[chris_seven]

There are a number of different issues at stake here and from a design point of view some of the mistakes are understandable.

If you consider the basic design of an Aluminium Sand Cast Engine case and the standard steel stud then there are very few issues.

The design is robust, has a good service factor and engines with Biral Cylinders and steel studs have very few problems.

If we then move to Magnesium engines with Biral cylinders even the 2.4S has few issues and has worked well.

The 2.7 with Thermal Reactors started to be an issue as we had high forces generated by expansion combined with a material that was losing strength with age due to the temperature of the thermal reactor and its impact on the stress relaxation of the magnesium Alloy. This leads to studs pulling out of cases.large the engine beyond its basic capability whilst trying to reduce emissions.- just a step too far I am afraid.

As far as I know Dilavar was first used on Turbo Engines where peak cylinder pressures were higher and maybe expansion was greater due to higher temperatures but this is a guess on my part. It is surprising that Dilavar was selected with a Alusil Turbo case and again the mistake is probably due to a rushed effort to react to the thread 'pulling' of mag engines.

It is not surprising that this material was initially considered as a solution for the problem of expansion.

Dilavar almost certainly will reduce the forces generated by expansion but sadly Porsche did fail to recognise its susceptibility to stress corrosion cracking in the presence of chlorides.

Is this a surprise? We must remember that although the Academic World had some evidence of this type of mechanism back in the Sixites the first commonly available published guides began to see the light of day in the early Eighties following a significant amount of research and testing, particularly in the case of Austenitic materials.

In fact very few Undergraduate Metallurgy Courses were not teaching Fracture Mechanics until the mid to late Seventies and then only as an optional part of a course.

If we add that the fact that Germany/Switzerland and Austria apply salt to their roads in a very sparing fashion it is not surprising that this fact was simply overlooked.

The Northern States of the USA, however, use vast quantities of salt which has not only helped to break Dilavar head studs but has also destroyed huge numbers of road bridge decks.

It is an irritating problem but and it's solution, with hindsight, is straightforward particularly with an Aluminium case.

Dilavar studs are now coated and should last many years even in poor environments as the salt will not penetrate the coating. I would also say that standard steel studs in Aluminium cases seem to work well and are generally trouble free.

The comment about using SAE 1020 is interesting and although I agree in principle that many of the exotic materials being used are not really justified some basic calculation will show that this type of steel isn't up to the job.

The force applied to a typical steel stud fitted on a engine using a Nikasil cylinder is around 6500 lbs.

This works out to a stress of around 580MPa (84 000psi)

Sadly the tensile strength of SAE 1020 is only around 400MPa (58 000psi) as a guaranteed minimum.

It may just about survive being torqued to the initial preload but it just wouldn't manage to deal with the expansion of the cylinder.

If you increase the shank diameter of the stud to try to deal with the tensile stress you would be fighting a losing battle as the force due to expansion would increase in scale with the area and be much more likely to pull out of a magnesium case.

The Aluminium cases seem able to withstand the increased expansion forces of a larger diameter shank but I don't really see the benefit.

I would like to see a stud made from a material with a basic yield stress of at least 750MPa (110 000 psi)