Thread Repair Tips
From: Perry Ruiter

One constant headache restorers of vintage iron can count on is that they will encounter rusty and damaged threads.  A dozen years ago, when I was tackling my first restoration, my initial attempt to address the thread problem was to buy a cheap tap and die set.  I learned my lesson about cheap tools and along the way I also learned taps and dies are often not the best tools to use for thread restoration.  The tools I now reach for first are thread files and thread restoration taps and dies (also sometimes called thread chasers).

For a number of years I had heard or read about thread files, but never understood how they worked so resisted buying some.  Now, I don't know how I got along without them!  Simply put, a thread file is a (usually) square piece of hardened steel that has ridges cut into each side.  The file is usually wide enough that there are at least half a dozen ridges cut and the spacing of the ridges varies on each side of the file (and often at both ends too).

To use, measure the threads per inch of the damaged thread you wish to restore.  Select the side of the file whose ridges are cut with the matching threads per inch spacing.  Position the file so that the bulk of the ridges are on good threads with one or two ridges on the damaged area.  Now, draw the file back and forth.  The ridges in the good threads will guide the file maintaining the proper thread per inch spacing while the ridges in the damaged area are restoring the threads.

A practical example of their use might be an axle that had its nut removed and was then driven out with a hammer.  The end has become slightly mushroomed so that the nut will no longer start, nor can a die be safely started (if you even have one in the required size).  This can easily be cleaned up with a thread file.

The advantages of thread files are that they are inexpensive (one file will often have eight different thread spacings.  I only own two and have never been stumped yet), they work equally well on left or right handed threads and lastly, they handle virtually all obsolete threads (since you're not concerned with diameter, only with threads per inch).

Thread restoration taps and dies are similar in construction to regular taps and dies, but look more like special nuts and bolts
than traditional taps and dies (granted hex dies are not uncommon).  However, rather than being designed to cut new threads they are designed to  "chase" existing threads.  As such, they should only be used to clean up existing threads.  They will be damaged if, for example, you try to use them to extend the existing threads on a partially threaded bolt.

Thread restoration taps and dies are my first choice for cleaning up bolts and nuts with minor nicks or rusting of the threads in order to prepare then for parkerizing or cad plating.  In fact I often use them in conjunction with thread files.  Clean up a bolt's initial threads with a thread file and then the rethreading die starts easily.

Advantages of rethreading taps and dies are that they are quick and easy to use.  There is never a worry that additional material will be removed (always a concern with traditional taps and dies).  A disadvantage is that each diameter or thread per inch change requires a separate tool and "chasers" are only commonly available for the standard fractional sizes (I often want to clean up 10-32NF and that's too small for my rethreading die set which starts at ¼).
 

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