Counting and fixing external threads

Thursday, February 3, 2011

For quick shop work, the thread files are the best tool when it comes to counting or repairing external threads

by KEITH BERGLIND

Thread files are a handy dual-purpose tool around any repair job. They come in imperial and metric thread and are not very expensive. I keep one of each in each of my workshops. The most common tool source is the K-D tool display you see in many hardware and auto parts stores.

Of course, the main intent of a thread file is to file damaged external threads. That's an easy and obvious job. Each square file has four different thread sections on each end, so one file will cover a wide range of eight thread pitches.

To use, you just keep trying the short sections, one by one, until you find the matching file section. Then just file away.

Of course, these only work on external threads such as bolts and shafts. There are many machinery applications where a large threaded shaft becomes damaged while removing a bearing. You can buy a new nut, but the shaft is expensive and not easy to change. So get out the thread file and go to work. With a little practice, you will develop the skill to push the file sideways to straighten threads that had been pulled over by a bearing puller.

Experienced mechanics often use mechanics blue dye to identify high spots. Put a small amount on the work and try the new nut. Tight spots will show up shiny, showing where you need to file. The same mechanics blue may help identify any two matching surfaces that are interfering, such as a ball bearing on a shaft.

These thread files are only for external threads. Internal threads may be repaired by thread chasers or threading taps. For large threads, larger than the standard tap and die set, there are large universal thread chasers that go over the part you are fixing. Thin v-shaped blades are turned into the base of the best threads. Then, as you turn it out, the damaged threads are re-cut.

These tools are often used on jobs like the front hubs of four-wheel drive trucks, where the hub is over two inches and has fine threads.

The most common job I use my thread files for is counting threads. All too often, we encounter metric threads without realizing it. And, since it is common for metric bolts to have odd-spaced threads (fine, medium and coarse), it pays to know for sure what the thread pitch is.

When in doubt, I just grab the metric thread file and do a quick check. This is why I keep a set of thread files in each shop. And I always end up buying the right nuts and bolts.

I also have many thread gauge packs, but I find for quick shop work that the thread files are the best tool. Of course, the proper machinists thread gauges will work on internal threads, such as nuts.

My favourite thread file job is for where I used a hammer on the end of a threaded shaft, such as an alternator pulley or a water pump hub, usually after I thought one more light tap would get things apart, and I ended up with a mushroomed shaft end. I have thread-filed myself out of many such self-created situations. BF