Then there was the type us agricultural folk used, I have 2, one metric and one imperial. A section of square bar cut abit like a file there were 4 sizes on each end. Marvellous for taking burrs off when you beat the bolt out with a hammer. Mine were made by sykes – pickavant. Ah – the good old days ! Noel.

This is probably what you are remembering, taken from an Armours’ 1997 catalogue. Sadly they do not seem to be offering special tools anymore.

I can’t see how that should work, basically you need to add material or displace it from the outside.

I have a thread rolling tool made for bicycle spokes in the size that my mountain bikes used. It was for rolling new thread so that an overlong spoke could be used. with double butted spokes there is usually about 25mm of full diameter at the threaded end of the spoke which allows some length adjustment. The recommendation was to run the rolls twice over the new section.

I wonder if a fluteless tap would work as a thread restorer in a cut thread that was damaged? When I was working making aircraft parts, if the female threads were tight on the gauge, a tap with the cutting edges removed would be screwed down and often enabled the go to screw in without removing any metal. I have lots of these butchered taps in one of my toolboxes still.

Another method to restore male threads is to snap a split die in half, the four holes type are best and fit the halves to the thread inboard of the damage which is usually at the end of the bolt. Unscrewing will clean up the thread. The oldstyle square halves as already mentioned are even better, but less likely to be in a thread commonly used today.