H ANDS ON HINTS FOR HOMEBUILDERS
Heli-Coil Use and Installation BY RICHARD KOEHLER
THERE YOU ARE, TIGHTENING the exhaust nuts on your Lycoming and one of the exhaust studs pulls right out of the cylinder. The pullout is so complete that there is no possibility of using one of the various oversize studs to solve the problem. What do you do? You can pull the cylinder and take/send it to an overhaul shop where they will weld the hole and retap it. A much easier alternative is to replace the threads in the hole with a Heli-Coil steel thread insert. The insert provides a new set of threads, compatible on the inside with your bolt/stud and holding on the outside in the base material with special threads you cut. Threads on bolts and studs are defined by the diameter and the number of threads per inch. On our homebuilts we commonly find two basic systems of threads, fine and coarse. Typical fine threads are found on our AN bolts, whereas most of the hardware on Lycomings and Continentals are coarse thread, particularly if the bolt or stud screws into the aluminum case or cylinder head. In the example above, the exhaust studs on most Lycomings are 5/16-18, meaning the stud diameter is 5/16 inch and the threads are 18 per inch. Another common size is 1/4-20 (1/4 inch diameter by 20 threads per inch), which is the size of many of the case bolts on Lycomings and Continentals and the exhaust studs on many Continentals. To make the repair you will need to buy a Heli-Coil kit. They can be found at many aviation supply houses and at your local auto parts store. The kit will include steel thread inserts, a special tap for cutting the threads for the insert, and an insertion tool. You will need to
A Heli-Coil kit will include steel thread inserts, a special tap for cutting the threads for the insert, and an insertion tool. You’ll need to buy a special sized drill bit.
102 Sport Aviation October 2011
buy a special sized drill as specified for the insert you will be installing. For instance, for 5/16-18 you will need a 21/64-inch (0.3281) drill bit, and for 1/4-20 you will need a 17/64-inch bit. These are not common, so you may need to special order them. You will also need a tap handle. If you have a tap and die set, you probably have one that will fit. If not, a small adjustable wrench or a socket may work, but not as well. The first step is to drill out the hole with the special bit. Be careful to keep the hole square and centered. Next, use the provided Heli-Coil tap to cut the special threads for the Heli-Coil insert. Start the tap slowly and keep it centered and square with the surface. After you get it started, back it out 1/2 turn or so to clear the threads. Continue this process of cutting in about a turn and reversing to break and clear the threads as you cut them. A little oil will help the process. Once the threads are cut, clean out the hole of chips and prepare the insert by screwing it onto the insertion tool. Note that one end of the insert has a tang in it that comes across the center of the insert. This tang engages a flat on the insertion tool, allowing you to screw the insert into the hole. The insert should be screwed in far enough that the top of the insert is 1/4 to 1/2 turn below the surface. If the hole is deep enough, you may want to put in two inserts. Once the insert is in place, remove the insertion tool and use a punch or rod (diameter should fit snugly into insert) to break the tang at a preformed notch on the bottom of the insert coil. The punch or rod must have a square end (no chamfer). Place the rod or punch into the insert until it rests on the tang. Holding the tool squarely, strike it with a hammer to break the tang free. After removing the tang, the Heli-Coil insertion process is complete. The part can be reassembled with the bolt/ stud firmly gripping the Heli-Coil and an expensive trip to the repair shop averted.
ILLUSTRATION BY PIERRE KOTZE
