The reason for offering replacement studs stems from the fact that our testing found that no available head stud was compatible with our Nickies™ or any aluminum cylinders for that matter, with exception of the 911 models equipped with aluminum cylinders from the factory. In most cases, the factory head studs are unable to perform satisfactorily under the increased expansion of an aluminum cylinder, in most cases leading to head leaks and fastener fatigue.

On 911 models, certain engines came fitted with Dilivar head studs, which were designed to match the expansion of the cylinders, but also suffered from hydrogen embrittlement, which is a condition that results from the accumulation of hydrogen gas in the atomic structure of the metal. This gas flows to the point of high stress (stress risers) and causes microscopic cracks. The hydrogen then flows to the “new” crack tip and causes it to crack further. In this way the crack moves across the part, because the crack-tip IS the stress riser. Finally the crack gets so large that the section is not large enough to support the load. Hydrogen embrittlement can take place without tensile stress. ARP® employs a baking process that purges hydrogen gas from the steel, eliminating this problem.