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Studs that are broached to recieve an allen key are easily removed to allow for head removal where bolts would otherwise be the only practical solutiom. In the pix, you can see this. The reduced head nuts provide for clearance when needed, otherwise, hex nuts are available.
Stretch bolts have two advantages, cheap and self limiting preload. The disadvantages are variability in yield strength. When torquing these by hand, one can feel ones that are sustandard. When using automated torquing equipment, such will not be detected.
With a non-stretch bolt, the clamping pre-load is determined by the torque procedure, not the yield strength of the material. With these, torque + angle cannot be used without extensive studies, as the elasticity of the head and compressability of the head gasket affect the outcome. So for one-off applications, these should be simply torqued. ARP offers suggested torque to get 75% of yield stress with W30 oil or their own moly assembly oil. Examle for 12mm stud, 97 ft-lb for W30, 77 with their moly lube. Those levels might more than one would want for an alloy head vs iron. And 12mm studs are very heavy for a gas engine. The studs are course threads for the block and fine pitch for the nut end. The studs are put in finger tight, not bottomed out. The nut should be run to torque 5 times to burnish the threads and then final tightened. This normalizes the thread friction which determines the final fastener preload.
A stud or bolt that has a higher preload will better maintain force if the HG settles. A stretch bolt will have lower preloads. Preloads are better controlled with a torque wrench and careful hand assembly then stretch bolts. Stretch bolts preloads are not controllable. I do like your 80 ft-lb rejection critera! A fastern that is undercut, with a shank that is narrower than the thread diameter, will stretch (elongate elastically - no yield-) more. Now, stretch bolts are a very bad term, as these are torque to yield bolts. So an under cut faster, will have more elastic stretch. The material must be stronger and the stresses higher than the same fastern with a full section shank. This means that the material must be stronger and that increases cost. Undercutting a fastener shank is also an added operation that increases costs.
Studs are more practical to manufacture than bolts. Hot heading a bolt head can be quite expensive. Studs can be cenerless ground to ensure perfect geometry. You can't readily do that with bolts.
If you were to use a non-yield stud in place of a stretch bolt, if the nut end threads are finer, as they often are, then the same T+T+angle method would be way off base as the thread pitch is so different. And the amount of turn takeup with a non-torque-to-yield bolt could create preloads that are too high.
posted by 68.93.19...
, Sat, 16 Oct 2004 13:32:00, Sat, 16 Oct 2004 17:04:20, Sun, 17 Oct 2004 05:13:19
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