DieselPower said:
It's not only Metric that has more than two sizes. There are four in SAE. Coarse, Fine, Extra Fine and National Special. I also on rare occasions when working on industrial type manufacturing machinery (usually import equipment) run across a whole other type thread, British Whitworth and oh let's not forget Acme thread.
Here's a example of how screwed up the US thread system is. Let's use a 3/8 as the example of all the available thread pitch's.
3/8 - 16 Coarse
3/8 - 24 Fine
3/8 - 32 Extra Fine
3/8 - 18, 20, 27, 28, 40, 48 National Special
3/8 - 12 Acme
Of course, let's not forget NPT (National Pipe Thread) that are the tapered pipe threads. According to my 21st edition of Machinery's Handbook, we have the following pipe threads:
NPT - American National Standard Taper Pipe Thread
NPTR - American National Standard Taper Pipe Thread for Railing Joints
NPSC - American National Standard Straight Pipe Thread for Couplings
NPSM - American National Standard Straight Pipe Thread for Free-fitting Mechanical Joints
NPSL - American National Standard Straight Pipe Thread for Loose-fitting Mechanical Joints with Lock-nuts
NPSH - American National Standard Straight Pipe Thread for Hose Couplings
Then there is:
NPTF - American National Standard Dryseal Taper Pipe Thread for Pressure-tight Joints
...and the variations of:
NPTF-SAE Short - Dryseal SAE Short Taper Pipe Thread
NPSF - Dryseal ANSI Standard Fuel Internal Straight Pipe Thread
NPSI - Dryseal ANSI Standard Intermediate Internal Straight Pipe Thread
Let's not forget square threads, which are similar to ACME; but aren't.
To paraphrase Mr. Spock from the episode "I Mudd", "There is a coarse thread, a fine thread, an extra fine thread, a taper thread, a dryseal thread, a whole plethora of threads; but only one ACME thread."
I wonder how many of these thread types are still in existence. Does anyone have the latest (27th I believe) edition of Machinery's Handbook to verify?