Are there any sources of square thread taps? I am looking for a 5/8"-6. I have tried Ebay and online. Another option is using Acme 5/8-8. Any leads would be appreciated.

McMaster Carr has Acme taps.

Hi John
Here are a few more you can try,
Tapco Cutting tools website, http://www.catalogtapotaps.com

MSC DIRECT http://www.mscdirect.com good luck, Pat

Just curious, are you sure you want a true square thread ?

They are very rare and hard to make. Normally you would use a special single point internal threading tool specially ground for a square thread. This would require a lathe with thread cutting gears.

Here is info on cutting square threads with a lathe (mostly external, but can do internal threads that way);

https://www.practicalmachinist.com/vb/g ... ead-94695/

Some old rifle barrels used a true square thread, but that was WWI vintage. They were replaced by the Acme thread or the modified 10 degree square thread a long time ago (50 plus years).

Most clamps, vises, etc. that look like a square thread are actually the modified 10 degree square thread.

I do have one old reference about taps that will cut a true square thread, but it takes a set of 5 taps that cut progressively deeper until a square thread results. And I do not believe you can even buy those sets of 5 taps anymore.

Might be easier to replace the thread and the nut with an Acme thread, almost as strong and you can get threaded rod and nuts already made for Acme threads.

Threaded rod and nuts with Acme threads in 5/8"-6 and 5/8"-8 are both available from McMaster Carr.

There is also a "Tandem Acme" tap out there (Try Victor Machinery). This is two taps on one shank, the first part does a "roughing cut" and then the second part (further up the shank) does a finishing cut.

You can always get someone to make a custom tap for just about any thread size you want, but the costs will make your eyes water fer sure...

Cheers, Kevin

The Tamdem tap Kevin mentions is really the one you want, especially for a thread of this size. What is your application?

I am duplicating the water stuffing box and valve spindle for a Sellers injector, which is a square thread. I was trying to avoid using an Acme thread which would give us a mix of square thread and Acme threads in our inventory. The square thread appears to have gone the way of telegraphed train orders. Apparently the square tap requires a special kind of grinder-no doubt old technology to grind the profile.
Acme or modified Acme is probably the direction we will have to go.
Thanks to everyone.
John