Assuming you're in the US and using unified thread then the correct standard is ASME B1.1
I'm not at work today so can't reference it but I suspect the size you want, if you can't adjust it to a standard size, will be a special thread. For a unified form this will be UNS designation.
Callout would be something like 1.95-20 UNS – 2A for the screw and 1.95-20 UNS – 2B for the nut.
If you don’t know the A or B designates male/female while the -2 is the tolerance class. There are other classes. For instance if it needs to allow for fine adjustment then class 3 may be appropriate.
My checker makes me add the major/minor and pitch diameter & tolerance underneath the callout for UNS threads. I’m not in 100% agreement with this but he has around 40 years more experience than me so I do it as he’s more likely to be right than I!
This website can calculate them for you, punch in the size you need in the UNS section and the results come up as a pop up window. The info you need will be at the very bottom of the pop up. Obviously note the disclaimer that anything you get off the net for free should be verified.
You should also reference the relevant thread standard on the drawing. While this is not exactly common practice it is recommended in the standards, I know because this question came up before and I looked it up, can’t remember where in the standards it is off the top of my head though. A note up with the standard drawing notes saying something like “ Threads in accordance with ASME B1.1” should do it.
As namdac implies if you just put “1.95x20” you’ll probably get what you want from most machine shops, but is ‘probably’ adequate?
A Drawing is effectively a legal document in the event of a dispute between you and a supplier. In this case just saying 1.95x20 may not be considered fully defining the thread form & tolerance so you may be out of luck in a court case.
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