1. In most cases of worn or damaged screw threads at least one of the cooperating threads needs modification or replacement to adapt it to the repaired screw thread of the other part, which, because normally recutting of the screw thread is necessary, will be of a different diameter than the original.

2. For damaged screw threaded holes it is common practise to insert threaded bushings or to use so-called "helicoils", which are particular appropriate solutions for restoring damaged plug holes or threaded holes for stay bolts because new plugs and stay bolts, if these also need replacement, are easily available.

3. In case each of the damaged parts is difficult to replace or to manufacture it is also possible to use a bushing which is adapted to both new inner and outer screw threads, recut with the same or different pitch screw threads and replacing the original screw threads.

However, manufacturing of such bushes is not very easy because of the accuracy involved. Moreover such bushes normally get relatively thick due to the in- and outside screw threads between which enough material must be left to avoid interference of the respective tops of the screw threads and cutting through the bush.

4. In cases where stresses are low, such as in screw threads for valve spring covers or other none structural parts of a motorbike (Harley Davidson), a practical solution for repair of damaged screw threads was found which is easy to handle and does not take much place.

For example in valve spring cover screw thread repair it is often possible to restore the cover casing screw thread by "deep" cleaning only, or by cutting the original screw thread a little deeper.

The next step is to simply cut away, using a lathe, the screw thread on the valve lifter body in order to obtain a cylindrical clean surface. Then machine a tube with the required screw thread on the outer surface to match perfectly with the reclaimed screw thread of the valve spring cover and a smooth inner surface of a diameter slightly smaller than the cylindrically cut outer surface of the valve lifter body.

This work can be done in a screw cutting lathe which gives the advantage that any screw thread can be perfectly matched.

After finishing the screw threaded tube, the required length can be parted off from it with a parting tool. The resulting screw threaded cylindrical ring is mounted by interference fit on the valve lifter body. Fixing also may be done by soldering or, which proved very convenient, with "loctite". If carried out with some attention to detail it is quite difficult to see that a repair was made.

5. It will be clear that, in case of real damage to the valve spring cover screw thread so that recutting would leave not enough material left, it is also possible to replace the valve spring cover screw thread by a threaded bush instead of the valve lifter body's screw thread, in essentially the same manner as described above.

6. The great advantages of this method of repair are that not much place is needed and that it is almost impossible to see that a repair was carried out as well as is that it can be done on any parts, plated or painted without that the parts surfaces are damaged since neither high machining forces nor high temperatures (when using loctite) are necessary for the repair.