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Hagerty Employee

DIY: Understanding taps, dies and threads

Cross thread, damaged thread, need a new thread? All three are encountered often enough when working on a classic car or motorcycle. Female threads are cut using taps and male threads with dies, both of which can which can be bought in sets or individually.
Advanced Driver

Good article on threaded fasteners.
Keep in mind that many cars have a mixture of metric and Imperial, SAE, threads. It is real easy to mix up a 5/16-18 and a M8x1.25 bolt or stud.
I once had a Cadillac that had studs that were 5/16-18 on one end, and M8x1.25 on the other.
Metric threads are measured by pitch, but SAE threads are measured by the number of threads per inch.
It has been many years since I had to work with Whitworth threads, thank God. Among other oddities, the threads are called out by the size of the nut. That is, a Whitworth bolt that looks like a 5/16 bolt is actually called out as a 1/2 in bolt. English Fords of the 60s used both inch and Whitworth bolts.
Really old cars, teens and earlier, don't use unified threads. Thread pitches and profiles weren't standardized until the 1920s. The moral here is don't lose the fasteners.
Happy screwing.

My great uncle was a man born in the 1890’s. He saw the first flight news up to the moon landing in his life. Yeast as a Engineer he stressed to me how the SAE fastener standard was the greatest thing that ever happened. 

He said all these other great things would have delayed and in some cases prevented due to no standardization. 


These are the he tools that fix our mistakes. If you don’t have them you are going to need them at some point. 



   Few things in fabrication (at least to the fairly simple level of fab that I do) are more satisfying that creating usable threads where none existed before (or fixing those that were bungled beyond use).  I consider my taps and dies to be "most favored status" and would never lend them out.  I have been known to shed a tear when breaking a tap (and if you've never broken a tap, I submit that you haven't used yours enough),,, 😊

Intermediate Driver

I mainly use my taps and dies for cleaning up rusty and crusty bolts and nuts. I always try to clean up the old fasteners and reinstall them in my vintage cars. Something just more satisfying about that.

Yep.  A T&D set plus a wire wheel on one side of your bench grinder can do wonders on those old fasteners!  😊


"HSS are the highest quality and least prone to wear, but they are also pricey and more suited to occasional use." I suggest HSS are more suited to regular use.

Very cool article. Having done a tiny bit of this I can say it is quite satisfying.
Advanced Driver

I find it hard knowing (and finding) which bit to use for the pilot hole.

Also Kyle, " Cut in a few turns then unscrew half a turn to unclog the tap."
My high school shop teacher said to back off a quarter turn with every turn, to break the chips from the mother metal.
Advanced Driver

P.s. (is there an edit function for our comments?) I know there are lots of charts on the WWW which point out which bit to use for the pilot hole, but I've found them confusing at best.
Community Manager

Hey Kyle;

Since you start out this missive with; "Cross thread, damaged thread" I think it should be pointed out here that using a tap or die to repair threads is a bad idea. Or as our British friends might put it; "Bad Form". 🙂 Taps and dies are for cutting threads. Many, possibly most, bolts in industry are formed rather than cut . That means the metal of the bolt shank is moved around to create the threads, not cut. One removes the metal and the other simply moves the metal to create a thread. The same is true of female threads, in fewer cases. Formed threads are stronger and tend to last longer.
Now if you have a thread that just needs to be straightened, use a re-threader, rather than a tap or die. A quality re-threader will also tend to re-form the threads rather than remove metal. Re-threader sets (they tend to come with both threaders for both male and female threads) are fairly inexpensive and should be in everyone's tool box.
Now if you have a bunged up thread, but don't have a re-threader tap or die you can make one. In the case of a bad female thread, just take a new bolt, with good threads, and use a Dremel or similar cutting disc to cut lined along the length of the shank. This will create reliefs that give a place for gunk or loose material to go when you are re-threading a hold. Use some lubricant when re-threading just like when tapping. Any good tapping fluid will work here.
Got one that is particularly stubborn? You can always turn the bolt or re-threader into the hole till it gets tight then tap the head with a hammer, then turn a little more. The shock loading will tend to straighten more damaged threads and help them to re-form back into useful threads. Yes, it takes patience and time, but when done properly it will save threads and greater aggravation over time.
The same rules and techniques work on trying to straighten out male threads, if you can't find a good new bolt.
Now if the female threads are just too far gone, then it's time to break out the Threaded Coil insert as sold under several brand names, but I think that would be another great article at a different time.