Tech Tip 13 Heli Coil Installation Dont be caught threadless
Heli-Coil Installation Don’t be Caught Threadless!
This is what an aluminum head looks like after a bolt or in this case an exhaust stud strips out. The cobble repair is to use an American (SAE) coarse thread bolt that is larger and will “self-tap” new threads into the aluminum. The correct method is to install a Heli-Coil stainless steel thread insert.
Apply lubricant to the tap and “cut” new threads into the enlarged hole. When the tap is removed the lubricant will hold the aluminum shavings to the tap in the flutes.
The insert is threaded into the hole until the end (red circle) is just below flush. Note the position of the tang (red arrow). Now insert a punch smaller than the hole and place the flat end of the punch on the bend of the tang (blue arrow) and hit it with a blow, shearing off the tang. In this case removal of the tang isn’t necessary.
Double nut the exhaust stud as shown and install the stud, threaded into the hole until it is quite snug. To remove the nuts, hold the bottom nut with a wrench in your palm and the top nut wrench in your fingers of the same hand. Have the top wrench offset one flat and squeeze the two wrenches together, using your fingers to pull the top wrench towards you (which should be in the direction to loosen the top nut, in this case it is from left to right) and the top nut will “break free” and then both nuts can be removed with your fingers.