Depending on the application, threads can become dirty, rusted, or unusable over time, or may have damaged threads due to wear and tear. Smaller components like individual screws or nuts can be easily replaced. But what about large, expensive, or highly specialized parts?
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If you're dealing with damaged threads, there are various methods you can consider for thread repair. Your choice depends on factors like the type and extent of damage, the component or material involved. We'd like to introduce you to a tool for thread repair that you might not have heard of before: the thread file. The thread file does exactly what it sounds like—it files threads. Let's explore how it works and whether the thread file is a suitable method for thread restoration.
It may appear that the thread file is specialized due to its design. However, it is an essential tool for precise work and not just for hobbyist applications. The thread file has the shape of a square rod and is covered with sharp teeth responsible for the actual filing. The tooth geometry matches common thread standards, and the spacing between the teeth is identical to the corresponding thread pitches. To maximize versatility, there are teeth with two different pitches on each side of the square.
To use a thread file, it has a smooth handle in the middle. Some files may also have a wooden handle, but using these may mean sacrificing half of the pitches. There are also sections with individual rows of teeth at the outer ends of the file for working on internal threads.
Thread files are mainly used for expensive, specialized, or hard-to-reach components. The type of damage also plays a role. Thread files are more suitable for dirt or light damage. Moreover, they can remove small irregularities, such as burrs or other remnants from the manufacturing process.
Before you can use the thread file, you need to select the correct pitch. You might already know this, and if so, you can find the right side of the thread file with the corresponding labeling. Alternatively, you can use the file itself to determine the pitch. To do this, gently press one side of the file against the thread and see if the teeth fit into the threads. Repeat this process until you find the correct spot on your thread file.
Now, move the file back and forth with light pressure, just as you would use any other file. For internal threads, press the end of the thread file against the thread and move it around following the thread inside the hole.
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How does the thread file stack up against other thread repair methods? Unsurprisingly, thread files have some significant advantages and disadvantages that you should consider before using them.
Unlike a screw tap, for example, a thread file is not limited to a specific thread size. With various pitch options on one tool, you can work on many of the threads you encounter in daily life. They are very handy tools and can be easily stored without taking up much space. This makes them readily accessible and unobtrusive when not in use.
Thread restoration tools, are flexible and can be used for different sizes. However, in comparison to these thread repair tools, a thread file is much more cost-effective. Especially if you only occasionally need to touch up a component, the file offers a very low barrier to entry in terms of cost.
Even though the purchase price may be enticing for beginners, inexperienced users might encounter challenges when using a thread file. Working with the file is not as straightforward and requires some practice to achieve satisfactory results. Incorrect usage increases the risk of causing additional damage compared to using a thread tap. Additionally, the file requires considerable effort and time, especially for internal threads, to reconstruct the threads effectively.
Apart from post-processing threads, the thread file serves another purpose that may influence your decision. We've already seen how to find the side of the thread file that perfectly matches your thread. Of course, no one forces you to actually use the file afterward. This means that thread files can also be used to determine the pitch of a thread. If you need to do this frequently, there are faster and more reliable methods for determining the pitch. However, from time to time, a thread file can be useful in this regard as well.
Thread files are inexpensive, compact, and suitable for a wide range of thread sizes. However, their use can be laborious, and depending on the issue, they may provide less satisfactory results compared to alternative methods.
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Having a thread file in your toolbox for specific cases won't hurt. It's readily available for removing dirt from threads or filing down minor burrs. However, for severe damage or heavily stuck dirt and rust, other tools like thread taps or thread restorers are a better choice.
One of the most talented mechanics I know observed me struggling with damaged threads and dispensed an opinion: “Stripped thread repair is one of the most basic skills everyone should know how to deal with.” That’s not revelatory. However, when I pronounced the part I was working on junk, he made a complex thread repair that at the time, to me, bordered on art.
I still have the part. It hangs on the shop wall in a position of honor as an admonition that I have two ears and one mouth and I should use them in that ratio, but it also serves as a constant reminder of the importance of threads in the daily work of a mechanic. Think of how many miles of threaded items you’ve turned over the years, how critical their applications, how meager their maintenance. Threads (and the fasteners on which they reside) are literally holding the vehicles together.
Thread repair can be one of the most difficult tasks in all of auto repair. I’ve gotten much better at it with practice, but I’d like to present a few products and practices that have helped me. But first, let’s cover why there’s such a wide spread of skill levels on this type of work.
Thread repair is not typically within the realm of common repairs for an auto mechanic. I mean, obviously we all need to know how to handle it as my mentor stated, but it’s difficult knowledge to acquire. Factory manuals don’t cover it. Most technical schools don’t, either. Nor is thread creation and repair really the focus of most adult education courses like welding or machining.
Threading operations are often the domain of the tool and die maker, a dying trade in this country. It’s related to machining, but it’s focused on creating tooling in a toolroom rather than production. Unfortunately, thread repair, especially on an item that is in situ, can be especially difficult, and most machining texts don’t account for poor tool access.
Many mechanics are not well-versed in threading operations for the same reason they’re not proficient welders—this is more the domain of the fabricator than the repair mechanic. Don’t feel bad if this isn’t your strong suit. I’m not a trained machinist, though I practice making chips fly when I am able. Remember, if you can get the subassembly free from the vehicle, you’ll often be money-up to bring the workpiece to a professional. There’s no shame at all in knowing the limit of your capability.
Whenever a repair goes awry and the service manual no longer is in play, I generally try to use an escalation of force, if that makes any sense. So in a “damaged threads” scenario, I’ll attempt to first salvage existing threads, and only if that doesn’t work do I look into overboring and cutting operations. Sometimes you must move to a drastic final step right at the beginning. Remember, though, that if you make several attempts in the correct order, you have given yourself multiple chances to engineer a repair that works. Personally, I like them odds better.
Thread files are an excellent initial repair tool in some scenarios. They are, effectively, a file the shape of which mimics the correct thread pitch and angle. While I am sure there is some magician out there who can use these babies in female threads, I have found them helpful for male threads, specifically on large fasteners with damaged sections that aren’t within the leading threads. (On large fasteners, a die may not be easy or affordable to acquire.) I’ve had good luck “sneaking up” on the damage by getting the file to run in the good threads as it restores some of the damaged ones; that way I know the metal is being removed from the right spots.
If these are something you’d like to explore, you should know split dies exist, too, that work similarly. The file is a little more universal, though—usually a metric and SAE will take care of most fasteners you’ll see in common service.
Tap sockets are particularly helpful for avoiding that angular rotation issue mentioned earlier. They’re effectively a tap-holding implement that allows you to use standard square-drive tooling you already own to run ‘em in.
In addition to the benefit of the reduced swing, the tap socket allows you to use extensions, creating axial distance, too. Ironically, that ability can often mitigate the need for something that can operate with reduced tool swing.
You can buy individual sizes or sets. (You can also try your twelve-point sockets in a pinch. It works, but the 60-degree corners are obviously not the same as the 90s on your taps.) I picked up a set of modestly priced adjustables (just two cover most taps I own) a few years back and by now, they definitely don’t owe me any money.
Taps and dies are designed to cut threads, of course. To do so, they have sharp edges. Most common taps sold for general use are plug taps, which have a bit of a taper to them, as do corresponding dies. Thread restorers (sometimes casually referred to as thread chasers) are a bit different. The taps are almost like a blunt bottoming tap, in that there is little to no taper, and the threaded section isn’t sharp. The idea here isn’t to remove metal as standard taps and dies do, but to move the metal. These tools push the metal back where it came from; they re-form (restore!) the threads. A set is very handy in a service bay!
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