How can I fix stripped threads in screw holes, especially in wheels (for wheel covers)?
In some cases it may be possible (and permissible) to drill and re-tap the stripped hole to the next oversize. A more suitable, more common, and more acceptable repair is to install a “Helicoil”. This will restore the threads to the original fastener size, and will enable ongoing removal and replacement of the fastener without thread damage.
Helicoils may be purchased in kits, either for a specific size, or for a range of sizes. A kit should contain the correct size drill bit, the correct Helicoil thread tap, the correct length Helicoil (for the length of the fastener and hole), and the correct insertion tool, and instructions. Once properly installed, a Helicoil provides permanent heat-treated steel threads, in otherwise soft material. They can also be used to repair stripped holes in cast iron and steel. In many cases the manufacturer will install Helicoil-like steel thread inserts in original parts; for example, spark plug holes in aluminum cylinder heads.
Here is some additional information regarding standard-thread hole taps. While the nomenclature sometimes varies, taps are generally categorized as “starting”, “plug”, and “bottoming” styles. A starting tap has an extra-long nose taper, intended to help ensure that the threads are cut straight and square in the hole. A plug tap is the standard tap that you get when you buy a single tap or standard tap set somewhere. They are most suited for cleaning out and restoring damaged threads, and to follow up a starting tap prior to use of a bottoming tap. A bottoming tap is cut almost square on the end; it has just enough bevel to be able to cut without chipping the tap teeth in steel stock. Its purpose is to cut threads all the way to the bottom of the hole, so that a fastener can get more thread engagement in shallow stock.
If you are doing specialty work in shallow stock, you can buy three-tap sets in any given size. For light aircraft work, sizes 6-32, 8-32, and 10-32 are those you are most likely to need in the specialty styles. Standard plug taps will usually do the job in the other sizes. A good brand of three-piece sets that can usually be found is “Hanson”.
Some words of caution. If you do not have experience with precision tap work (as opposed to just cleaning/restoring threaded holes), you would be wise to get some help from an expert, or do a lot of expert reading. It is surprisingly easy to snap off a heat-treated tool-steel tap in an aluminum hole, and then you are really “in for it”. You need to develop a “feel” for when the tap is binding; you need to use a back-and-forth motion to keep the chips broken up and “flowing”; and you need to use the proper cutting/tapping lubricant for the material you are working on.
And to wrap up, for this kind of work you need a set of quality drill bit stops. Not some junker cheapies that will slip when the bit tries to dig itself in a bit further; you want some that clamp securely on the drill bit, and will positively stop it at the desired depth.