What’s a thread repair kit?

By Dr. Jody Muelaner

Thread repair kids allow damaged threads in a hole to be removed and replaced with new threads of the same size. They may also be used to provide strong threads in soft materials such as aluminum.

A thread repair kit consists of a tapping tool, an insertion tool and some thread inserts. A drill and a tap wrench will also be required to use the thread repair kit. The drill bit may or may not be included in the kit.

The most common type of thread insert used in a thread repair kit is the helical insert, commonly referred to by the trade name Helicoil. A helical insert looks like a tightly wound spring. It consists of a helix wound from a wire with a diamond cross section.

The helix has a small return at one end that engages with the insertion tool, allowing the insert to be screwed into the hole. A small notch in the wire allows this return to be snapped off once the insert has been installed.

The process of repairing the threads in a damaged hole, using a helical insert, has the following steps:

• Drill out the damaged threads using a drill bit of the correct size.
• Tap the hole with a special thread to accept the threaded insert. The tap required is included in a thread repair kit.
• Screw the threaded insert into the hole, using the insertion tool, to just below the surface of the part.
• Unscrew the insertion tool. The threaded insert will expand slightly as the torque is released from the insertion tool. This will cause it to lock into the part.
• Snap off the tang where the thread insert wire returns across the hole. A punch or hex tool can be inserted through the new thread, to rest against the tang. A tap with a hammer will snap it off.

Threads can become damaged in a number of ways. If a male thread becomes damaged, such as on a bolt or screw, it can often be cleaned up by running a die over it. When threads are lightly damaged, it may be possible to reform the threads by simply running a nut up and down, but there is a risk that the nut will fuse to the thread, a process known as galling. It is therefore safer to use a die in the first instance.

When threads are stripped out of a hole there are a few options. One option is to drill out the hole and tap it with a larger diameter thread. This can be a good option if there is room for a larger fastener. When it isn’t possible to create a new larger tapped hole, a thread repair kit can be used.