Posted by JaSAn on October 05, 2000 at 11:08:20:
In Reply to: Re: Head Bolts, Grade 8 replacements? posted by Rob Steller on October 04, 2000 at 23:32:17:
There are two reasons for reducing the diameter of
a bolt shank. One reason is to
make the shank the weak point of the bolt rather than
the threads. This makes it more likely that the
shank will shear if the strength of the bolt is exceeded.
The assumption here is that the shank will be sticking
out of the threaded hole and therefore easier to get
hold of. The reduced diameter is smaller than the minor (smallest)
diameter of the threads. This usually works well on bolts in
good condition, but weakness caused by cutting threads, material
weakness, rust, heat cycling can still cause bolts to break at
the threads.
The other reason is that in tightening, a bolt will
stretch. Most of this stretching will occur at the
weakest (smallest diameter) point of the bolt. With
this being the shank, the threads do not stretch as
much leaving more threads holding the load.
can ca;u