I just pulled this stud out of my HiPo head.
The stud does not have normal threads (see tap).
What thread style is that?
I'm sure there is a proper and specific technical engineering name affixed to this thread profile, that is with the square minor or root profile, but as I stated previously, most of the O.E.M. examples are as such, an "interference" thread. Also one may encounter a similar thread profile coupled to a triangular shaped O.D. of the stud's diameter.
In this application I feel there are a couple of intended benefits for this interference thread; first the obvious value of fastener retention as discussed previously, but also since the cylinder head is tapped for a different thread profile the softer cast iron material is then displaced as it engages the stud, this not only providing the mechanical locking feature, but also disruption of the clearance between the two thread surfaces which would otherwise act as a fluid channel (leak). Oh, and since the minor diameter of the stud is increased, a probable increase in the stud strength as most often the fasteners failure begins in the root of the thread.
So yes, one should run a "thread forming" tap thru the holes before attempting to run the yes "rolled" but different thread of say an ARP fastener in.
Also for consideration is if in the operation of the placement of the O.E.s interference thread intention, with the require process of displacement of material accomplished, what effect is presented on the turning torque in order to acquire the same fastener preload sum, and that effect on the as specified by the O.E. torque specification, when installing a non-interference thread fastener such as the ARP, with something on the threads?
Think about it!
Scott.