Quick ARP question

[Turbinepowered]

That doesn't involve asking about part numbers.

What's the difference? I see an option for "Hex Nuts," "12-point nuts," and "12-point nuts with undercut studs."

What's an undercut stud? And why is it better/worse than a non-undercut stud?

[Kudagra]

Fatigue failure is frequently caused by localized stress risers, such as sharp corners. In bolts, this would correspond to the notch effect associated with the thread form. It is well known that the maximum stress in an engaged bolt occurs in the last engaged thread. By removing the remaining, non-engaged threads, the local notch effect can be reduced. This leads to the standard configuration used in most ARP rod bolts: a reduced diameter shank and full engagement for the remaining threads. Providing a local fillet radius at the location of the maximum stress further reduces the local notch effect. Thus this configuration represents the optimum with respect to fatigue strength.

The reduced diameter shank is helpful in another sense. It reduces the bending stiffness of the bolt. Therefore, when the bolt bends due to deformation of the connecting rod, the bending stresses are reduced below what they would otherwise be. This further increases the fatigue resistance of the bolt. A typical bolt configuration is shown below.

That is from ARPs tech page.

[Turbinepowered]

Sweet deal. Also went digging through their catalog and found the actual term "undercut studs."

You will also note that ARP offers specially undercut studs for several engines. This procedure (done only to the shorter studs) more equalizes the “stretch” of both studs, which makes for a more consistent clamping force – one that compensates for head gasket compression when the head is installed.

So apparently "undercutting" makes the shorter stud act like an effectively longer stud. Neat deal.