Light Aircraft Association

I had an idea the other day which I thought I'd pass on. The problem was a damaged thread in a casting. The top bit was "bruised" but the lower part was OK. The snag with retaping is I sometimes find the new thread doesn't line up with the old one causing further damage.
My idea, and I've no doubt someone will tell me it's been done, is an expandable tap. The idea is something like a Rawlex expander bolt but with cutting teeth on the jaws. Drop it in the hole, expand it to engage in the existing threads and wind it out, hopefully giving a perfect thread.
So has this been done and did it work?
Very difficult to think of something really new but if someone wants to make them, best of luck
Rob

Hi Rob, nice idea but I wonder how often you would need such a tool. Don't want to teach granny to suck eggs but for those not aware, taps come in three configurations, 1st, 2nd and plug, 1st having a longer taper lead in than 2nd and plug (what you usually get when you buy 'a tap') having virtually no tapered lead in. If you had a damaged thread as you describe, the tapered sections of tap 1 or 2 should generally engage the lower threads so that the damaged start of the thread is cleaned up in alignment. The only time you might have a problem is if the thread is quite shallow, or if the start of the thread has been really mullered and burred over. In that case I'd clean up the face of the hole very lightly with a countersink to remove the worst of the burred edge and then proceed with a tapered tap.

You're right,a taper tap should engage the existing threads unless it's very bad. Could explain why nobody has made one

They do, taps come in sets of three or if you have good ones they do.
Do not buy cheap ying-yang ones, but a quality set and all will be revealed.
The other difference is that quality is in sizes and material, they are both good as against something like. There should be taper , second and plug and they follow each other in use, using plenty of the correct lubricant for the material.
Graham

or of course a helicoil insert will create a stronger thread in alloy, epsecially where removal of the bolt is frequent. Depends on the application of course

In the shallow thread scenario Brian mentions I think I'd helicoil it rather than risk having only half of a short thread.

Far better than helicoils is a product called "time-sert" from Wurth.

Problem with helicoils is the reduced "wall" thickness if they are close to an edge.