"Drill Rod"
These pins are called drill rod because they are offered in the same sizes as the drills in your 1-60 (and larger) drill index, using the AWG (American Wire Gage) nomenclature.
Many people think "drill" and immediately equate that with hard and brittle - both of which are wrong in this case. The yield point of the O-1 rod is 50 ksi - not much more than A36 mild steel, and the ductility is adequate as referenced by the low Rockwell hardness rating. These pins are quite a bit "softer" than those that come with the hinges, but with careful use can provide easier installation because of their small reduction in diameter as the number size increases.
I have used drill rod for my cowling installation for more than 25 years and 3000+ hours and have not had one break. YMMV!
Distributors typically don't tell you this, but drill rod is (usually) made in 12' lengths and then cut into shorter sections for retail sale. McMaster-Carr sells them in 3' lengths, but as a special order, will cut them any length you want - up to 12' (or at least they did the last time I ordered some). For my Rocket I needed 4 footers, and just for the heck of it I got some 6 footers.
Drill rod can be heat treated to a "tougher" condition, but careful treating processes are necessary to insure straightness after treatment. It has to do with the "dunking" into the cooling solution - lowered vertically at a controlled rate, you get a straight rod. Lowered horizontally, or worse yet, haphazardly dropped into the solution without regard for orientation, and you get a waviness that looks like a ruffle potato chip - ask me how I know! After my failure at having them treated I reverted back to using them as delivered, taking care as to how they are installed and removed.