I am sure that everyone is aware of the arguments surrounding the use of Dilavar and other aftermarket studs.

There are also arguments about Timeserts versus Case Savers and the debate just keeps on going around and around.

Standard steel studs seem to be unpopular, Dilavar has a bad reputation for failure and the other studs available are all materials that are more at home in the Hot Section of Fighter Aircraft Engines than in basic IC engines.

It seems that the common opinion is that the more exotic, hence expensive, the better the stud.

I have been concerned for some time that many of the ‘high end’ studs that are strongly promoted increase clamping forces when a Nikasil cylinder expands and do cause a significant increase in clamping force and increase the risk of pulling studs out of Magnesium cases.

There also seems an opinion that the extra clamping produced by cylinder expansion can be a good thing and prevents head leakage.

I don't agree with this idea, the head needs to be clamped correctly when cold so they seal effectively. The clamping doesn't need to be increased as the engine heats up as combustion pressures don’t change much from hot to cold.

To try to come to a sensible conclusion some basic estimates to see what increase in preload is developed by some of the different studs available seems a reasonable idea.

The properties of the stud that influence pull out forces are quite straightforward and easily summarised:

a. The difference in the Coefficient of Expansion between the stud and the cylinder.
b. The Elastic Modulus of the material used for the manufacture of the stud
c. The cross-sectional area of the stud

To keep the calculations simple I assumed that the cylinder and head are infinitely stiff (not quite true but a reasonable first approximation).

This assumption means that the numbers produced shouldn’t be used for any real modelling but are valid to rank the various studs in terms of their potential to damage a Magnesium Engine Case.

I have also assumed a Delta T of 120 degK

Standard Steel studs have a shank diameter of 7mm. The Elastic Modulus of the steel used for this type of stud is around 205GPa and the Coefficient of Expansion is approximately 10.8 ppm/degK.

This produces an increase in clamping force of around 2500 lbs per stud.

Early Dilavar studs also have a shank diameter of 7mm, the Elastic Modulus of Dilavar is around 195GPa and the expansion is 18ppm/degK. This results in an increase in clamping force of just less than 500 lbs which is much safer than the steel stud.

Aftermarket studs vary in material between Inconel 718 and 17-4PH. These steels have similar expansion to a conventional steel and an Elastic Modulus which is around 198GPa.

Many of these studs have an larger diameter – 9.2mm is typical- This increase is always justified by a claim of improved performance but seems very similar to the core diameter needed for thread rolling which also has a strong impact on the cost of manufacture due to a reduction in machining cost.

A large diameter stud of 9.0mm would see the pull out force increase to around 4200lbs – a 65% increase from the steel stud and a 8-fold increase from Dilavar.

I am not sure this type of stud is a great idea for early and increasingly difficult to replace Mag cases even when fitted with Time Serts, Big Serts or Case Savers – although these parts will help.

I don’t think I would install this type of stud in a 1969 911S case as they are becoming too hard to find.

The high clamping force will also have a detrimental effect on the long term performance of the engine case as the increased clamping force will tend to promote stress relaxation in the case which will reduce the effective strength of the threads in the magnesium.

Dilavar seems to have, at least in some circles, become unpopular due to cracking issues. This seems to have been solved and the current 993 studs don’t seem to fail but at a cost of around $60 each ($1600 per engine set by the time you add nuts and washers) they should be good.

There are two alternative strategies.

The first is to use a stud with a smaller cross-sectional area. This will reduce the influence of expansion on the increase in pull out force. It will weaken the stud BUT on early Mag engines we only use 24lbsft so why do we need materials with a strength of 200ksi.

Using a 5mm diameter shank would reduce the increase in pull out force to around 1100lbs – still more than Dilavar but much less than either standard studs or the larger diameter ‘special’ studs.

The other way is to use a material with a lower Elastic Modulus.

Titanium Alloys have a Modulus of around 115GPa which is around 50% of the steel used for the standard stud.

A 7mm diameter Ti Stud will generate an increased pull out force of about1200lbs, still a sensible figure.

A suitable Ti alloy has a tensile strength of around 180ksi which is more than adequate and is available in grades that will function well even in Salt Water environments and would be resistant to crevice corrosion.

By using a nut made from a different material the affect of galling that can occur with Ti fasteners can also be eliminated.

As we are currently building a 1969 S motor for an FIA spec rally car we have decided to make a few sets of Ti studs and test out how they perform. The cost should be much lower than 993 Dilavar studs. They should be ready early in the New Year.