Shorten GT3 Head Stud for an engine rebuild?

[Elferhelfer]

Hello,
I have a question. Actually I’m rebuilding a 2.2 S engine and would like to upgrade the head studs. My preferred would be Dilavar ones....

Now, I heard you could use the Dilavar head studs from a 996 GT3 (PN 99610117270) and shorten them. The length of one is 185.5mm, M10 full threads like from the 993. I have access to machine tools, so a correct job could be done. It would be one of the last evolution stages of Porsche studs and a lot of engine rebuilder use this opportunity. Moreover it may be price attractive, you can get them for 12€ / 14$.... ( Unfortunately some people may have to add some additional taxes from today... )

It would be great, if somebody could give a feedback about this opportunity to shorten the GT3 ones.

[gled49]

I vote no on anything other than factory upper and lower steel head studs.

[raspy2point2]

I also vote for only factory steel studs. I have been reading a lot of posts on the Pelican site about after market studs getting loose after use and tricky thread procedures on bottoming out the studs in the case. Keep it simple. Chris

[chris_seven]

What to do really depends on the type cylinders your are planning to use.

If you are using new Nikasil Aluminium Alloy cylinders then the additional force generated by expansion may cause some problems in regard to the case threads.

Old magnesium cases can suffer from stress relaxation and this can cause the material around the studs to have weakened over time increasing the risk of failure.

Even if you are using Biral cylinders I would fit Timeserts to any Magnesium case as a precaution. (I know many people seem to use Case savers but I still prefer a Timesert)

If you use a Biral Cylinder then factory steel studs will work well and shouldn't create any issues.

There are two different Part Numbers available in the PET. 901.101.172.03 and 911.101.172.00 - both of these studs are steel and the 901 Part Number has the lowest price and is dimensionally identical to the 911 part.

If you buy steel studs I would only buy them from a Porsche dealer as the ‘class’ of thread is generally superior to any aftermarket stud.

Dilavar Studs are 993.101.170.51 and are coated in a resin to avoid the cracking problem of the original Dilavar 930 studs.

993.101.172.02are steel studs.

If you plan to use a Nikasil cylinder then the increase in the force trying to pull the stud out if the case will be significant and it may be worth considering Dilavar.

Metallurgically Dilavar is a type of Precipitation Hardening Austenitic Stainless Steel compared the more commonly used Martensitic or Ferritic Steel Studs used in the majority of engines.

Austenitic steels have a different crystal structure to Martensitic/Ferritic steels and this difference in crystal structure results in a greater Coefficient of Expansion.

Typical Values would be

Martensitic/Ferritic Steels - 11-12 x 10^-6 per degK

Austenitic Steels - 16-18 x 10^-6 per degK

A Dilavar stud would tend to exhibit similar levels of expansion to the High Silicon Alloys used for the manufacture of Nikasil and Alusil Cylinders hence their ability to control the 'pull out' forces.

If we consider these basic details there are some simple conclusions.

A ‘901’ factory steel stud (around $8.00 each) will work well with a Biral Cylinder .

Using a modern Nikasil replacement cylinder will increase the risk of threads pulling out of old cases and I would be inclined to consider using Dilavar studs.

I would have to say that if I were offered ‘Dilavar’ studs at $15.00 each I would be very concerned that they weren’t ‘genuine’ and would probably not buy them.

Austenitic Alloys are always Non-Magnetic so Dilavar studs should never exhibit any magnetism.

There are aftermarket ‘Dilavar’ studs that claim to be Magnetic, which is, from a metallurgical standpoint, a very unsound and one which I would never believe. If they are magnetic they aren’t Dilavar.

I would generally avoid studs which are described in this manner as any magnetic steel will have the lower expansion levels of a Martensitic/Ferritic steel so why take a risk with a material description that is so inaccurate.

It is also possible to buy studs manufactured from A286 which is an Austenitic Stainless Steel and will have high expansion. Studs of this type are around $14 each.

A286 can also be prone to Stress Corrosion Cracking in the presence of Chlorides – hence the reason they are advertised as being ‘coated’.

A286 is commonly used for fasteners in the Hot-Section of Gas Turbines. I am surprised that they can be effectively produced at this very low price and again I would be conservative and not be keen to use them in a customer engine.

Some of the other Aftermarket studs being sold have an increased ‘shank’ diameter when compared to a standard ‘901’ design of stud.

An increase in the resulting cross sectional area is commonly around 80% which will generate a very significant increase in the force trying to pull out studs when the cylinders expand

As a basic engineering concept increasing the stiffness of a fastener will always increase the forces generated by expansion and increases of this stiffness should be avoided in critical situations.

I would never use studs of this type on early magnesium engines using Nikasil cylinders as they must increase risk of case thread deformation and even very low levels of deformation will result in a loss of clamping force.

We have been making our own studs in a Titanium Alloy (170 ksi) for about 4 years now. We use them in Mag Engines with Nikasil cylinders and the low modulus of Titanium helps to reduce the forces due to expansion and so far they seem to work well.

We make them with a ‘dog’ point so they can be tightened into the case and evenly load the threads.


We use an M10 x 1.25 12 point flange nut and make the washers for both early and late heads in 17/4PH Stainless Steel

[Elferhelfer]

Hello together, thanks a lot for your feedback. I wasn’t talking really about an aftermarket head stud. I mentioned an OEM from Porsche, especially the ones for the GT3 of a 996/997. They are also Dilavar and epoxy coated, but they are longer than the 993 ones. They cost around 14 USD… I have some in the garage and will post some pictures tonight.

[chris_seven]

Have you checked them with a magnet?

They just seem to be too low in price compared to a 993 Dilavar Stud.

If they are Dilavar then you may be able to use them without shortening.

We use a 12 point open nut and the studs we make are 175mm long and there is plenty of space above them for a longer stud.

You could use a K Nut as an alternative and just add the 'running torque' to the preload figure.