I called the man on Thursday and he hadn't ordered the #2 Plastite screws. He apologized that they had been moving their office and promised he would order them. He said he would probably sell them 100 for $3.50.
I asked him a number of questions that I had collected.
1. He verified that #3 sheet metal screws have 28 threads per inch (same as #2 Plastite). Last week he told me the diameter was .102 - .106 inch, compared to .089 for #2.
2. Sheet metal thread types: "A" is a coarser thread pitch with a pointed tip, used only on larger sizes, considered obsolete today but still made. When you go to the hardware store and there are two distinctly different thread pitches in the same bin, the coarser one is "A". "AB" is the finer thread pitch and the only pitch in smaller sizes, with a pointed tip. "B" is the finer thread pitch with a blunt tip.
3. He verified that they do stock the #3 X 5/16" sheet metal screws along with the 3/8" and 1/2" that are in the online catalog. He verified that the 5/16" is a "servo arm screw" with a blunt tip. It didn't occur to me at the time, but now I wonder if the "servo arm" application might mean it is not completely threaded, so part of the length serves as a shaft? He also said that he has recently bought #3 X 5/16" sheet metal screws with pointed tips because customers requested them - he said they use them in wood. But I understood that he still has the blunt ones in stock. The catalog number is SMPP0305B, 100 for $4.50. BTW, he said #3 sheet metal screws are harder to find than Plastite screws!
4. I asked him what is the proper hole size for Plastite screws and as I expected, it depends on the material. From his reference he gave me two hole sizes for each of three screw sizes. The two hole sizes define a range. The smaller hole is for "soft" material and the larger is for "brittle" material: #2 - .076" - .080", #6 - .122" - .128", #8 - .149" - .158" These dimensions are for the original hole and will allow the screw to cut (actually "roll") the best thread.
5. I asked him if he could recommend any good books about screws. He said there were lots of books but no good ones! He did recommend a magazine, "Fastening Technolgy", which he said recently had a 20 part series on screws for plastic! He mentioned another thread type used for plastic, which I recognized from the name: "high-low". I've seen these in equipment, they have two intertwined threads, one deeper than the other.