U.S. Coarse Thread Size

Machine screws and bolts come in lots of sizes and with a different number of threads per inch.

Example in a small machine screw:

#10-24 x 1 1/2"
(The #10 is the size, the 24 is the number of threads per inch, the 1 1/2' is the length)

#10-32 x 1 1/2"
(The #10 is the size, the 32 is the number of threads per inch, the 1 1/2' is the length)

The 32 threads pre inch is a smaller thread and called "fine"
The 24 threads per inch is a larger thread and called "course"

I do believe in this case the guest is refering to a 1-1/2" diameter bolt with 6 threads per inch. This particular fastener is commonly denoted as "1-1/2-6"

Not quite. The correct designation for a bolt with this particular thread is

1-1/2-6 UNC-2A and for the corresponding nut or tapped hole 1-1/2-6 UNC-2B. The UNC denotes Unified National Coarse and the 2A and 2B indicates class of thread fit. The class of thread fit defines the dimensional allowance between mating threads.

Standard U.S. Threads and thread classes are covered by ANSI Standard B1.1.

Screw thread chart: http://www.engineersedge.com/screw_threads_chart.htm

And general info: http://www.efunda.com/designstandards/screws/screwun.cfm

http://homepages.tesco.net/~A10bsa/bswgo.htm