When a bolt hole strips out in aluminum or magnesium, there must be a way to repairing the bolt hole. One repair option is to drill an oversize hole for a larger bolt, but the larger bolt may not be the correct size for an application. Another repair option is to chase the stripped thread, but the chased threads may not last long in the repair.
Wire thread inserts are one solution to repairing a stripped bolt hole in aluminum or magnesium; wire thread inserts will restore the original size of the bolt hole and add more further strength to the repair. A wire thread insert is simply a coil of stainless steel wire that you place into an oversized tapped hole. The load from the bolt are distributed over many contact point between the bolt and the wire thread insert.
How to Use Wire Thread Inserts to Fix Stripped Bolt Holes
The load is distributed such that the load is not placed on the first few threads of the stripped bolt hole. By distributing the load over many contact points, the wire thread insert can better distribute the load and provide more pull-out strength to the repaired component. Additionally, wire thread inserts acts as a buffer against galvanic corrosion between the steel bolt and the aluminum or magnesium components.
Wire thread inserts come in several different style. Free-running coils are one style of wire thread inserts that is used for everyday repairs on components. Free-running coils are easy to install into the stripped bolt hole and provide clean and precisely threads.
Screw locking wire thread inserts include at least one deformed coil that grip the bolt when it is under tension. A technician use screw locking wire thread inserts where vibrations is present in the component to avoid the bolt from loosening over time. Tanged wire thread inserts have a driving tang that allows the installation tool to push against the insert.
Tangless wire thread inserts has no driving tangs and are used in locations where loose metal fragment should not be left behind, such as in a clean room. To install a wire thread insert, drill the hole to the specified tap drill size. Cut the threads in the component using a special STI tap to ensure the threads are of the correct size.
Wind the wire thread insert into the tapped hole such that the top coil of the wire thread insert is flush with or slightly below the surface of the component. Do not over drive the wire thread insert into the component as this will damage the component and create a binding issue for the bolt. Finally, break the tang of the wire thread insert using a punch tool.
Skipping any of these step may result in a loose wire thread insert or damaged bolt hole in the future. The wire thread insert material must be chosen correctly for the installation environment. The most common material for wire thread inserts is 304 stainless steel as this steel resist corrosion and can withstand temperature change.
For environments exposed to salt water or chemicals, use 316 stainless steel wire thread inserts that is more corrosion-resistant. For applications where weight is an issue, use titanium wire thread inserts. Lastly, use phosphor bronze wire thread inserts for components where magnetic interference are an issue.
There are two common length for wire thread inserts. A 1D wire thread insert is a short wire thread insert for use in thick material. A 1.5D wire thread insert have a longer shaft and is used in aluminum or magnesium components for better holding power.
The longer wire thread insert distributes the bolt load over a greater distance then the 1D insert; thus, it is used in structural components. Additionally, 1.5D inserts requires more depth in the component than 1D inserts. There are several mistake that can occur when installing wire thread inserts.
One of the most common mistakes is the use of a standard tap instead of an STI tap; the threads cut by a standard tap is too small for the wire thread insert. Another mistake is dry tapping of the thread; dry tapped threads have rougher surfaces than tapped threads with lubricant and thus have less pull-out strength for the wire thread insert. The third most common mistake is leaving the tang of the wire thread insert inside the component; this can damage the component and cost the organization alot of money in repair costs.
To avoid any of these mistakes during installation, use a go/no-go gauge to check the threads of the component. This gauge will ensure that the threads of the component are of the correct size for the wire thread insert.
