From the workshop
Maintenance
Repairing stripped threads
by
Josh Giumelli,
KONDININ GROUP
S
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Stripped threads are a common problem in farm
machinery and can be difficult to fix. Take care
when tightening bolts in soft metals to avoid
stripping threads.
tripped thread bolts can be frustrating and
cause machinery downtime but there are
options available to repair the damage.
Stripped threads occur when the threaded
portion of a bolt hole separates from the rest
of the component, often becoming embedded
in the bolt threads.
This usually occurs when a bolt is overtightened but also can be caused by machinery
wear or if insufficient threads are engaging in
the bolt to carry the load.
Stripped threads are more common in
components made of material which is softer
than the bolt such as cast iron or aluminium.
In stronger metals, the bolt is more likely to
snap if it is too tight or overstressed.
This article outlines several methods to
overcome stripped threads.
Ways to fix stripped threads
Re-drill
then re-tap
Damaged
section
Re-tapped
oversize
Damaged
section
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Drill out the stripped thread and tap a larger
diameter thread in its place. This is one of the
simplest methods but it has drawbacks: the hole in
the component might need to be drilled larger,
which is not always possible; and to increase
thread size both holes will need to be made larger,
which can weaken the component.
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5
It could be possible to weld the bolt hole, re-drill
and re-tap the thread back to its original size.
This is more convenient on larger holes and is
easier with a metal inert gas (MIG) welder than a
stick welder because there is no slag to interfere
with welding. The difficult part is centering the drill
bit in the welded hole.
Longer
bolt
Damaged
section
Built up
with weld
Drilled and
tapped to
suit thread
insert
Undamaged
section
The thread and coil kits contain several thread
inserts, a handle for screwing in the inserts and a
thread tap for creating the outer thread. The kits
cost about $70 but this still can be cheaper than
replacing an original component. Unfortunately,
they are usually bought to repair one thread and the
other inserts are never used. If the inserts are all
used, it is possible to buy replacements for less than
the cost of a complete kit. Inset: This workshop uses
a kit for repairing a 3/8 UNC thread, which has 16
threads per inch (see Farming Ahead No 140, page
18, for information on threads). The kit label also
specifies the correct tapping drill bit needed to drill
the insert hole. Ensure the exact drill bit is used, in
this case a 9.9-millimetre drill bit is specified.
Thread
insert
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3
Use a threaded coil or sleeve insert to re-thread the
bolt hole to its original size.
Use a longer bolt and pick up any undamaged
threads below the stripped section. This is only
possible if the hole is deep and threaded all the way
through. The bolt could become stripped again if
there are not enough threads to engage in the bolt.
Using a coiled thread insert
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The stripped thread pictured is in an aluminium
manifold. Because the hole is close to the edge
and does not pass through the manifold entirely,
most repair options outlined are not possible.
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Damaged
section
Nut
4
If the threaded hole passes through a component,
a nut can be placed on the other side.
14
Threaded coil inserts are made from stainless steel
and resemble a spring. The coil’s internal thread is
matched to the original bolt thread and the outside
thread is tapped into the damaged bolt hole.
Different lengths are available in metric coarse
and fine, unified national fine (UNF), unified
national coarse (UNC), British standard pipe (BSP),
British standard whitworth (BSW) and sparkplug
thread sizes.
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Before drilling and tapping the thread, check the
tap against the bolt to confirm the correct sized kit
has been bought. Although the tap will have the
same pitch or threads per inch as the original bolt,
its diameter will be larger to accommodate the
insert. Unfortunately, this means the tap cannot be
used for other general thread tapping duties.
FA R M I N G A H E A D
No. 142
November 2003
Maintenance
From the workshop
11
With the correct drill bit size, drill out the damaged
thread as shown. This is best carried out using
a bench drill but if a hand-drill is used ensure
the pieces are square. The size of the hole drilled
will be smaller than the corresponding hole,
which needs to be drilled if fitting an oversize bolt.
This will not weaken the component significantly.
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When nearing the end of the coil, take care not to
screw it in too far. The coil should sit about half a
thread below the surface as shown.
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12
Tap the thread into the hole using tapping fluid if
necessary. Use a tap wrench to turn the tap rather
than a spanner as this will help keep the components
square. Do not form the thread in one pass but back
off the tap after a few turns before continuing the
process. This will help break up and clear metal chips
to avoid breaking the tap, especially on smaller holes.
Remove the tang on the bottom of the insert (inset).
Use a hammer and punch to snap off the tang.
A sharp blow will snap off the tang at a weak point.
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If the tang is difficult to break off, bend it backward
and forward with long-nosed pliers until it snaps.
Ensure it is removed from the hole.
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21
Sleeve inserts can be used on most metals and are
ideal for putting threads in wood and plastic.
They are made from case-hardened steel and the
slotted end inserted first cuts its own thread.
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Use the correct drill bit size when drilling out the
damaged hole. The information is found on the
sleeve packet.
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Ensure the hole is straight (not oval). Remove all
chips before installation.
When the new thread has been cut, clean out the
metal chips. When working on engine blocks,
hydraulic systems or gearboxes metal particles can
cause significant damage if they enter critical
areas. Grease can be used while drilling and
tapping to keep all of the particles together.
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The repair is now complete and the component
can be returned to service.
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Place the coil over the insertion tool, with the tang
between the two halves and set the depth. This tool
can be used to place the coil a set distance down
the hole or stop it being wound too far.
Screw the coil into the thread slowly (as shown).
There should be little resistance as the installation
tool will compress the coil slightly. After installation
the coil will spring into place, locking it into position.
FA R M I N G A H E A D
No. 142
November 2003
Thread the nut and sleeve onto the bolt as shown,
and lock it into place with two spanners (inset).
Using a threaded sleeve insert
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An alternative to the coil insert is a solid thread
sleeve insert, which is cheaper and available as a
single insert. Inserts such as the ‘tapper’ include a
bolt and nut, which are used as the installation tool.
The drawback is that they need a larger hole, which
may weaken the component being repaired.
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Screw in the insert, applying downward pressure
to help the threads bite. Ensure the insert goes
in straight. A cutting compound or grease could
be needed for this operation. Screw in the sleeve
until it is flush with the top of the hole (inset).
For hard steels a tap could be needed to thread the
hole before inserting the sleeve.
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