What are the disadvantages of self tapping repair inserts?
A2. Not using a tap to create the receiving hole for the new thread repair insert is similar to screwing a wood screw into a piece of wood. The thread cutting ability is very limited due to the shape of the flute and cutting edges of the insert so the threads are mostly formed much the same way the threads are rolled. This works fairly well in thin walls but has reduced ability to work in deeper holes. The biggest problem comes in attempting to install them into cast or ductile iron. A severe spreading pressure is created when the insert is forced in that can easily crack the casting. Just like screwing a wood screw into wood too close to and edge will cause the wood to split.
