Repair damaged threads with a Helicoil thread repair kit

One of the parts most likely to be damaged during periodic and repeated maintenance of an engine, or during successive repairs are the threads, especially those practiced in the different parts of the crankcase, motor block and its elements. In modern engines these threaded holes are usually made directly on the material of the component (usually aluminium or steel alloy), and the screws that penetrate them as a general rule are of a harder material.
The insertion, tightening, loosening and subsequent tightening of these screws usually wear out the thread little by little, being more soft, arriving in extreme cases to completely spoil it and leave it useless.

The problem and the solution
When we face the problem of a thread with the spoilt threads, or "passed" as it is commonly said we will no longer be able to properly perform the set torque, as the screw will not have enough grip surface and will rotate if exercising the necessary force to join the elements or perform their function.
We can choose to revisit the hole with a "male thread" larger, although we will need to install a larger screw in diameter. The thread is a very hard cutting tool that will again create threads in the hole where it is used.

The problem is that we will not always be able to install a larger screw, as many times we will be limited by the characteristics and physical limits of the pieces (i.e., not always "fits" a larger screw).
The other option, much more recommended is the installation of a threaded insert Helicoil type, and part of allowing the installation of the original screw will improve the original thread.

The Helicoil kit
The Helicoyl-type threaded inserts are a kind of reinforced steel spring, which is threaded into the damaged hole and allows to create a firmer bond, providing the following advantages:

-It allows us to use the original screw size.
-Reforce the thread, usually always stronger a Helicoil thread than a thread practiced in the block. It also eliminates stress and improves the accuracy of the tightening.
-It improves the quality and duration of the thread, it can even exceed the duration of the same screw.
The components of the Helicoil kit are the bit, the screw, the inserts, and the installation tool.

How to use
We are going to explain in a practical way how to repair a damaged thread using a kit of Helicoyl type inserts, in this case we have a fairly common example, where the threaded of the clutch cable of a Harley-Davidson Sportster has deteriorated just where the cable is inserted into the top of the primary chain, causing a small oil leak through the hole.

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

The first step is to go over the hole with the bit included in the kit, which will leave the appropriate diameter for the next phase. Then we will start the cutting of the threads for the insert with the male thread, it is a critical step, it has to be very precise and to go little by little, advancing ¼ of turn and unscrew 1 turn, in this way the excess material will go falling out of the hole and the thread will be of good quality.

Then we can already insert the insert into the installation mandrel,

and helping us with the compression tool go by gently threading until I completely get into the hole.

The same insert acts as a brake when expanding, which will cause the screw to be unscrewed.

The last step is to break the insert's installation tab,

If the hole is blind we will do it by a blow dry with a button, and if the hole is through as in this case we will simply cut the leftover with some pliers.

Completion
We visually check that the insert is correctly located and we can already proceed to reintroduce the clutch cable, without forgetting to install a new O-ring to ensure the tightness of the assembly, and we tighten it to the indicated torque: 4.1 to 6.8 Nm.

With this we already have our primary chain cap repaired and our bike again ready for the road.

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

Frank Burguera

#mecanicaharley