993 Head studs, two types?
 

Hi There

Can anyone confirm that there are two types of head studs used on the 993 depending on how early the engine is? My car is a 1994 and has part threaded studs, but I heard that later cars have fully threaded head studs. Is this correct?

Many thanks

Berni

The original dilivar were 10mm threads with shaft that is 8mm, sealed black coating.
The later studs were 10mm threaded for the entire length, coated black and very expensive.
I always read nice things about the 993 late style.
Bruce

I think there are three types of 993 studs. There are the two dilivar types that Bruce mentions and then there are the plain steel ones. The steel studs are part number 993.101.172.02 and only $7 each here on Pelican.

I think the all thread ones are the 993 twin turbo studs?

Add to that list, the only stud using a modern day stud design. Designed and built with no cost constraints.
The Supertec head stud kits are available from our host.
Pelican Parts - Product Information: SPTC-HSK-1
Buy now and buy often:SmileWavy

BTW: we also produce a new version suitable for use in GT3, twin turbos and custom builds requiring a longer stud length.

When I did a rebuild of my engine Supertec was not an option. Why? I have read every thread here on Pelican about head studs and I am not convinced. Maybe it is the way the supplier write about the product. It is a litte bit "too much" for me.

I see that the Supertech design is very good but I still am in the differing thermal expansion rates camp. The Supertech steel is very strong I am sure but I do not think they have engineered the thermal expansion rate to more closely match Magnesium.

Hi

Here on PP the 993.101.172.02 are shown as being replaced by 993.101.172.03 but there is no price or availability listed. So now I am even more confused.

Ivath, which studies did you use in the end?

I may just clean up my old steel ones!

Tks

Berni

So Flieger as an engineer you must have calculated the actual expansion rate of the material involved in order to make your determination. So how much do they change?

What we a talking about is a temperature change of 100-120 degrees C randomly applied to multiple material including studs that are heated unevenly. Right?
So if the Dilivar studs change (grow) an appreciable amount (even under these small temperature range) and they are not heated evenly wouldn't that create an uneven clamping pressure on the heads?

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.
I have the Supertec studs in three of my engine builds (including a 450+hp turbo) and have never had an issue. Something I can't say about Dilivar.

I'd have to look up the Dilivar thermal expansion properties, but then it would be a simple calculation. [thermal expansion rate * temperature change * normal length] / [elastic modulus * area of the stud (pi*diameter^2/4)]. That will give you a change in force due to temperature, which you add to the static stud stretch force to get the total force.

You would just compare the change in force between steel and Dilivar. Anyone have the elastic modulus and thermal expansion coefficient for Dilivar?

That would be a first approximation, assuming there is no change in length allowed. Reallly, there is a significant change in length since the Aluminum and Magnesium heat up and expand, but those materials are not changing so each change in force should be adjusted by the same multiplier constant.

I believe steel expands about 1/2 as much as Aluminum and Dilivar is about halfway between steel and Aluminum.

I do agree that up to a point tighter is better than looser. Aluminum is ductile enough that it could yield the first time a stud is too tight, relieving the stress at the high temperature but then meaning too loose cold. Magnesium is not quite as ductile so could cause cracks. This is in the extreme situation.

Porsche would have done the calculations back in the day and determined it was within the material's strength. But then they did subsequently invent Dilivar.

The uneven cooling should not mean significantly uneven stress since both the Aluminum and the steel are exposed to the unequal cooling equally. (:)) It does still mean the cylinders go bannana shaped.

They did some interesting things on the 917 like the fiberglass jackets for the studs so that they would stay warm on the intake side.

Before Supertec (and maybe 993 factory) studs, the choices were steel, Dilivar, Raceware, and ARP. The Raceware guy had nasty things to say about ARP, claiming they used the wrong alloy and thus got the wrong expansion rate. But ARP employs engineers, too. Hard for us unwashed to know what to think.

I like the little extra features of the Supertec, like being a bit longer for more case thread engagement, and using finer threads up top. To get that with case through bolts I laid out some bucks to get the later throughbolts. Testimonials like Jim's didn't hurt when I was making up my mind and getting off the pot, so to speak, about head studs. If I recall rightly, Henry had engineering help with his materials selection.

Is it Steve or Henry who uses Supertech studs in everything except Magnesium cases?

When I last inquired, Steve had not used Supertech, but that was because he had used one (expensive) flavor of 993 stud and it had always worked on lots of high strung motors he built for customers. Engine builders can get conservative, because broken motors are bad for business.

Yeah, I'm in the expansion rate camp as well. But I've been wondering, what exactly lead Porsche to develop Dilivar? Sometimes the engineers over-think things and the field guys just make it work. So what if you use a high-strength alloy and the cylinders get distorted by a few microns when the engine is hot? There are plenty of other variables that affect engine longevity.

There is a LOT more to this that meets the eye. :)

Nothing was 'over-thought' at the factory on this since maintaining proper clamping pressure affects head sealing and cylinder concentricity at all temperatures. Its not a strength issue since after all, the steel ones are plenty strong.

The principle is maintaining consistent head torque under a very wide range of temperatures and IMHO, Dilavar excels at this, based on my personal experience. Naturally, this isn't as critical with low-compression, smaller bore, modest output engines, however large bore, turbocharged or high-compression motors have different requirements.

The modulus of Diliavar is likely to be on the 195-200GPa range as it is basically an iron based alloy.

Iron has a nominal Modulus of 207GPa and a 25%Cr/20%Ni Alloy such as HK40 is 198GPa so alloy content has a limited influence on this property.

I believe Supertec studs are 17-4PH which is a precipitation hardening Martensitic Stainless Steel.

The coefficient of expansion will be around 10.5mm x 10^-6/m/degK.

This is virtually identical to a conventional steel stud.

Dilavar appears to be an Austenitic alloy as they are non-magnetic and if they are precipitation hardening in nature the Coefficient of expansion is likely to be around 18-19mm x 10^-6m/m/degK and could be higher than this.

I think it is reasonable to assume that the stresses developed will be directly proportional to the DeltaT and the Coefficient of Expansion. I don't think the length has much influence.

The expansion of the cases is constant so conventional steel studs and any martensitic steel studs will produce higher stresses in threads than Dilavar.

The main disadvantage of the Dilavar type of alloy is that they can be sensitive to Stress Corrosion Cracking particularly in the presence of chlorides hence the resin coating.

I am also not sure if there is much evidence of pulled threads in Aluminium cased engines.

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

If the steel studs were used and while cold were given a preload that would overcome the peak cylinder pressure force then when hot do you think it would be possible that the preload would increase enough to locally yield the Aluminum or Magnesium of the case/threads? If so, then the preload when the engine became cold again would be too little and could cause the head to move or lift and combustion gasses to escape.

If the cold preload is set so that it would not yield the Aluminum when hot then when cold it may be too loose anyway.

Therefore, Dilivar may allow a more aggressive warm-up period?

The choice of head studs doesn't really affect warm-up,.....lubrication issues are the main concern. I caution our street & race clients NOT to run past 5K RPM until oil temps are at 185F and the thermostat(s) are completely open. Oils work much better at those temps as well.

I realize it was a poor choice of words. I meant "less worrying and babying". But I always keep the revs below 3500 when dead cold, as it starts to warm periodic instances of 4000rpm just before shifting.

I think I have Dilivars, fwiw.

You are a good man,....:) :)

Smart fellow,....:) :)