most bolts use the top three or four threads to hold the load, strain, torque the fastener is trying to clamp. Studs are suppose to distribute the clamping load across all the threads, not the top two to four threads
Another thing to consider is the studs are threaded on both ends, course bottom threads like stock and fine threads above the part being clamp or held, which is providing more clamping surface per inch of threads than the course threads do
When I have found a head bolt(or any other bolt) that starts to stretch to much before reaching the torque values needed the bolt has stretch on the top two or three threads getting thinner there which is telling me it pulling apart at those threads
IHTHs
Close but not quite there.
Proper thread engagement for a high load would have thread contact that is 1-1/2 times the diameter of the bolt. A 1/2 inch bolt should have about 3/4 inch of threads in contact. Even at a 12 tpi pitch you'd have 8 threads engaged.
In theory, 8 threads will give you about 95% of the maximum strength on most conventional fasteners. From there on out you get in to a diminishing return. Each progressive thread becomes less beneficial to the point where you get zero return for adding more.
You'll notice that nuts are usually as tall as the threaded hole is wide. That gives you a proper amount of threads. The extra half a diameter in length of a threaded hole is done for cheap insurance as well as flexibility of installation.
If you're seeing damage or wear to a couple threads it is likely from improper installation, bottoming in a tapered threaded hole, or a damaged bolt.
Most common thread taps (taper taps) have 7 tapered threads on the bottom. Tapping a hole will result in the bottom 7 threads not having full depth. You can easily damage the threads if your fastener is too long.
There are also plug taps (3 tapered threads) and bottoming taps (no tapered threads) but milling threads in a CNC machine is very common and can result in threads stopping just before the bottom of the hole.
As for the strength of the studs, that comes from the installation. Studs are not cranked in hard like a head bolt. Instead, they get run into the holes snugly and left at that. When the nut gets torqued down it will tighten the stud but it's a bit easier on the block than bolts. There is much less wear on the block's threads, too.
