Quote:
Originally Posted by Turbo_pro
?All of this theory is an interesting exercise but most of it is just much to do about nothing. I prefer a static expansion that increases the head clamping pressure. Of course there is a limit but I have seen the Supertec studs in 800hp engines running nitrous and the heads didn't show signs of movement when disassembled for inspection. Dilivar studs show movement in NA street engines.
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Head studs need to have enough preload to overcome stresses created by the peak cylinder pressure otherwise they may suffer from fatigue failure and this would give rise to significant failures.
The clamping force needed would generally be calculated using room temperature data. It is unlikely that the design of the joint would use the increase in preload caused by expansion.
It is very unlikely that peak cylinder pressures cause the head to lift if it had been correctly fitted as this would have a very damaging influence on performance and reliability.
If the stud, howver, has too much preload and peak cylinder pressures are too high then there is the likelyhood that it can and will pull out threads.
If there is head shuffling it is much more likely that there has been a loss of preload from the bolt due to some visco-elastic behaviour of the magnesium or aluminium castings.
Aluminium/Silicon alloys are notoriously poor in this respect especially at temperatures in excess of 220 degC where creep can occur in very short timescales and Magnesium is even worse.
There is reasonably good evidence to show that prestress reduction of up to 50% can occur within a few weeks due to either hot spots or casting variation.
There is a very good paper on this subject:
Chen, F. C., Jones, J. W., McGinn, T. A., Kearns, J. E., Nielsen, A. J., and
Allison, J. E., 1997, “Bolt-Load Retention and Creep of Die-Cast Magnesium
Alloys,” Characteristic and Applications of Magnesium in Automotive Design,
SAE International Congress & Exposition, Detroit, pp. 13–21