This is what I did to a broken BSW tap to use it for my backplate threads. I wanted 8 TPI BSW x 1.75", this started off as 1" x 8 BSW tap. If you could get a scrap 6 TPI tap with just one good tooth it would make a useable tool.

Martin C

If you go down the tapping route you will need to screw cut it first so why not just make a single point internal screw cutting tool. I'm pretty sure you will not get a screwcutting tip for your S1 2M- SCLCR09 boring bar

Tony

It’s nigh on impossible to cut this size thread in steel with a tap without first screw cutting a close impression of the thread.

i cut lengths of alloy studding this size and run a die down to clean it up after screw cutting and do it in the lathe and the motor (3hp) at 50 rpm is close to stalling and using all my strength to hold on the die holder

As already mentioned, a tap would be quite usable, but at that size, would be very difficult to turn. I bought a much smaller 3/4 x 5 ACME tap from Tracy Tools, it had a very long taper, and I tried it out in an aluminium test piece. I found it so hard to turn that I single pointed the actual nuts using an 8tpi ACME running at the 5 tpi and took out as much metal as possible from the gunmetal. The threads were about 3/4 finished before tapping which was still hard, but doable.

I think I would prefer to tap cast iron than steel using this method.

John, Good luck with it, just to clarify my earlier post, on the assumption you would still screw cut the thread first, you could then bring it to size using a tap, which I can lend you.

Chris Gunn