Star Pressure Screw Nut Thread Size

[garandsrus]

Hi,

I am making a replacement Pressure Screw Nut so that I can directly add an air tool fitting to the nut. I have already bored and threaded the top of the nut for the air fitting.

What internal dimension should I bore the bottom of the nut (attaches to the sizer) to prior to internal threading it? The ID of the nut, measured with a caliper is 1.197. Measuring the OD of the threads on the cylinder of the sizer gives me a dimension of 1.236.

I need to allow for the internal threads in the cap, so I am guessing that I should bore the hole to 1.197 and then thread it. The threads would be about .020 deep, which looks about right. These are very shallow threads.

Is the thread for the Pressure Screw Nut a standard size, or something custom? Is this a 1.250 nominal thread? I think it must be custom (which isn't a surprise with the Star) as the standard thread pitch for a 1.250 nominal size is 7 (coarse) or 12 (fine). I measured the Star threads as 20 TPI. Can someone confirm this?

I have never cut internal threads before, so this should be fun :)

Thanks,
John

[deltaenterprizes]

You answered your own question, 1.197.

[manomet]

There is a great post in casting equipment that creates the whole air feed system with a different approach, I would suggest you look at that first, sounds alot simpler.