Standard Practices
Teledyne Continental Motors, Inc.
TM
C-10
IOF-550 Permold Series Engine Maintenance Manual
1 April 2007
C-6.
Helical Coil Insert Replacement
Helical coil inserts are made of wire with a diamond-shaped cross section forming both a
male and female thread. Helical coil inserts are factory-installed in various tapped holes
of some engine components. Stainless steel helical coil inserts of special design are
installed in all spark plug holes.
Tools, inserts, and information are available through HeliCoil®, Emhart Fastening
Teknologies. The latest revision of the manufacturer’s bulletins 959A, 995, 943, T4000,
and 1000 list manual and power-driven installing tools, tang break-off tools, special taps,
plug gauges, and tap/drill information.
Helical coil inserts are available in both National Course and National Fine series in
lengths equal to 1, 1½, and 2 times nominal diameter and in pipe thread sizes. They are
made of carbon steel, phosphor bronze, or stainless steel, as specified by part number.
They are supplied with or without a notch above the driving tang. The notch is provided
to facilitate breaking off the tang in open holes.
When compressed into a special tapped hole at the widest part of the wire between male
and female threads, the diameter of the insert is equal to the nominal screw size. The
special finishing taps size the threaded hole to allow the pitch diameter of the female
thread of the installed insert to conform to Class 3 fit with standard bolt threads or Class
4 (tight) fit with standard-size studs. The difference in fit is due to a difference in pitch
diameters of bolts and studs.
Only one set of helical coil special taps is required for installing these inserts in both bolt
holes and stud holes. Tap drilling depths and tapping depth for helical coil inserts to be
installed in blind holes must conform to the recommendations relative to inserts of length
equal to 2 times nominal diameter, as tabulated in the latest revision of the
manufacturer’s bulletin numbers 1000 and T4000.
Run helical coil tap drills and special taps perpendicular to the machined surface to
follow the alignment of the existing hole.
For drilling and tapping aluminum alloy castings, use a commercial-grade cutting
lubrication oil to prevent overheating of the metal and tearing of the thread.
Helical coils are prohibited in certain areas; verify that a helical coil repair for a certain
area is approved prior to installing the helical coil.
Replace helical coils in approved areas when they are damaged in accordance with the
manufacturer’s instructions.
