There are many times when disassembling old parts that you end up stripping a bolt or nut and ruining your whole day. New shiny replacement hardware is often available, but looks 50 years newer than the piece you are attaching it to, and a lot of original hardware items are no longer made. Using re-threading tools to restore existing hardware can be a viable alternative to new parts or when replacements are not readily available to complete the task. Re-using the original hardware is also preferred during restorations in order to keep the car’s patina earned over years of driving.
There are several ways to restore threads on original hardware, most obviously through the use of taps and dies. The taps most consumers buy to restore stripped nuts or holes in metals are nut or plug taps. A “nut tap” actually takes the place of two separate or serial taps that are used when the hole or nut is first threaded. The thread cutting surfaces on these taps create a progressively better-threaded hole to receive the bolt or stud of corresponding size and thread pitch.
The first of these serial taps that are used is an “entering tap”; the majority of the aggressively chamfered threads on this first tap do about 60 percent of the thread cutting in the hole. Once the bottom of the tap has done the cutting, upper threads on the tap are designed as a following thread that keeps the tap centered in the hole as the lower cutting threads set the thread pitch.
A second plug tap is then run in after the first; the plug tap has less-chamfered threads and more following threads, and does about 30 percent more of the cutting. The design of the plug tap continues the thread pattern further down into the hole.
The final tap used is a “bottoming” or “finishing” tap that has very few chamfered edges at the bottom of the tap, and a majority of the tap has following threads. Bottoming taps are used to finish the lower end of the hole.
Nut taps can be used in place of both the entering tap and plug tap to finish holes that are drilled all the way through; however, bottoming taps are recommended for holes that stop inside the material. Nut or plug taps have a tapered, pointed end, while bottoming taps are flat, and the wider chamfered threads start at the flat base.
It is recommended you purchase one of each or a set that contains both types for each thread size. Your taps will be found with several different tops on them. The standard configuration is a four-sided head. In this case, the set should include a T-shaped tap handle, designed to hold the tap by its square head. The handle enables you to keep the tap at 90 degrees from the hole, ensuring a straighter thread once cut. T-handles can also be used to multiply the force needed to cut into the unthreaded hole.
Other taps will have a hex-shaped head that can be turned using a wrench or socket on a ratchet. Several custom thread taps for spark plugs can also be found with a 3/8-drive square hole for a ratchet.
Most taps will also indicate on the stock which size drill bit is recommended to make the hole wide enough to allow the tap to do its jobs without breaking. It is important when using a tap that you do so gradually, turning the tap into the hole a few turns, then backing the tap back out to allow the cut material to travel up the threads and out of the hole. One of the toughest removal jobs you can encounter during any repair is removing a broken tap. They are very stubborn to twist out once they break and very difficult to drill out. Cutting oil is also recommended to keep the tool from overheating or dulling. The cutting oil will also help grab onto the metal chips that are produced during cutting, and help carry them up the threads and out of the hole.
Externally-threaded bolts, spindles and studs can be restored using dies. Thread dies restore the original pitch using chamfered cutting threads with following threads at the back to help guide the tap perpendicular to the stock being threaded. Dies are usually easier to use and more accurate when held in a die holder with twin handles that allow the application of additional force where necessary. In the case of damaged threads on a spindle or wheel stud, larger re-threading dies are available for these operations, and they function in the same way.
Thread files and thread chasers can often be used in cases where just a few threads on a bolt or nut are damaged. A thread file is a square file with teeth cut in various thread pitches on each side of the flat surface. Usually double ended, a thread file can contain eight different thread pitches and are offered in both metric and standard thread pitches.
Line the thread file up using the remaining threads that are still good and draw through the bad threads as you would a regular file to straighten the bad threads or re-establish the thread pitch that will allow the nut to continue onto the lower or inner threads.
Thread chasers are used in a similar manner to a tap, but are more like a hardened bolt that you use to force internal damaged threads back in line with the other threads in the hole.
Should you encounter a hole that is damaged beyond the point of re-threading, there are a few other alternatives at your disposal.
A Heli-Coil kit uses a drill bit, oversize tap, thread insert and inserting tool that allows you to re-establish the thread hole. The taps contained in the kit are matched to the size of the external threads on the inserts; the internal threads of the inserts are the same size and thread pitch as the original hole. Once the hole is drilled out, the tap then cuts new oversized threads. The insert is installed into the inserting tool, and both pieces are screwed into the newly threaded hole. Once the insert has bottomed out, the inserting tool is then removed, and the hole is ready for a new bolt.
Another trick that can be used when original threads are too damaged to repair, is to rethread in another size. Often, U.S. threaded holes can be oversized using a metric tap (and matching bolt) of a slightly larger size and vice versa. Five-sixteenths-inch diameter threads can be tapped to 8 mm, 3/8-inch threads can be oversized to 10 mm, 5/8-inch bolts can be tapped to 16 mm, etc. This doesn’t apply to all U.S. sizes. For example, it would be difficult to find 13 mm hardware to replace a 1/2-inch bolt, but there are close equivalents that allow this switching between continents to be successful when repairing some threads.
Thread holes in thin metals can be repaired using poly nuts. A poly nut is a rivet that is popped into the hole using a special rivet gun that has new internal threads for a bolt or stud. Poly nuts are available in many sizes and thread pitches but are usually used in holes smaller than 3/8-inch.
Whenever replacing new or used hardware, it is recommended that a thread lock product be used to ensure the bolt or nut does not vibrate loose or back out. The use of a thread locker will also preserve the threads from damage, should the connection have to come apart at a later date.