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Easy thread repair

Overcoming stripped threads

by Julian Edgar

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We’ve all had that sinking feeling when the bolt that we’re tightening suddenly ‘gives’- the thread is stripped. If that thread is in a nut, well, then it’s just a case of replacing the nut (and probably the bolt too). But what if you’re screwing your bolt into a tapped hole - say in an alloy casting? Then it can be a real downer.

But help is at hand – and cheaper now than ever before. You need a thread repair kit – either one to suit just the thread you’re repairing, or a general purpose kit.

Click for larger image

This kit cost me just over AUD$100 on eBay. It can be used to repair M5, M8, M10 and M12 threads. (Kits designed for imperial threads are also available.) The kit comes with the correct drill bits (more on this in a moment), taps, installation tools and three different lengths of thread inserts for each thread size.

So how do you use the kit to repair a thread? Let’s take a look.

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The need for the repair came about because I stripped this M6 thread in an alloy casting. In fact, while careful inspection shows that the thread on the left is ruined, the one on the right isn’t far behind either!

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Using the correct size drill, the first step is to carefully drill out the damaged thread. Note that if you are drilling into a blind hole, you should wrap a piece of tape around the drill bit to show the correct depth to which you need to drill.

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One advantage of buying a kit is that the drill bits are correctly sized for the taps. This drill bit, for example, is 6.3mm in diameter. It suits the M6 tap.

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The next step is to use the correct tap to from a thread for the insert. The kit I bought did not have handles to drive the taps. In this case, where access was best from directly above, I used an Allen key T-bar to drive the tap, adapting the Allen key to the tap via a small socket.

When tapping the hole, it’s imperative that the tap be at 90 degrees to the top face, in both planes. Using the long Allen key allowed me to easily check that the tap was being inserted correctly. If required, lubricate the tap with some thin oil while tapping. Note that if you are tapping a blind hole, withdraw the tap frequently to remove metal swarf.

Click for larger image

The threaded inserts are made from stainless steel. They use a wire that’s diamond-shaped in cross-section, and have a lower tang that allows the tool to screw them into place.

Select the length of the insert to match the depth of the hole.

The insert is placed on the end of the tool, tang engaged with the slot in the tool.

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The thread insert is then carefully screwed into place. Ensure that the top of the insert ends up a little below the outer face.

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The break off tool is then inserted until it touches the tang and a quick hammer blow then used to break off the tang (the tang is pre-cut to allow this to easily occur). When doing this, ensure that the tool is vertical and so is not snagging on any of the threads.

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The finished job – with both threaded holes repaired (and the cast alloy bracket off the engine to allow a better pic!).

Conclusion

Whether you buy a kit to repair just one job, or a general purpose kit so that you always have thread repair capability, it’s amazing how much less depressed you feel when a thread strips!

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