Repair damaged threads with a Helicoil thread repair kit

One of the parts most susceptible to damage during periodic and repeated engine maintenance or repairs are the threads, especially those found in the various parts of the crankcase, engine block, and their components. In modern engines, these threaded holes are usually made directly from the component material (usually aluminum alloy or steel), and the bolts that penetrate them are generally made of a harder material.
Inserting, tightening, loosening, and then tightening these screws tend to gradually wear down the thread, as it becomes softer, and in extreme cases, can even completely damage it and render it useless.

The problem and the solution
When we are faced with the problem of a thread with stripped threads, or "loose" as it is commonly called, we will no longer be able to correctly perform the determined tightening torque, since the screw will not have enough gripping surface and will turn without exerting the necessary force to join the elements or perform its function.
We can choose to rework the hole with a larger tap , although we will also need to install a larger diameter screw. The tap is a very hard cutting tool that will recreate threads in the hole where it is used.

The problem is that we won't always be able to install a larger screw, since we will often be limited by the characteristics and physical limits of the parts (that is, a larger screw won't always "fit").
The other option, much more advisable, is the installation of a Helicoil type threaded insert, since apart from allowing the installation of the original screw, we will be able to improve the original threading.

The Helicoil kit
Helicoil threaded inserts are a type of reinforced steel spring, which is threaded into the damaged hole and allows for a firmer joint, providing the following advantages:

-Allows us to use the original screw size.
-It strengthens the thread; a Helicoil thread is generally stronger than a thread drilled into the block. It also eliminates stress and improves tightening precision.
-It improves the quality and durability of the thread, and can even outlast the life of the screw itself.
The components of the Helicoil kit are the drill bit, tap, inserts, and installation tool.

How to use it
We are going to explain in a practical way how to repair a damaged thread using a Helicoil type insert kit. In this case, we have a fairly common example, where the clutch cable threading on a Harley-Davidson Sportster has deteriorated right where the cable inserts into the primary chain cover, causing a small oil leak through the hole.

After removing the cover and the cable, we can visually check the damage to the threads, which no longer allow proper tightening.

The first step is to rework the hole with the drill bit included in the kit, which will give it the correct diameter for the next step. Then, we begin cutting the threads for the insert with the tap. This is a critical step. You must be very precise and proceed gradually, advancing 1/4 of a turn and backing out 1 turn. This way, the excess material will fall out of the hole and the thread will be of good quality.

We can then insert the insert into the installation mandrel,

and using the compression tool, gently thread until it enters the hole completely.

The insert itself acts as a brake when it expands, which will prevent it from coming out when the screw is unscrewed.

The last step is to break the insert installation tab,

If the hole is blind, we will do it by means of a sharp blow with a punch, and if the hole is through, as in this case, we will simply cut off the excess with pliers.

Ending
We visually check that the insert is correctly positioned and we can now proceed to reinsert the clutch cable, remembering to install a new O-ring to ensure the assembly is watertight. We then tighten it to the indicated torque: 4.1 to 6.8 Nm.

With this, our primary chain cover is repaired and our bike is ready for the road again.

By the way, if you need to repair the thread of the Harley-Davidson clutch cable, you can see the helicoil kit HERE .

Frank Burguera

#harleymechanics