Specialty Racing Products has made it's reputation in the design and distribution of Titanium replacement bolts for bicycle specific applications. These bolts are made from Titanium 6-4, which is to say that the Titanium alloy used in these bolts is comprised of 90% Titanium, 6% Aluminum and 4% Vanadium. Since most bike parts are made from Titanium 3-2.5, not 6-4, you should know why. Fundamentally, though Ti 6-4 is used widely in the aerospace industry, it isn't commercially available in tubing form, but it is available as rod, or in billet and ingot form. Ti 6-4 is better suited to making bolts, because it has a higher tensile and yield strength than Ti 3-2.5.
Tests measuring the two alloys show their differences. In the annealed state, Ti 3-2.5 has an Ultimate Tensile Strength of 94,000 pounds per square inch, while Ti 6-4 runs up to 134,000 psi. Yield Strength Comparisons, in the annealed state, show Ti 3-2.5 will yield at 84,000 PSI, while Ti 6-4 runs all the way out to 126,000 PSI. Because of this, Ti 6-4 may not exhibit the modest type of flex required of bicycle parts for even the slightest shock dampening. Again, Ti 6-4 is not commercially available in tubing form.
The bolts are measured in millimeters from the top of the "shank", which is just below the head, to the bottom of the threads. The threads on all SRP bolts are cold rolled into the bolt, rather than cut. These are the reasons that a rolled thread is preferred. Cutting of threads actually leads to a weaker bolt, because more material is removed, to a deeper depth, than rolling threads into the side of the piece. In rolling threads, the material displaced from the trough portion of the thread, is pressure shaped into the thread peak.
Because of the sharpness of the tool used to cut a thread to the proper depth, the "base" of the thread is narrower, rolled threads produce a thread base that is broader, giving the thread more foundation to grip by. Lastly cutting threads produces burrs on the threaded material, that make installing the bolt more difficult. Bolts are described as have a thread and pitch. The thread is measured as the outer diameter of the threaded area, and the pitch is how far apart the peak of each thread is from the next. For example, an 8mm by 1.25 thread pitch bolt, has threads that measure outer-outer of 8 millimeters, the distance between each thread, on center 1.25 millimeters.