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,....:) :)

The preload has to be high enough to overcome peak cylinder pressure or the stud would have to be fatigue rated rather than designed for a static load.

I think it is unlikely that the increase in stress caused by expansion will cause yielding in a conventional sense.

The stresses caused by peak cylinder pressure does, I am sure cause studs to pull out on magnesium cases.

The traditional technique of trying to solve problems by overtightening fasteners will make this more likley.

I am not sure that using a much higher strength stud and therefore increasing the static load is a great idea as this will also mean that the sueprimposed cylic stresses will be much more likely to damage the case and cause stress relaxation which will loosen the bolt.

Whether this chould be called yielding or creep is arguable but it is certainly inelastic.

I would have thought that the 'best' engineering approach would be to use the minimum load needed to avoid fatigue but then the issue would be trying to either estimate or measure peak cylinder pressure which will vary significantly between atmospheric and Turbo charged motors.

The original design was for a 2.0 litre motor which even in race trim was about 200HP.

Now we have 700HP+ 3.8 litre engines so is it surprising there can be issues. IMHO the problem is really with the case not the stud accepting that early uncoated Dilavar studs could suffer from SCC problems and break for no apparent reason.

I am sure case savers which spread loads and reduce local stresses in the case will mean that unloading of the studs is less likely and the engine should be more secure.

Things are complicated, too, by the inaccuracies in measuring bolt preload with torque rather than stretch.

The Supertec stud has far more than just a superior (debatable for sure) material.
The fine thread 12 point nut is a far superior design both in material and functionality. No designer of high performance hardware calls out coarse thread unless threading into soft material. Rarely/never with a nut.
The extra thread length for more thread engagement in the case. Thread engagement to resist pulling
Spark plug clearance for twin plug application. Barrel nuts used from 64-89 will interfere with the spark plug in an twin plug conversion.
Corrosion resistant material that will never break (any fair jurist will admit that even new Dilavar studs break) and the nuts will never seize on the stud (common with Dilavar)
No installed length issues that slow down assemble time. Ask engine builders if they ever had an issue with stud install height. With the install length issue resolved you can install the piston and cylinder without the studs in place making pin and clip installation far easier. Without the studs, you can install the piston in the cylinder on the bench and install the P&C as a unit.

http://forums.pelicanparts.com/uploa...1324062473.jpg
http://forums.pelicanparts.com/uploa...1324062496.jpg


BTW: the complete set (24 studs, 24 12-point nuts, 24 hardened, ground washers, Loctite and anti seize) sells for nearly half (when all hardware is included) as much as the Dilavar stud option. $660 from our host.

After pondering the advantages of installing cylinders without the studs, I think I would want the two studs opposite the side I was working on to be there to keep the whole assembly from squirming around. Is there really inough more clearance laying the rod and assembly over on one side to make a difference?

I am recalling that the studs on the working side did, to some extent, get in my way on my most recent build. Because I don't know if I have ever been as frustrated as I was trying to get the wrist pin C clips installed on my J&E's.

And, armed with tales from others, I had the Stomski tool, too, which I never needed with the Mahle clips. I still had a lot of trouble. After various issues, including buying more clips because I had mangled some on installation (plus I had to pull the pistons to have valve pockets deepened a bit more), I finally started lubricating the insertion tool. That seemed to make quite a difference. The air color reverted from blue to a more neutral tone after that.

What I don't understand is why doesn't J&E cut their pistons for circlips with those nice little holes in them which allow the use of a tool to compress them easily, both for installation and for removal?

On my first twin plug motor we turned the barrel nuts down to a cone shape to gain plug socket clearance. And I screwed up the stud height, which led to other issues and eventually the demise of the case. Then I learned about flange nuts, which ever after I had used. Wondered why Porsche didn't use them as a matter of course. Or weren't they around in 1964?

Henry has thought this all through pretty well, so I think he is entitled to toot his own horn. I liked the package, and it gave me the opportunity to buy a moderately big ticket item from our host.

Here is what you get with Dilavar studs after 65000 miles.
This is a street driven turbo. So much for the constant sealing of Dilavar studs. BTW: no broken studs. The Pistons and cylinders showed little/no wear indicating that this engine was not abused.

http://forums.pelicanparts.com/uploa...1324166054.jpg
http://forums.pelicanparts.com/uploa...1324166064.jpg

Improperly installed? (Preload) If not broken then what happened?

The engine only had 65000 original miles. Factory installed Dilavar.
What you are seeing is what I've been seeing for 30+ years. Dilavar failure.
Even when they don't break they offer horrible head to cylinder stability.
If they actually worked to my standards I wouldn't have had to seek a remedy.

Are you suggesting that the studs are permanently deforming?

This would seem to suggest that there is a significant amount of stress relaxation taking place at around 200degC which from a metallurgical point of view seems a bit unlikely if you look at the basic composition of Dilavar.

It seems unlikely that Dilavar would creep at the preload being applied at 200-ish degC as this type of alloy would have time to rupture in the order of 10 000 hours at stress levels in the order of 100 ksi even at temperature of 1000degF.

Austenitic Stainless Steels re generally considered to have excellent creep properties and are commonly used for the manufacture of fasteners used in the high pressure stages of Gas Turbine Engines.

17-4PH on the other hand tends to be rated in terms of stress to rupture life of about 1000 hours at 70 ksi at temperatures of 1000degF.

This sort of says that if Dilavar causes problems due the a creep mechanism then martensitic stainless steel and conventional steels would be even worse.

A real puzzle.

My experience is in the field of Air cooled Porsche engine building (over 400 to date) and all the theory goes out the window when real life (practical) experience contradicts theory.
I am not suggesting anything other than under real life applications, my experience tells me that Dilavar is a horrible material.

There is other experienced engine builders that still use dilavar (993 turbo studs) with good results. Why don't they have the same experience as you? Very strange I think.

Perhaps I'm misreading your statement.
Your statement leaves me wondering how you know they (the other builders) don't have the same experiences as I do. The engine I posted is one of many engines that I've disassembled that show the same instability.
Are you suggesting that this factory built engine with leaking heads and Dilavar studs is unique to my experience?

BTW: Everyone gets to choose the parts and building process they deem appropriate for their requirements.
Dilavar studs are the safe choice because builders can blame Porsche for any failure. "I chose Dilavar because it's what the factory does". I choose something different and offer that option to others.
Like many other engine builders, I choose the push the envelope when building these engines because I want to build something special. The factory never cross drilled production cranks and yet most would have to admit that the improved lubrication offered by cross drilling is beneficial. We cross drill every crank. This is not to suggest that builders who don't cross drill every crank are doing something wrong, I'm just suggesting that there may be ways to improve on the factory process.

The point I was trying to make is that the well established behaviour of the family of materials that is being debated would indicate that they are extremely unlikely to suffer from stress relaxation.

If Dilavar suffers from creep why doesn't an alloy with lower properties exhibit the same behaviour?

There is some reasonably sound Science behind my comments about material selection which is why I provided some basic data. This is not a theory it is a practical measure.


It is quite clear that there are bolting issues with 911 engines I just don't accept that swapping stud material is guaranteed to solve the problem unless there is a sound reason.

..

I think I can provide another example of Henry's experience with Dilivar studs. My '79's engine was built with all Dilivar studs. It exhibited the same condition that Henry's picture shows. Leakage above and below the cylinders. Engine was very damp at many of the sealing surfaces. Cyls-to-case leaked, cyls-to-heads leaked, heads-to-cam towers not so bad.

The leaks weren't constant drips that would leave a puddle. Only a few drops- some at startup, some after cool-down. However the engine was always VERY wet. Upon tearing this engine down, I found the head stud nuts were inconsistently tight. Some came loose very easy, some required only a slight amount of effort. Other engines i've done, the nuts were typically a bit resistant and came free with a loud squeak. Maybe the oil leakage made its way along the studs into the threads, making the nuts easier to remove?

Not the greatest picture, but does show similar oil residue above and below the cylinders.

http://forums.pelicanparts.com/uploa...1324308012.jpg

The only problem with the Porsche studs was corrosion and stress risers (product finishing) and it has long ago been addressed.

regards

Also, it should be noted that my engine shown here is in a dedicated race car. It was rebuilt in Feb of 2005. So it is driven hard pretty much all the time once up to operating temp. I suspect that would aggravate the problem more so than a street driven engine.

I'm not trying to be argumentative here but come on!
If the stud issue is a thing of the past, how do you explain inconsistent head to cylinder sealing (leaking and movement) and broken studs. Even broken 993 TT Dilavar?

From an earlier thread on this topic:

As an mechanical engineer I am very interested in an explination of the modern engineering practices that is used.
Maybe the Supertec stud is an ok product, but I get worried when you say the best available solution from Porsche is horrible and yours is "perfect".

The arguement about Dilavar will, it seems, never end so lets try again to see if we cam make sense of the issues.

The fact that there is a bolting problem is indisputable as there are just too many engine with cylinder heads leaking which is very easy to observe.

The interesting point must be the cause of the problem and then finding a good solution.

It is also clear that the early Dilavar Studs suffered from breakages particularly in Continental USA.

Precipitation Hardening Stainless Steel does suffer from Stress Corosion Cracking particulalty in the presence of Salt. (Chlorides). The resin coating helps in this regard unless the coating becomes damaged.

Obviously if studs break then they allow the heads to leak.

If, however the stud doesn't break the only way that the head can leak is if the bolt stretches.

The only mechanism that can be responsible for a material deforming at constant load is known as creep.

I don't really believe that any rengineering steel will suffer from creep at the temperatures applied to 911 head studs.

Dilavar is basically a Precipitation Hardening Steel and its metallurgy means it is very unlikley to deform permanently at the temperatures and stresses involved in 911 head studs.

A basic estimate of the stress in a 911 head stud shows that for a torque of 24 lbsft the axial force would be about 3700lbs which would result is a stress of around 48ksi - not really very high.

My point about 17-4PH which is a Precipitation Hardening Martensitic Stainless Steel is that its creep properties are just not as good as the Dilavar family but still significantly better than standard steel bolts.

If Dilavar suffers from creep elongation so will the other materials being discussed. This is not a theoretical consideration it would be part of a considered approach to material selection using data established by significant test programmes.

As I don't think any of the bolts used are suffering from creep elongation there must be another explanation and as Magnesium and Aluminium Alloys are prone to stress relaxation at temperatures around 200degC it is possible that this is a part of the problem.

I also questioned the idea of using higher loads to 'solve' this problem and if there is an issue with stress relaxation occuring in the casting a higher load will only make it worse.

The last engine we had with leaking heads had conventional steel studs on the top and Dilavar on the bottom. Two heads were leaking quite badly (50-60psi compression) but the Dilavar studs were still tight it was the upper studs that were loose.

I am also not 100% convinced that a $10 CE ring is the best device available and there must be better ways to seal the heads.

My point was not to criticise any specific stud but to try to establish the real reasons for these problems as I think blaming Dilavar is too easy unless they have broken.

I have a lot of respect for the aftermarket and its capabilities. I tend to agree with Weissach as to the CE ring and its abilities as well as the integrity of the Barrel in the larger displacement engines. With the current design you are going to see evidence at some mileage of oil and the dirt it attracts with any of the current bolt and joint arrangements and except for the appearance the motors seam to run relatively well. Hence the cost effectiveness of replacing one for the other.

regards