Reference (Common Sizes)
The figures below mirror standard industry charts used by tool makers and shops; confirm against your kit and material notes before cutting. A good reference for imperial coarse and fine series is the Guhring page linked above.
| Thread Size | Tap Drill | Note |
|---|---|---|
| #10-24 UNC | #25 drill (0.1495″) | General hardware |
| #10-32 UNF | #21 drill (0.1590″) | Finer pitch |
| 1/4-20 UNC | #7 drill (0.2010″) | Common machine joints |
| 1/4-28 UNF | #3 drill (0.2130″) | Auto and instrumentation |
| 5/16-18 UNC | F drill (0.2570″) | Structural brackets |
| 5/16-24 UNF | I drill (0.2720″) | Thin nuts, fine pitch |
| 3/8-16 UNC | 5/16″ drill (0.3125″) | General fabrication |
| 3/8-24 UNF | Q drill (0.3320″) | High clamp, short grip |
| M6 × 1.0 | 5.0 mm drill | Metric standard |
| M8 × 1.25 | 6.8 mm drill | Metric standard |
Thin Sheet: When A Tap Won’t Hold
In sheet metal, a cut thread often leaves too little engagement for lasting clamp. Use a rivet nut sized to the sheet’s grip range and the screw you need. The tool sets the insert by crimping, which locks a full-depth thread to the sheet. If heat is off-limits and backside access is tight, this route beats welding a nut.
Through-Bolt As A Last Resort
If you can reach the backside, drilling clean through and adding a washer and nut gives full strength without cutting new threads in a weak boss. Add a medium threadlocker on the nut if the joint sees vibration. Mark the bolt so you can see any movement during service checks.
Step-By-Step: Your First Choice Repairs
Re-tap To The Next Size (Detailed)
- Confirm the original size and pick the next size up with a similar pitch style.
- Mark the center, drill to the tap drill size from the chart linked earlier.
- Chamfer the entry slightly for a clean lead-in.
- Start the tap square with steady pressure; half turn forward, quarter turn back.
- Flush chips, test the new screw, and set torque to a known value for that size.
Install A Coil Insert (Detailed)
- Drill to the kit’s size using a stop or depth collar.
- Tap with the kit’s tap; keep the tool aligned with a guide block.
- Thread the coil on the mandrel and drive it until a turn below flush.
- Break the tang; vacuum chips; test the screw for smooth entry.
- Add medium threadlocker if the assembly sees vibration or heat cycles.
Rebuild With Steel-Filled Epoxy (Detailed)
- Degrease and dry the part; scuff the cavity for better bite.
- Mix the putty to a uniform color and pack it firmly.
- Form a pilot with a waxed sacrificial screw or a release-coated plug.
- Let it cure by the product window; don’t rush this step.
- Drill and tap to the original size; test the fastener under light load first.
Quality Checks Before You Button Up
- Engagement: Aim for 1× diameter of thread engagement in steel and 1.5× in aluminum.
- Feel: Turn the screw by hand for the first few threads; any gritty feel signals chip leftovers or a nicked lead.
- Seat: Heads should sit flat with no rocking; a raised burr needs a light countersink pass.
- Mark: Paint-pen the head after torque; a moved line during service means re-check.
Common Mistakes That Wreck Repairs
- Skipping the pilot chart and drilling too small, which snaps taps.
- Starting the tap at an angle; use a guide or a drill press with a hand tapper.
- Installing a coil proud of the surface, which holds the head off and fakes torque.
- Driving key-locking keys before the insert is fully seated.
- Tapping epoxy before it reaches full cure strength.
What To Keep In Your Kit
A compact repair kit saves hours on the next job. Stash a fractional drill index, a UNC/UNF tap set, a metric set, a square tap wrench, cutting fluid, a coil insert kit for your most common sizes, a small key-locking insert kit, steel-filled epoxy putty, a countersink, a deburring tool, layout dye, a center punch, and a thread pitch gauge. Add a drill guide block and a torque wrench. With these on hand, most stripped holes become a short stop, not a full teardown.
When To Choose Each Method
Choose re-tapping when you can live with a larger fastener and have at least one diameter of depth in steel or 1.5× in aluminum. Choose a coil insert when you must keep the original screw and the base metal is soft. Choose a key-locking insert for high cycle loads or when the casting is tired and needs more bearing area. Choose epoxy if heat or thin walls rule out cutting, or when access limits tool swing. Choose a rivet nut in thin sheet. Choose a through-bolt when you can reach both sides and want a simple, strong clamp without new threads.
Last Word: Make It Repeatable
Use the same steps each time: confirm size, choose the path, set alignment, control chip load, and verify the clamp. Add two references to your bookmarks: the coil insert guide above for exact kit steps, and the UNC/UNF tap drill chart for pilots. With those and a small box of inserts, taps, and putty, a stripped hole is just another line on the job sheet.