Most stripped threads are caused by the person tightening them

These are listed in the order worth acting on, which with fastener torque and over-tightening is not the order they are usually presented in.

What tightening is actually doing

Tightening a fastener stretches it slightly, and that stretch generates the clamping force holding the joint together. Torque is used because it is easy to measure, though only a fraction of the applied torque becomes clamping force.

The rest is lost to friction under the head and in the threads, which is why lubrication changes the relationship substantially. This is why manufacturers specify whether a torque figure applies dry or lubricated, and why the distinction matters. Over-tightening pushes the fastener past its elastic range, after which it no longer springs back and the joint loses clamping force.

Where threads are most vulnerable

Threads cut directly into plastic have very little strength and strip at torques that feel gentle in the hand. Aluminium and other soft metals are similarly vulnerable, particularly with steel fasteners that are far harder.

Short thread engagement concentrates load on few threads, which is why thin panels strip more easily than thick ones. Repeated assembly wears threads progressively, so a joint opened many times becomes weaker each time. Thread inserts, which place a metal thread into a soft material, are the standard solution and a good design signal.

The consequences beyond a stripped thread

Over-tightening distorts sealing faces, which causes the leak the fastener was supposed to prevent. It crushes gaskets beyond their working compression, permanently damaging them and shortening seal life. It preloads brittle materials such as ceramics and glass towards cracking under thermal or mechanical stress.

It can bend flanges and housings, misaligning bearings and shafts in ways that produce noise and premature wear. On wheels and structural fasteners it is a safety matter, and specified torques exist precisely for that reason.

Getting it approximately right without a torque wrench

Use the tool the manufacturer expects, since a long lever applies far more torque than a short one for the same effort. Tighten until resistance rises firmly and stop, rather than continuing until something gives, which is the usual failure mode.

Compared like for like, tighten multi-fastener joints progressively in a crossing pattern, which distributes load and prevents distortion. On plastic, stop at the point where the parts are drawn together, since further tightening achieves nothing useful. A torque wrench is inexpensive relative to a stripped casting and is worth having for anything mechanical.

Thread locking and why joints loosen

Vibration causes fasteners to rotate loose over time, particularly where the joint sees reversing loads. Thread-locking compounds, spring washers, nyloc nuts and staking are all methods of resisting that, with different strengths. Using a high-strength locking compound where a low-strength one was specified makes future disassembly destructive.

A joint that loosens repeatedly usually has an underlying problem such as insufficient clamping force or a distorted face. Retightening the same fastener repeatedly work-hardens and weakens it, and replacement is sometimes the correct answer.

Manufacturer figures are measured under conditions the manufacturer chose.

Repairing a stripped thread

Thread repair inserts restore a damaged thread in metal and are a standard workshop repair rather than a bodge. In plastic, options are more limited and often involve a larger fastener, an insert or bonding.

In the small print, using a longer or wider screw as a field repair is common and frequently makes future proper repair harder. Where a stripped thread is in a structural or safety-critical location, professional repair is the appropriate response. This site does not provide repair procedures, so service manuals and technician guidance are the correct sources.

Everything above, in order of what to do first

1. What tightening is actually doing. Tightening a fastener stretches it slightly, and that stretch generates the clamping force holding the joint together.
2. Where threads are most vulnerable. Threads cut directly into plastic have very little strength and strip at torques that feel gentle in the hand.
3. The consequences beyond a stripped thread. Over-tightening distorts sealing faces, which causes the leak the fastener was supposed to prevent.
4. Getting it approximately right without a torque wrench. Use the tool the manufacturer expects, since a long lever applies far more torque than a short one for the same effort.
5. Thread locking and why joints loosen. Vibration causes fasteners to rotate loose over time, particularly where the joint sees reversing loads.
6. Repairing a stripped thread. Thread repair inserts restore a damaged thread in metal and are a standard workshop repair rather than a bodge.

The takeaway

Stop when the joint is clamped, because everything after that point is being taken out of the threads.

Buy for the failure you can live with, not the feature you will use twice.