What are "Undercut" head studs, and why would I want them??
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special machining
undercut on the left, standard on the right:
<img src="http://images.circletrack.com/enginetech/ctrp_0607_03_z+engine_fasteners+ARP_head_studs.jpg ">
The theory is that the undercut studs are stronger but I frankly dont think the difference matters.
FWIW I put undercut studs on my S6 and v8tt but wouldn't hesitate to have used regular studs either.
<img src="http://images.circletrack.com/enginetech/ctrp_0607_03_z+engine_fasteners+ARP_head_studs.jpg ">
The theory is that the undercut studs are stronger but I frankly dont think the difference matters.
FWIW I put undercut studs on my S6 and v8tt but wouldn't hesitate to have used regular studs either.
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The theory behind it...
is that with the regular cut stud (on the right) the highest area of stress is right at the end of the thread, where the smooth part of the stud starts. Ideally you want the stress spread across the whole stud evenly, which doesn't happen with the standard design.
With a proper undercut head stud, they are shaped through forging and are thinner in the stud area. Even after the threads are rolled, the stress is evenly distributed through the whole stud. Thus, even though they are thinner, they are stronger then a regular stud.
-Dave
With a proper undercut head stud, they are shaped through forging and are thinner in the stud area. Even after the threads are rolled, the stress is evenly distributed through the whole stud. Thus, even though they are thinner, they are stronger then a regular stud.
-Dave
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Well... close....
Problem with that is, that the weakest link will always be the weakest link. The theory more has to do with the stretch in the bolt than the overall strength. When you neck / undercut the bolt, you are making it weaker than before. This is desirable because it makes another part of the bolt appear stronger.
Short fasteners are very stiff and have very little stretch capability. If a sealing gasket loses its compression over time as it seats (or otherwise), the bolt loses its pre-load torque w/ respect to the fastener (clearly not a good sign...esp in a head gasket)
The undercut shank provides a better elastic region than another stud of same material without the undercut, but will not be able to withstand as high of an initial torque, dep on radius of necking.
To sum, the use of the undercut stud helps control part-to-gasket clamp force better than conventional studs. BUT - a stretch bolt designed to yield w/o undercut would be even more desirable... =)
Short fasteners are very stiff and have very little stretch capability. If a sealing gasket loses its compression over time as it seats (or otherwise), the bolt loses its pre-load torque w/ respect to the fastener (clearly not a good sign...esp in a head gasket)
The undercut shank provides a better elastic region than another stud of same material without the undercut, but will not be able to withstand as high of an initial torque, dep on radius of necking.
To sum, the use of the undercut stud helps control part-to-gasket clamp force better than conventional studs. BUT - a stretch bolt designed to yield w/o undercut would be even more desirable... =)
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