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.
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.
