Occasionally threads, both internal and
external, are only partially stripped.
These can be readily cleaned up using
a die or tap.
In cases where there are no threads a
tap does little good. In these cases a
stainless helical wire insert called a
“Helicoil” can be used.
Thermostat housings, cylinder heads,
exhaust manifolds are a few of the
places that Helicoils can be used.
Helicoiling begins by drilling out the
old threads with a specified drill bit for
the insert size to be used.
Drill straight is easier if you use a
machinist square to serve as a guide.
Each Helicoil requires a specific tap.
(part of a kit) This will cut „oversize
threads to fit the insert.
Use cutting fluid, tapping fluid, WD-
40 or even a little oil when tapping
aluminum to avoid galling and
stripping the thread often leaving a
Clean out the shavings and hole
Using the insert tool, carefully wind the
insert in to the threaded hole.
Insert so that the Helicoil is below the
surface and embedded into the recut
The tail is snapped off with a punch
once the insert is in position.
Answer the following questions:
1. What is a “Helicoil”.
2. What is the purpose of using a
machinist square when drilling?
3. How are oversize threads made?
4. Why should you use cutting oil?
5. What two things will tell us if the
Helicoil has been inserted correctly?