I realize that for many of our members this may be a redundant topic, but for the rest of us, helicoils are a rather intimidating process. Just thinking about it can be enough to put one off of the project for a very long time. I know, cause I am one who was very intimidated by the process and had Ellie's final changed at CubFest Northeast NB 2009 rather than helicoil the damaged implement mounting hole. Unfortunately, due to some rather bad luck etc., that final came loose and as indicated in another thread required replacing again. Since I did not have another final that I could utilize except for Ellie's original final, I had no choice but to finally proceed with the helicoil repair. What follows is my attempt at documenting the process in the hopes that it might help others.
One of the things I realized was that the final needed to be supported somewhat securely so that the drilling would not go awry. A 1/2" drill is a pretty powerful unit, and the older ones like the one pictured below have a heck of a lot of torque. Ellie's final is supported on a 20 liter/5 gallon pail.
A 21/32" drill bit is required for the STI tap. The drill bit is marked with a rudimentary depth gauge - a bit of duct tape to indicate the depth of the bolt hole so that the drill does not actually bottom out.
Ensure that the drill bit is perpendicular to the final/bolt hole in at least 2 axis and keep it that way. It may not seem easy, but it really isn't too difficult. A little practice in other materials may help give you confidence. It is important to keep the rpm's of the drill very low. This may include having to trigger the drill often for very short periods of time. This will keep the rpm down as well as counter the tendency of the drill to grab into the cast and possibly cause damage/injury to the casting/your body parts Pay attention to the depth gauge and stop drilling when the duct tape reaches the same level as the implement mounting boss.
Apply some Rapid Tap paste to the STI Tap and carefully insert up to the end of the starter threads keeping the tap perpendicular to the hole in 2 axis. Ensure that you carefully start the new threads.
Stop often and back out the tap cleaning it as you go.
This is about as far as you can go with the tap. After this point you require a bottoming tap. (I made a bottoming tap from this particular STI tap and finished up the threads - see the Grinding A Bottoming Tap thread)
Finish up the threads with the bottoming tap. There will be a fair amount of debris in the casting after creating the threads. The Rapid Tap will trap some of the debris. To get the rest, utilize compressed air (either from your compressor/canned air) and a magnetic pick-up tool.
Thread a helicoil insert onto the installation tool. Apply a locking thread sealant to the outside of the - I used the Loctite 272 brand. Thread the insert into the threaded hole.
Insert carefully - take your time.. it is not a race. The thread locker provides a lot of working time.
Ensure the insert seats fully.
Remove the insertion tool. I did not try to break off the tang as the insert pretty much bottoms out.
Clean the excess and allow to dry. Maximum strength is after a relatively short cure time.
One thing I learned is that there really is nothing to be afraid of in the helicoiling process. Like any skill, if you take your time, proceed methodically, pay attention to the proper steps/setup etc., the end result will be great. I no longer will be reticent about this process. In fact, I ended up spinning another bolt in the same final, so I am going to have to do this again. Only difference is the final will still be on Ellie
Here are the items needed:
Loctite 272 High Temperature, High Strength Red Threadlocker
HeliCoil Thread Insert Kit, 5/8"-11 x .938