When working on old motorcycles, you’ll eventually find yourself dealing with damaged threads at one point or another, and a threaded insert is a great way to repair them.
The most common reasons for damaged threads are:
- Overtightening bolts – when too much force is used to tighten down a bolt the excess pressure will cause the threads to become deformed.
- Undertightening – screwing down a bolt or screw too loosely can also damage threads because it allows for unnecessary movement or vibrations to occur.
- Soft metal parts – the threads on some parts, especially older ones made out of softer materials, can easily get worn down over time after repeated use. Using bolts made out of stronger and harder materials with soft metal threads also increases the risk of damaging the threads.
- Corrosion – rust and corrosion causes threads to become weaker and breaks it down much quicker.
- Incorrectly sized bolts – using bolts that are too large or too small can damage threads just like over or undertightening.
- Cross threading – inserting a bolt at the wrong angle then tightening it down causes a lot of damage to the thread.
Threaded Insert Kits
Repairing damaged and stripped threads is a common task that every mechanic and DIY enthusiast should be familiar with.
You simply need a couple of relatively basic tools and a threaded insert kit. You can buy thread repair kits or threaded inserts from well-known brands like PowerCoil and Heli-Coil, but you can also pick up extremely affordable kits from China and other lesser known generic brands.
Typically, the more expensive and high quality kits from PowerCoil and Heli-Coil are sold in specific sizes, so you’ll have to find the right kit size for the thread that you are trying to repair. Check out our article on Threaded Fasteners if you need more information about threads and how to identify them.
Each kit comes with everything you’ll need to perform the repair. Inside the box you’ll find a drill bit that lets you create a pilot hole for the threaded insert, a tap to create the threads for the threaded insert, a hand installation tool that lets you hold on to the threaded insert’s tang and screw it in manually, and a tang break tool that lets you break off the tang once you’ve finished installing the threaded insert.
The kit also comes with several threaded inserts, typically ranging from 5 to 20 inserts depending on the kit you have, which you can also buy separately once you’ve run out of inserts.
What is a Threaded Insert?
A threaded insert kind of looks like a small spring or more specifically, it’s a small metal cylinder with threads on the inner and outer surfaces. The internal threads have the exact same size of the thread that you are trying to repair.
For example, an M10 x 1.5 threaded insert will fit a 10mm bolt with a 1.5mm pitch.The threads on the outer surface of the threaded insert are the same size as the tap that’s included in the kit, and will be a little bit larger than the original thread.
Inside the threaded insert, you’ll see a small metal tang that’s used to hold on to the insert as you’re screwing it in the threaded hole.
Installing a Threaded Insert
To install the threaded insert, you’ll first have to use the supplied drill bit to drill out and completely remove the damaged threads.
If the damaged part is too large to fit under a drill press or in table clamp, such as when working with an engine block or cylinder head, it’s safer to use a hand drill so you can work slower and have an easier time keeping the drill bit straight.
The drill bit will create an oversized hole and a fresh metal surface with no traces of the original damaged thread.
Now, you can create new threads for the insert using the supplied tap. To make this step easier, use a tap handle so you can get more leverage on the attachment. Before starting, use some tapping fluid to lubricate the tap. If you don’t have any tapping fluid, you can also use engine oil or light machine oil to make this process go a bit smoother. For aluminum parts, you can also use kerosene as a lubricant.
During the threading process, you’ll want to keep the tap as straight as possible and work slowly to avoid any mistakes. Turn the tap clockwise then back it out a bit to break off the chip, otherwise known as swarf or shavings, after the first few turns.
Keep going clockwise until the tap has reached its full depth. Once you’re done, remove the tap fully by turning it counter clockwise then remove any metal shavings by blowing it out with some compressed air or just flush it out with some brake cleaner or WD-40.
Make sure to clean it out completely as you don’t want any shavings to damage the threads you have just made.
To screw in the threaded insert, slip it over the hand installation tool and place the tang inside the tool’s notch so that the threaded insert turns with the installation tool as you’re screwing it in.
The freshly made threads should match up perfectly with the threads on the outer surface of threaded insert and you should be able to screw it in with very minimal force.
Once you’ve fully screwed in the threaded insert, the tang should end up at the bottom of the part.
The final step is to break off the metal tang. Use the tang break tool to snap off the tang, which will just end up being an obstruction for your bolt if left in place.
In a lot of cases, you can just snap off the tang by hand, or you can use a hammer or a similar instrument to make the job much easier.
So there you have it. Installing threaded inserts is a fairly straightforward process that can salvage broken parts that would otherwise be hard to replace.
Everything you’ll need is pretty much supplied in the kits that you can easily find at any auto parts store, and no expensive tools or trips to the machine shop are required.
MENTIONED IN THIS VIDEO:
How to identify threaded fasteners: https://krankengineering.com/tech/threaded-fasteners-bolts-nuts/
RELATED TO THIS VIDEO:
Other videos and articles in the “Engineering for motorcycles” series:
- Basic properties of metals: https://krankengineering.com/tech/basic-properties-of-metals/
- Introduction to torque: https://youtu.be/StIjMMMYpnU
- Introduction to limits and fits: https://youtu.be/ZKJn4-TVky8
- Introduction to bearings: https://youtu.be/KKfGV_tHsz4
- How to pack bearings: https://youtu.be/8fAiGGvz49Q
- How to identify bolts: https://youtu.be/c25uegkBkB0
- Top ten shop fluids and lubricants: https://youtu.be/HnhpslOUYSk
- Shop Safety Tips, plus “”How to set yourself on fire with an angle grinder””: https://youtu.be/VTiQqj0yHNo