A fine thread bolt or nut does NOT give you additional surface area to grip.
The cut depth of the bolt threads is less for a fine thread bolt than it is for
a course thread, but due to the number of threads the surface area is almost
identical. Fine threaded bolts are used most often in high clamping force
applications because it offers less drag when torgued therefore applying more
force to the item being clamped. A fine thread bolt is no less or no more
prone to vibration failure than a course thread. A fine thread bolt merely has
to turn a greater number of times to fall out. Either way it is easily
remedied with the use of a thread locking compund or a simple lock washer. A
grade 8 bolt could never be considered brittle. Grade 8 bolts are used in
automotive main and rod cap applications and are less likely to strech under
high torque than a lesser grade. By the way 9 out of 10 bolt and nut failures
are not due to the thread but to the shank of the bolt streching, weakening
then failing.

A grade 8 bolt is made from grade 8 steel and it is not a surface hardening.
The same is for a grade 5. A grade 3 bolt, a bolt without markings, can be
made of any material less than a grade 5. Some grade 3 bolts will fail under
only slight pressure while others will hold under greater.

If you don't know what you're talking about leave the matter to someone who
does.

Dennis
ASE Master Auto Technician and
Marine Engineer in training

But of course. Why else would there be so many different threads?

You want fine thread. The engagement, or amount of material 'gripping'
between bolt & stud, will be approx twice as much. Stronger and less
tendency to vibrate loose.

BTW hardened steel may not have been the best choice either. High
hardness usually corresponds to more brittle. It may just be surface
hardened which wouldn't make a huge difference.

Fresh Breezes- Doug King