The screw thread gauge that I use most often is a thin strip of polished stainless. The strip is graduated on each edge, one edge being graduated in 1/32 inch intervals, while the other edge is graduated in
1mm intervals. For convenience, 1/16ths, 1/8ths, etc., are marked out on the inch side, and centimeters are marked out on the metric side.
It works dandy for any thread pitch less than 6", although you sometimes need a magnifying glass (our loupe) for finer threads.
For some reason, though, instead of being called a "universal thread gauge" it's called a "ruler".
What sort are you looking for? The kind with folding leaves and teeth or the ones with threaded holes? I have both sorts as well as the threading gage for setting up lathe tools. The one I have that I use the most is Starrett, it's triangular and goes up to like 84 tpi. I have a metric one I inherited, the screw plate sort I bought from Brownells back when they were much more reasonable in price. Also has the bastard threads they use for scope mounts on it, one reason I have it.
Just looked up the Starrett, it's #472 in the catalog, no idea of current price. Listed under "Screw Pitch" gauge, if you want to actually check if your cut thread will fit in a tapped hole, get the screw plates.
Two -- one metric, and one inch. There are inch ones available (mine is from Starrett) which have three groups of gauge leaves, and if that doesn't cover what you need, it will probably be a coarse enough thread to measure by a steel ruler -- and too coarse to cut on a home lathe.
Just get one which covers the maximum number of *reasonable* threads.
The Starrett No. 472 which covers from 4 TPI to 84 TPI (51 leaves).
You could add the No. 155, which covers from 28 TPI up to 2.25 TPI -- but -- why would you need to measure one that coarse, since you can't cut it at home anyway.
The Starrett No. 156M covers 28 Metric pitches, from 0.25 mm to
Perhaps you *could* add a No. 159M for threads from 1.00 mm to
11.5 mm, but you are not likely to need that for any threads which you can cut on a home lathe or with a reasonable sized tap or die. The 2.5 mm which is the coarsest of the previous one is about 10 TPI which is coarse enough for most purposes. The 11.5 mm thread pitch would translate to about 2.2 TPI.
Essentially -- *think* about what you will be capable of working with. (Check what range your lathe can thread for example. I think that the No. 472 and the No 156m should do more than you really will need.
My Harrison M300 does 2TPI as standard and it has been needed on one occasion when my neighbour needed to cut an internal twin start square thread for the nut on his dining room table jack as it had finally wore out after about 200 years. He couldn't do it as his Myford Super 7 doesn't go that coarse so he came around and did it at my house.
And how big is the Harrison M300? The original poster has to live with machines which he can take upstairs to his apartment -- by himself. My 12x24" Clausing can only manage 4 TPI maximum coarseness, and there is no way he would be able to get it up to his apartment.
And the thread pitch gauges which he is considering would not work for your square threads anyway. But the steel ruler which I mentioned above in my first still quoted paragraph *would* do just fine for determining the pitch of the 2 TPI thread.
How about something like the 27 TPI common for microphone stands? Pretty difficult to use with a rule graduated as you suggest. Even with 64ths graduations (which you don't mention) you wind up with
2.3704 64ths -- and it is pretty difficult to distinguish from a 1 mm thread (27 TPI winds up at 0.9407 mm).
Having exchanged a lot of e-mails with him, as well as posting exchanges here, I would be willing to bet that he is talking about a thread pitch gauge. A bunch of sawteeth folding out of a handle like feeler gauges do.
If the original needs replacing the crests probably aren't sharp enough to read that closely. I have an old transit with a 27TPI tripod mount thread that was quite difficult to measure due to its condition. You can hold the screw and sawtooth gage up to a light and easily see a slight mismatch between them.
The Harrison M300 is a 13" lathe and comes in IIRC 24" and 40" centre distances, mine is 40". You must be recalling information the OP mentioned in his past postings about his lathe size as in this thread gauge post he doesn't say anything about his lathe in his original posting, I only recall a reading some of his posts about some drill press he was trying to repair.