How to make a threaded repair without it all unravelling
How to master the dark art of repairing stripped threads
1 Prevention better than cure
It’s all too easy to strip a thread, especially when there’s corrosion present or you’re not using a torque wrench. Bikes usually have steel bolts tightening into alloy threads and it is often the alloy parts that give in as they are softer than the steel bolt. Make sure bolts are free from corrosion, they should initially drive in smoothly with almost no resistance.
3 Learn a bit of tap dancing
Reviving damaged alloy threads can only be considered a temporary fix. The fact that some of the alloy thread material has already been lost in transferring to the bolt suggests the overall strength of the fastening has been compromised. Ideally, use a ‘chaser’ or ‘forming’ tap to revive the threads, as these will try to reform what is left of the existing threads.
2 File away
A bolt with damaged or corroded threads can be revived with a thread file. Costing £12, buying one is often dearer than replacing a bolt, but if the bolt is expensive or no longer available then it’s invaluable. The file has eight pitch sizes. Select the right size then slowly draw the file across the bolt, each stroke clearing and reviving the threads.
4 Prepare for blast off
Once you have cleared the threads out they need to be cleaned thoroughly. Use an airline and contact cleaner to blast away the dirt and debris. It’s really important to do this thoroughly as any left over swarf will get caught up into the threads again and cause more damage. Re-fit the bolt or stud and torque up to the correct spec.
5
Insert or helicoil?
An insert kit is a worthwhile purchase if you have a project on the go. The repair kit replaces the aluminium threads with a threaded steel insert and comprises a drill bit, shoulder cutting tool, tap, forming tap and a selection of inserts. They are more expensive than a helicoil set, but give a better, more permanent repair.
7 Get flush
The insert has a shoulder at the top which prevents it from tightening into, and sinking to the bottom of, the hole. It also makes the repair flush, which is especially important for sealed mating surfaces. With the shoulder tool held in a tap wrench, cut the shoulder to the permitted length of the tool.
9 Lock and load
Next, you need apply some permanent thread lock on the outside of the insert and wind it into the threaded hole with the tap wrench. The forming die will start to give a bit of resistance as the insert shoulder tightens to the surface. Keep winding the tap in and it will press the threads on the outside of the insert into the alloy threads of the hole.
6 Get drilling
Some repairs can be done with the part in situ, others will require removal. Drill out the damaged threads using the kit’s drill bit, which oversizes the hole, removing traces of the threads. Make sure the hole is at least as deep as the insert. Ideally drill a bit deeper so the forming tap can run through the insert.
8 New threads
Use the tap in a tap wrench to cut new threads, apply cutting fluid or light oil to lubricate the tap as you work. It’s important to make sure the tap is at the correct angle. Work the tap backwards and forwards progressively, cleaning out any swarf or cuttings as needed. The new threads need to be a little deeper than the insert.
10 Back stronger
With the insert now in place you can rebuild straight away. If the new insert is a repair on a component that has many fasteners, check the workshop manual for specific tightening sequences and torque values. The steel insert will be stronger than the original alloy threads but do not take advantage of this by thinking you can tighten it up to a higher value.