A loose fastener in a metal part usually means stripped threads, an oversized hole, or the wrong screw for the base material.
Why A Metal Screw Loses Grip
Threads need a matching profile, the right pilot, and enough engagement. When any of these fall short, the head keeps turning while the joint stays loose.
Quick Fact On Thread Engagement
As a rule of thumb, aim for engagement about one diameter in steel and closer to two diameters in soft alloys. This keeps load in the threads instead of the tip.
Rapid Checks Before You Grab Tools
Confirm the screw type. A machine screw needs a tapped hole or a nut; a self-tapper needs a tight pilot. Match pitch with a gauge or a known nut. Check sheet thickness; thin stock often needs an insert.
Fast Diagnosis
| Symptom | Likely Cause | First Move |
|---|---|---|
| Screw spins with no clamp | Female threads stripped | Try a longer screw to reach fresh threads; if none, plan a thread insert |
| Screw cuts in but backs out | Pilot hole too large or too small | Measure and drill to the correct size for the screw type |
| Screw binds or squeals | Galling between like stainless grades | Use lubricant and mixed materials or coated hardware |
| Screw rocks side to side | Hole elongated from wear | Upsize the fastener or add an insert with a proper OD |
| Head seats but joint still loose | Stack too thin for the grip | Add a washer, spacer, or switch to a rivet nut or clinch nut |
Pick The Right Fastener For The Base Metal
Machine screw with nut: ideal when you can reach both sides. Self-tapping screw: good for thin steel or aluminum when a nut is impossible. Thread-forming types need a tight pilot; thread-cutting types clear chips in thicker sheet.
Set The Pilot Hole Size Correctly
A pilot that is off by a small margin can ruin holding power. Charts from fastener suppliers list the pilot for each size and style. Use a sharp bit, square the drill, and deburr lightly if a tapping screw will form threads; many charts advise a clean hole with no large chamfer.
Mind The Thread Pitch And Series
Metric and inch hardware can look similar. A mismatch lets the screw run in loosely and strip the threads on the way back out. Always match the pitch with a gauge or the correct reference nut before you drive power tools.
Watch For Galling With Stainless
Like-on-like stainless steel can cold-weld under friction. The symptom is a squeal, rising torque, then a frozen shank. Mix materials, add a proper lubricant, slow the driver, and avoid back-and-forth starts that heat the joint.
Get Enough Thread Engagement
Grip length matters. In steel, about one diameter of full threads works well; in soft aluminum or magnesium, aim closer to two. In thin sheet, add an insert or a nut. See the NASA threaded fastening standard for engagement guidance and projection of threads past a nut.
Step-By-Step: Fix A Stripped Tapped Hole
- Confirm the screw size and pitch with a gauge or matching nut.
- Choose a thread repair insert sized for that thread. Helical inserts are common and widely stocked.
- Drill the hole to the specified diameter for the repair kit. Keep the drill square to the surface.
- Tap with the special insert tap, using cutting fluid in metals that need it.
- Install the insert flush or just below the surface. Break the tang if the kit calls for it.
- Refit the original screw and torque to spec.
Step-By-Step: Add Full Threads To Thin Sheet
- Decide whether you can access the back side. If yes, a nut or a clinch nut is strong and quick.
- If back side access is blocked, choose a rivet nut with a grip range that matches the combined sheet stack.
- Drill the mounting hole to the catalog size. Avoid large chamfers that reduce bite.
- Install with the proper tool so the insert collapses evenly. Check that it seats flush, then test with a sample screw.
- For vibration, add a mechanical lock like a serrated flange nut, a prevailing-torque style, or a non-permanent threadlocker.
Torque, Threadlocker, And Vibration
If the same fastener walks loose after every repair, confirm torque, add a lock feature, or both. Medium-strength threadlocker keeps a machine screw in place while allowing service with hand tools. Red grades are near permanent and need heat for breakaway, so choose them only for hardware that never needs routine removal. See the Loctite threadlocker guide for cure, removal, and grade selection.
When Upsizing Makes Sense
If no threads remain, upsize and re-tap or use a larger insert. Keep edge distance near two diameters to limit tear-out in sheet.
Pre-Drill Tips That Protect Holding Power
Use the right bit size and a rigid setup. Mark the center with a punch. In aluminum, run slow with a sharp bit; in steel, use cutting fluid to hold size.
Pilot Sizes For Common Self-Tappers (Reference)
| Screw Size | Threads Per Inch | Pilot Drill |
|---|---|---|
| #6 Type AB | 18 | #32 (~0.116 in) |
| #8 Type AB | 15 | #29 (~0.136 in) |
| #10 Type AB | 12 | #21 (~0.159 in) |
| #12 Type AB | 11 | 3/16 in (~0.188 in) |
| 1/4 in Type AB | 10 | 7/32 in (~0.219 in) |
When A Self-Tapping Screw Is Wrong
Thread-forming styles work only in ductile sheet; brittle alloys crack. In thicker steel, a thread-cutting type clears chips and avoids jams. If the panel is soft, a clinch nut gives tougher threads than a formed path made by a small screw.
Avoid Overtightening
Spinning the driver to chase a last half turn often strips the female threads. Use a torque spec from the kit or from a recognized reference. Lubricated threads need lower torque for the same clamp. The goal is clamp load, not a number on the tool.
Edge Distance And Hole Quality
Keep the centerline away from edges and bends. Near an edge, sheet pulls and the hole eggs out. Drill clean, hold the work, and use a backup block when a burr on the exit face would interfere with a flush head.
Material-Specific Advice
Aluminum
Pick coarse thread where possible and add engagement. In thin sheet, use an insert.
Steel
Coarse or fine profiles both work. Fine pitch needs alignment. Add coating and a seal where corrosion is a risk.
Stainless
Reduce speed, use lubricant, and mix grades to cut galling. Seal exposed threads in marine areas.
Zinc Die Casting
Use a slow feed and a clean pilot. Inserts designed for soft castings work well here.
Locking Choices That Work
Prevailing-torque nut, nylon-insert nut, serrated flange, safety wire in racing use, or chemical threadlocker. For serviceable joints, a medium-strength anaerobic compound balances retention and future disassembly.
Decision Tree: Your Best Fix
Hole stripped in thick metal: install a thread insert and keep the original screw. Hole stripped in thin sheet: add a rivet nut or a clinch nut. Hole oversized from wear: upsize and re-tap if material allows, or switch to a larger insert. Threads intact but still loose: match pitch, set pilot size, and use a lock feature.
How To Size A Pilot Hole Without A Chart
No chart at hand? Measure the outside thread diameter and the root. For thread-forming styles, pick a pilot near the root so the screw displaces metal. For thread-cutting styles, go slightly larger to leave room for chips. Test in scrap and record the bit that gives a firm drive without tearing.
Pick Between Helical And Solid Inserts
Helical wire inserts restore the original screw size with little added diameter and work well in aluminum and magnesium. Key-locking solid inserts resist pullout in soft or worn castings and suit parts that see many service cycles. They need a larger counterbore. Both styles place a hard internal thread that outlasts the base metal.
Driver Technique That Saves Threads
Start by hand to catch the first turns true. Hold the driver square, use short pulses, and let the screw cool between tries. On stainless pairs, a dab of anti-seize stops heat build-up. On coated screws, a dry fit keeps the coating intact for corrosion resistance.
When The Metal Is Too Thin
Some housings and panels offer less than a single thread height. In that case, add a clinch nut or a rivet nut sized for the sheet. Each catalog lists a grip range and a minimum sheet hardness; matching both keeps the insert from spinning under load. If the edge is close, choose a smaller insert with a backing plate.
Field Repair Kit: What To Pack
Carry a compact bit index, pitch gauges, blue threadlocker, cutting fluid, a hand rivet nut tool with common mandrels, and helical insert kits for your usual sizes. Add a short torque wrench and a clutch driver. With that kit, most loose metal joints are a one-stop fix.