Don’t cry over Stripped threads
We’ve all done it but wrecking a thread doesn’t need to be a costly disaster
Stripped? Don’t flip 1
There’s nothing worse than tightening a bolt and feeling it getting looser. When steel bolts go into alloy threads, it’s often the alloy that fails. But it doesn’t mean disaster – stop, remove the bolt and assess the damage. If the aluminium threads have failed they often stick to the bolt. Firstly, make sure the bolt is the correct one by checking the threads are the right pitch and the bolt is the right length.
File away the decay 2
You can clean up the bolt using a thread file (these can be bought from any good tool shop for around £15). A thread file usually has a choice of eight pitches, four on each end. With the correct pitch selected, place the bolt in a vice and slowly draw the file across the threads, each stroke clears and revives the threads. You need to turn the bolt in the vice one bit at a time until all of the threads are clear.
Tap into fixed threads 3
When trying to revive damaged alloy threads you should only consider it a temporary fix; the fact that some of the thread material has already been lost by transferring to the bolt suggests the overall strength of the fastening has been compromised. Before you start, blast away debris using an airline or aerosol cleaner, then use a ‘chaser’ or ‘forming’ tap to revive the threads.
Flirt with an insert 4
An insert kit is a worthwhile purchase if you’re repairing damaged threads. The kit replaces the original threads with a threaded steel insert, and comprises a drill bit, shoulder cutting tool, tap, forming tap, and selection of inserts in various sizes. In many cases the steel insert provides a superior fastening than the original alloy threads.
Get some shoulder room 6
The insert has a shoulder at the top, this prevents it from being over-tightened and sinking to the bottom of the hole. But first you need to cut some space fit for the shoulder and for that you’ll need the shoulder cutting tool. The tool is used in conjunction with a tap wrench, and the shoulder needs to be cut flush or slightly lower than the surface.
Insert for strength 8
Next, screw the insert on to the forming tap. Apply a permanent thread lock such as Loctite to the hole and wind the insert into the new threads using the tap wrench. The forming die will start to give a bit of resistance as the insert’s shoulder tightens against the surface. Keep winding the tap in and it will press the threads on the exterior of the insert into the alloy threads of the hole.
Drill before you fill 5
First, you need to drill out the worn threads using the drill bit in the insert kit. The hole needs to be oversize in order to both drill out the old threads and accommodate the steel insert. Make sure the hole is at least as deep as the insert. Ideally you need to drill a bit deeper so the forming tap can run through the insert.
Cut some new threads 7
With the tap in the tap wrench, start to cut new threads in the alloy using a cutting fluid or light oil to help lubricate things as you go. It’s important to make sure the tap is perfectly level. Work the tap backwards and forwards progressively, cleaning out swarf as needed. The new threads need to be a little deeper than the length of the insert.
Repaired and ready for use 9
With the insert now in place the hole is fixed and ready to take a bolt. Ideally you should use a new bolt with a bit of light oil or grease applied. This will not only prevent corrosion, but will also give better feel when you torque up the bolt. 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.