Quote:
Originally Posted by Lapkritis
My guess: The middle cylinders will be hottest sandwiched between the other two... more heat=more thermal expansion and stress on the fastener. The cams on these engines must have bearing wear like crazy with this design.
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If the studs are Dilavar then the increase in stress caused by expansion will be small and unlikley to be the cause of failure.
The stress in a Dilavar stud is about 45ksi even when peak cylinder pressure is taken into account.
The standard steel studs and the Martensitic Stainless Steel studs will develop higher stresses - which is due to differences in expansion- but only to around 75ksi.
None of these stresses are likely to cause stud failure unless macroscopic defects are present and the failures don't indicate this type of problem.
It is much more likely that failure is due to Stress Corrosion Cracking and precipitation hardening Austenitic Stainless Steels such as Dilavar can be prone to this mechanism, particulalry in the presence of Chlorides, when strengthened to levels greater than 160ksi.
The addition of high levels of Molybdenum used in Dilavar should be helpful in eliminating this problem but avoiding certain temperature ranges in the heat treatment schedule is likely to be more important, and perhaps not well controlled.
I am aware that this expanation is not widely accepted but the levels of stress developed in head studs is just too low without an additional failure mechanism being present.
